Mg−X二元系新規化合物の高圧合成とその水素化特性 (X= Co, Zn, Y)
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言語: English
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公開日: 2020-12-15
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キーワード (En):作成者: ブラポーンポン, スィリー
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https://doi.org/10.15118/00010331

High-pressure synthesis of novel compounds in Mg–X
binary systems and their hydrogenation properties(X = Co, Zn and Y)

Mg−X 二元系新規化合物の高圧合成とその水素化特性(X= Co, Zn, Y)

Siree Burapornpong
Muraran Institute of Technology

A dissertation submitted in partial fulfillmentof the requirements for the degree ofDoctor of Engineering
2020

Contents
Chapter 1 Introduction
1.1 Overview of high-pressure synthesis

1

1.1.1 High pressure science1

1.1.2 Effect of the high pressure2

1.1.3 Exploration of novel functional materials by high-pressure technique

11

1.2 Magnesium and magnesium alloys for hydrogen storage material

12

1.3 Synthesis of Mg-based materials by high pressure synthesis

16

1.4 Purpose of this study

18

References

19

Chapter 2 Experimental methods
2.1 Sample preparation

28

2.2 High-pressure synthesis

29

2.3 High-pressure cell

33

2.3.1 High-pressure cell assembly without internal H source

33

2.3.2 High-pressure cell assembly with an internal H source

33

2.4 Characterization method

35

2.4.1 X-ray diffraction analysis (XRD)

35

2.4.2 Scanning electron microscope with energy dispersive X-ray spectrometry

38(SEM/EDX)
2.4.3 Differential scanning calorimetry (DSC)

40

2.4.4 Thermogravimetry-differential thermal analysis-mass spectrometry42(TG-DTA-MS)
2.4.5 PCT measurement43

2.5 Crystal structure analysis

45

2.5.1 Rietveld refinement analysis
References

45
48

Chapter 3 High-pressure synthesis of novel compound in the Mg – Co system
3.1 Introduction

50

3.2 Sample preparation

51

3.3 High-pressure synthesis of novel Mg – Co compounds

52

3.4 Crystal structure refinement55

3.5 Effect of pressure on the presence of the Mg44Co7 phase57

3.6 Thermal stability measurement62

3.7 Hydrogenation properties

63

3.8 Summary

70

References

71

Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn system
4.1 Introduction

75

4.2 Sample preparation

77

4.3 High-pressure synthesis of novel Mg – Zn compounds

77

4.4 Crystal structure refinement82

4.5 Effect of pressure on the solid solubility limit of Zn in Mg

83

4.6 Thermal stability measurement87

4.7 Hydrogenation properties

89

4.8 Summary

93

References

94

Chapter 5 High-pressure synthesis of a novel compound in the Mg – Y system
5.1 Introduction

96

5.2 Sample preparation

97

5.3 High-pressure synthesis of novel Mg – Y compounds

97

5.4 Crystal structure refinement99

5.5 Effect of pressure on the presence of the Mg3Y phase101

5.6 Thermal stability measurement

106

5.7 Hydrogenation properties

108

5.8 Summary

112

References

113

Chapter 6 Conclusion

116

List of Achievements

119

Acknowledgement

120

Chapter 1
Introduction
Alternative energy has an important role in slowing down global warming. The alternative
energy has been expected to replace fossil fuel to decrease emission air pollution andgreenhouse gases. However, the production and the use of alternative energy have limitations
depending on weather and topography for natural resource energy, such as wind energy, solar
energy, and hydropower energy. Additionally, the energy storage material properties of some
types of alternative energy are improper for efficient use in some applications, such as
thermoelectric energy, magnetic energy, and hydrogen energy.
Many research studies emphasize the enhancement of energy material properties for
obtaining higher use efficiency. One of the key challenges to enhancing the energy material
properties is the development of novel energy materials.

1.1 Overview of high-pressure synthesis
1.1.1 High-pressure science
High-pressure technology is widely used to develop frontiers in many research fields ofphysics, chemistry, biology, geology, and materials science by applying the external pressure
on materials. Pressure (P) is a factor that effects extremely on the changes in physical andchemical properties in the materials. In the field of organic materials such as pharmaceuticaland biological science, the uses of high-pressure technology are mainly in the range of MPa[1–4]. In the field of inorganic materials such as materials science and geology science, highpressure technology is used in the pressure range of GPa [5,6]. The effect of pressure inducesa change in atomic size, atomic distance, and electron configuration in condensed matters,
which influences physical properties such as densification, electrical conductivity, andmagnetic properties of the materials. Therefore, applying pressure is widely used tosynthesize the materials with new functions such as super-hardness, superconduction,
thermoelectricity, magnetism, and hydrogen storage property [7,8].
High-pressure synthesis can be classified into 2 types: static-type and dynamic-type
technologies. For the static-type apparatus, piston-cylinder type and Bridgeman anvil-typeapparatuses are widely used [7,9]. The piston-cylinder type apparatus was developed to allow
maximum pressure up to 3 GPa for tungsten carbide (WC) pressure cell and up to 4.6 GPa for
NiCrAl pressure cell [10,11]. The Bridgeman anvil type apparatus can generate higher

1Chapter 1 Introduction

maximum pressure depending on the material of anvils, in which hard alloy anvils for 15 – 20
GPa, SiC anvils for 20 – 70 GPa, and diamond anvils for 100 – 300 GPa [12]. For the dynamictype apparatus, the pressure is generated by a shock wave technique that generates dynamic
pressure in the range of 100 – 1000 GPa in a short period for a microsecond to nanosecond on
a material [13,14].
Moreover, the pressure-transmitting medium of static-type high-pressure synthesis can bedivided into 2 types:
1. Fluid pressure-transmitting medium
Fluid pressure-transmitting medium is subdivided into 2 types that are gas media
such as argon gas, neon gas, and helium gas and liquid media such as the mixture ofmethanol and ethanol, and silicone oil [15]. Choosing the fluid pressure-transmitting
medium is necessary to consider temperature and pressure conditions, besides, thepotential interaction with the sample material is also considered [16].
2. Solid pressure-transmitting medium
Solid pressure-transmitting medium is also necessary to consider the endurance forhigh pressure and temperature. WC, SiC and diamond are used as a solid pressuretransmitting medium in agreement with their incompressible and high hardness
properties [12]1.1.2. Effects of the high pressure
Pressure is a thermodynamic parameter that determines changes in physical and chemical
properties. The effect of the high pressure on the elements is as follows.
1. Compression of atomic volume
Compression behavior of solids under the effect of high pressure can be described by
the second-order Birch-Murnaghan equation of state [17,18]. The relationship between
pressure and volume is given by

7

5

3
V0 ⁄3
V0 ⁄3
P = K0 [( ) − ( ) ]2
VV

Eq. 1.1

2

Chapter 1 Introduction

where,P= pressure

K0= bulk modulus

V0= reference volume

V= deformed volume

For conveniently, the change of volume is expressed as follows [17,19]:∆V
V − V0== aP + bP2
V0
V0

Eq. 1.2

where,P= pressure

V0= reference volume

V= deformed volume at pressure Pa= the isothermal compressibility at zero pressureb= the pressure dependence of the compressibility

The a and b values of each element are given in [19].
The change in the atomic volume of the elements corresponds to the change in their
atomic radius. The atomic radius of the elements is a factor which influences the stability
and occurrence of phases in term of the atomic radius ratio. For the Laves phases, theatomic radius ratio of the constituent elements in a range of 1.05 to 1.67, of which the ideal
value is 1.225, is limit for the phase formation [20].

3Chapter 1 Introduction

Fig. 1.1

Relationship between pressure and change in the atomic volume ofthe elements [19].

2. Increasing of coordination number (CN) and atomic packing factor (APF)
As the definition of a coordination number (CN), that is number of atoms around thecentral atom of a crystal structure. When pressure is applied to a crystal structure, an atom
is pushed closer to each other atom in the structure, that means coordination number
increases with increasing pressure. Furthermore, the increase in pressure also leads tostructural phase transformation to a higher atomic packing ratio [21].
The increase in pressure causes the decrease of interatomic distances and atomic sizes,
that is, the increase of density. The electrons in the outermost shell are allowed to movecloser to their nuclei. The change in the outermost electron shell, which moves closer tothe nuclei, causes the overlap between the electron shells [5,22]. This phenomenon also
induces the change in crystal structure due to rearrangements of atomic packing andphysical and chemical properties.
For example:-

Alkali metals in bcc structure are transformed into FCC structure at a pressure
above 7.5 GPa for Li, 65 GPa for Na, 11.6 GPa for K, 7 GPa for Rb, and 2.4
GPa for Cs [21].

-Si element has a diamond structure under ambient pressure. The crystal
structure of Si transforms to -Sn-type and simple hexagonal by the effect of

4

Chapter 1 Introduction

pressure with the increasing coordination number from 4 to 6 to 8, and theincreasing atomic packing factor from 0.34 to 0.55 to 0.6 [21].
-ZnS (CN = 4) transform to the NaCl-type structure (CN = 6) at 15.7 GPa [23].
The atomic packing factor of ZnS increases from 0.525 to 0.809 when it
transforms to NaCl-type structure.

-

NaCl (CN = 6) transform to the CsCl-type structure (CN = 8) when thepressure more than 30 GPa is applied on NaCl at room temperature [24]. The
atomic packing factor of NaCl increases from 0.67 to 0.87 when it transforms
to CsCl-type structure.

The examples of the structural transformation of elements and the change incoordination number with increasing pressure are given in Tables 1 and 2.
Table 1 Structural transformations of elements due to increasing pressure [21].
Element
Li
Na
K
Rb
Cs

Crystal structure
7.5 GPa
BCC

FCC→
65 GPaBCC

FCC→
11.6 GPa

BCC

FCC

→
7 GPa

BCC

FCC

→
2.4 GPa

BCC

FCC

→
12 GPaSi

Diamond-type→

13 GPa-Sn-type→

Simplehexagonal

5Chapter 1 Introduction

Table 2 Examples of structural transformation and the change in the coordination number,geometry, and crystal structure type with increasing pressure.
Pressure

Coordination
numberGeometry

Crystal structure type4

Tetrahedral

ZnS-type

6Octahedral

NaCl-type

8

Cubic

CsCl-type

Low

High
3. Increase in melting point
The effect of pressure on the phase transformation temperature has been revealed byClausius-Clapeyron relationship [25] that describes the relationship between pressure andtemperature of 2 phases in equilibrium. The Clausius-Clapeyron relationship is expressed
as follows:dP∆trs S=dT∆trs V

Eq. 1.3

where,∆trs S= the change in entropy of the system∆trs V = the change in volume that occurs on transformation
For effect on the melting point, the two phases are solid and liquid. The melting pointof the matter increases due to increasing pressure in case the matter volume increases on

6

Chapter 1 Introduction

melting (Vl > Vs). The entropy of melting (∆trs S) at T is∆trs HT, hencedP∆trs H=dT
T∆trs VEq. 1.4

where,∆fus V = the change in volume that occurs on melting (∆trs V is positive)∆fus H = the change in enthalpy that occurs on melting (∆trs H is positive)
When Eq. 1.4 is integrated from a pressure P0 to a pressure Pf ,Pf

Tf∆trs H 1∫ dP =∫ dT∆trs V T

P0

Eq. 1.5

T0

hence,

Pf = P0 +∆trs H Tf
ln∆trs V T0

Eq. 1.6

From Eq. 1.6, pressure induces the availability of solid-state reaction at a higher
temperature than the constituent elements' normal melting points.

Fig. 1.2

Change of melting point of the elements under pressure in GPa-range

7

Chapter 1 Introduction

4. Increasing of chemical potential
The chemical potential () is a partial Gibbs’ free energy. The chemical potential can be
written in term of Gibbs’ free energy (G) as
G = nEq. 1.7

where n is the number of moles.
The chemical potential is defined as the change in the internal energy (U) due to thechange in a mole of the substance of a system. The internal energy in differential form is
dU = TdS – PdV + dn

Eq. 1.8

From the Gibbs’ free energy (G) which is defined as

G = U + PV − TS

Eq. 1.9

The differential of Eq. 1.8 can be expressed as follows:dG = dU + PdV + VdP − TdS − SdT

Eq. 1.10

and then, Eq. 1.8 is replaced in Eq. 1.10, Gibbs’ free energy change can be written as
dG = VdP – SdT + dn

Eq. 1.11

At constant temperature and mole number (dT = 0 and dn = 0), term of SdT anddn become zero, thus Eq. 1.11 can be written as

dG = VdP

Eq. 1.12

When Eq. 1.12 is integrated from a pressure P0 to a pressure Pf ,8

Chapter 1 Introduction

Pf∆G = G(Pf ) − G(P0 ) = ∫ V dP

Eq. 1.13

P0

where P0 and Pf are reference pressure, and pressure at condition f was obtained.
In case of the ideal gas

Eq. 1.14

PV = nRT
Eq. 1.13 then becomes

Pf

Pf

G(Pf ) − G(P0 ) = ∫ VdP = nRT ∫
P0

P0

1
dP
P

Eq. 1.15

hence,

Gf = G0 + nRT ln

PfP0

Eq. 1.16

From the relation of the chemical potential and Gibbs’ free energy in Eq. 1.7, Eq. 1.16
can be derived as follows:
For, gas-state,f = 0 + RT ln

PfP0

Eq. 1.17

For, solid or liquid-statef = 0 + Vsolid or liquid (Pf − P0 )Eq. 1.18

where 0 and f are chemical potential at a reference pressure and a pressure atcondition f, respectively.

9Chapter 1 Introduction

From Eqs. 1.16 and 1.17, the Gibbs’ free energy and chemical potential increase withincreasing the pressure. Y. Fukai and H. Sugimoto studied the chemical potential ofhydrogen in metals under high-pressure conditions in the range of 0.1 MPa to 100 GPa[26–28]. Due to the effect of pressure on thermodynamics, the increase of pressure tohigher than 1 GPa induces the extreme increase of the chemical potential of hydrogen [27]and enhances the solubility of hydrogen in metals exceedingly [26]. Moreover, the Gibbs’
free energies and chemical potentials of solid and liquid-states have the same tendencywith gas-state with lower increasing rates because the change in volume of solid and liquid
are lower than that of gas [25].

Fig. 1.3

Schematic of (a) the chemical potential of the solid, liquid and gas phasesand (b) the effect of pressure on the chemical potential of the solid and liquid
phases, and melting temperature [25].

The effect of pressure in GPa-order on the structure and properties of the materials has
been interested in exploring novel compositions, modifying new structures of materials,
developing and improving some properties of materials that cannot be obtained by othertechniques ambient pressure.

10Chapter 1 Introduction

1.1.3 Exploration of novel functional materials by high-pressure technique
The high-pressure technique is utilized to synthesize novel materials, to modify the
structure of materials, and to improve some properties of materials.
1. Super-hard materials
Synthesis of super-hard materials is mainly focused on exploring new super-hard phasesof B, C, O, and N elements. The new phases of super-hard material have been expected tohave thermal and chemical stability more than that of a diamond and hardness higher thanthat of cubic-boron nitride (cBN) [29,30]. Cubic BC2N [31] and diamond-like BC5 [32]are examples of the synthesized super-hard materials which have thermal stabilities andhardness higher than diamond and cBN, respectively.
2. Thermoelectric materials
High-pressure synthesis technique is used to improve thermoelectric materials'
efficiency, which is indicated by ZT value. A large ZT value is an index of the efficiencyof the thermoelectric materials. Recent studies have shown that the ZT values of CoSb3based compounds have been enhanced by filling rare earth elements [33–35] and filling
Ba element [36] using high-pressure synthesis.
3. Hydrogen storage materials
Hydrogen storage materials are required to have lightweight and to be composed ofabundant elements. Aluminum and magnesium have been focused as a base element ofhydrogen storage materials. Those hydrides have high hydrogen storage capacity 10
mass% for AlH3, and 7.6 mass% for MgH2, however, they have thermodynamics andkinetics problems. Alloy is one of the most common methods to overcome the problems.
High-pressure synthesis, which is an effective technique to synthesize novel materials, hasalso been applied to explore novel Al-based alloys and Mg-based alloys for the hydrogenstorage materials.
AlH3 has not only high hydrogen storage capacity but also has low thermal stability. It
easily decomposes at a low temperature of 175 – 200 °C [37], however, re-hydrogenationof Al required at H2 pressure more than 2 GPa [38,39]. Alkali metal-alanates have been
focused on being hydrogen storage materials after Bogdanovic and Schwickardi reported
reversible hydrogenation properties of NaAlH4, Na3AlH6, and Na2LiAlH6 [40]. By highpressure synthesis which uses H2 gas as a pressure medium, K2LiAlH6 was synthesized at
70 MPa of H2 pressure [41]. Moreover, Al3TiH0.4 and Al2CuH were synthesized at 10 GPa

11

Chapter 1 Introduction

with internal H source [42,43].
MgH2, which has high hydrogen storage capacity and reversible hydrogenation
properties, has drawbacks of high decomposition temperature (high stability) and slowhydrogen sorption kinetics. Many new Mg-based alloys and hydrides synthesized by highpressure synthesis have been reported that their desorption temperatures are lower than the
MgH2 [44–47]. The details of newly synthesized Mg-based alloys and hydrides are
introduced in section 1.3.

1.2 Magnesium and magnesium alloys for hydrogen storage material
Mg is attractive for hydrogen storage materials, because it is a lightweight metal with adensity of 1.74 g/cm3, an inexpensive metal due to natural abundance in the earth's crust, andnon-toxic to the environment. MgH2 has some unsatisfied properties that are its high thermal
stability and slow hydrogenation/dehydrogenation kinetics. Mg and MgH2 require highoperation temperatures around 300 – 400 °C for de/hydrogenation reactions, as shown in Eq.
1.19. Moreover, the enthalpy of formation of MgH2 is 74 kJ/mol H2.

Mg + H2 ↔

MgH2

Eq. 1.19

In order to overcome these unsatisfied properties, many techniques have been proposed.
1. Improving the kinetics of hydrogen absorption/desorption reaction in Mg-basedhydrogen storage materials
To improve the kinetics of hydrogen absorption/desorption reaction in Mg-basedhydrogen storage materials, it needs to decrease the activation energy of the reactions. The
reduction of activation energy method has been proposed, such as particle size reduction
and addition of catalysts.
The particle sizes of Mg and MgH2 are one-factor affecting the kinetics of hydrogen
absorption/desorption reaction. Decreased particle sizes of Mg, MgH2, and Mg-basedmaterials using ball milling, hydrogen-plasma metal reaction technique, and thin-film
synthesis exhibited faster absorption/desorption kinetics [48–51].
The studies of the effect of metal oxides, metal fluorides, transition metals (TM), andrare earth metals (RE) as catalysts have been studied to improve the sorption kinetics of
Mg. The metal oxides such as Nb2O5, TiO2, Sc2O3, V2O5, Cr2O3, Mn2O3, CuO, Al2O3, and12

Chapter 1 Introduction

SiO2 have been reported that the absorption and desorption kinetics of Mg/MgH2 can beenhanced by them [52–55]. Oelerich [53] studied the influence of the metal oxides (Sc2O3,
TiO2, V2O5, Cr2O3, Mn2O3, Fe3O4, CuO, Al2O3, and SiO2) on absorption and desorption
behaviors of MgH2. Addition of Cr2O3 was the most effective in improving the hydrogen
absorption kinetics of the MgH2, and V2O5 and Fe3O4 were also effective in improving thehydrogen desorption kinetics at 300 °C [45]. Barkhordarian [52] has been reported better
results of both absorption and desorption kinetics of MgH2 by using Nb2O5. Furthermore,
the MgH2 catalyzed with Nb2O5 showed hydrogen absorption at room temperature [55].
The 3d-transition metals such as Ti, V, Mn, Fe, Co, Ni, and Cu have also been studied on
their influence on absorption and desorption behaviors of MgH2, and they showed betterboth hydrogen absorption and desorption kinetics than non-added MgH2 [56,57]. Amongof Ti, V, Mn, Fe, and Ni, the fastest hydrogen desorption and absorption were obtained byusing V and Ti, respectively [56], whereas, Ni most effectively lowered the hydrogen
desorption temperature and sharpened a peak of hydrogen emission [57].
Rare earth metals have been reported as an effective catalyst in the form of rare earthoxide and rare earth hydride, which presented during the material preparation process andhydrogenation process [58–60]. As an oxygen getter of the rare-earth metals, adding rareearth metal reduces the formation of oxide on Mg surface. Mg3RE (RE = La, Pr, Nd, andMm) prepared by induction melting can rapidly absorb hydrogen at room temperature due
to disproportionate into magnesium hydride and rare earth hydride [59,61,62].
2. Improving thermodynamics of Mg-based alloys
To improve the hydrogen sorption thermodynamics of Mg, the reduction of enthalpy offormation (∆Hf ) method has been concentrated such as alloying.
Reactions between Mg – TM-based alloys and H2 were studied, such as Mg – Cu andMg – Ni alloys [63,64]. Mg2Cu disproportionates to MgH2 and MgCu2 duringhydrogenation [63,65], as shown in Eq. 1.20 with a hydrogen storage capacity of 2.6
mass%.

2 Mg2 Cu + 3 H2 → 3 MgH2 + MgCu2

Eq. 1.20

The enthalpy of formation of this reaction is approximately -70 kJ/mol H2, lower thanthe enthalpy of formation of MgH2 (∆Hf = -74 kJ/mol H2 ) in Eq 1.19.
Mg2Ni reacts with H2 to form Mg2NiH4 with the enthalpy of formation of -64.5 kJ/mol

13Chapter 1 Introduction

H2 [64]. Although, the enthalpy of formation of the Mg2NiH4 becomes higher value thanthat of the MgH2, its hydrogen storage capacity declines to 3.6 mass%.
Mg2 Ni + 2 H2 ↔

Mg2 NiH

Eq. 1.21

4

Mg2Cu and Mg2Ni alloys are examples of stable Mg-based alloys which have beenstudied for enhancing thermal stability.
On the other hand, not only stable Mg-based alloys have been studied. Mg2FeH6 is oneof the Mg-based hydrides which is attended because of its hydrogen storage capacity of
5.5 mass%. Mg2FeH6 can be prepared according to Eq. 1.22 by sintering process underhigh pressure of H2 [66–68] and mechanical alloying process [68–71]. The enthalpy offormation is approximately -77.4 kJ/mol H2 [72,73].

2 Mg + Fe + 3 H2 →2 MgH2 + Fe + H2 →Mg2 FeH6

Eq. 1.22

For the synthesis of Mg2FeH6, Mg or MgH2 and Fe are used as starting materials because
there is no stable compound in Mg – Fe binary phase diagram. Mg2Fe alloy, which is an
unstable compound, has been tried to synthesize for using as starting materials of the
Mg2FeH6, but it has not been obtained. After dehydrogenation of Mg2FeH6, it decomposes
into Mg and Fe.

Mg2 FeH6 →2 Mg + Fe + 3 H2

Eq. 1.23

MgCo2 is the only one stable intermetallic compound in Mg – Co binary system. Its
hydrogenation property has not been reported under ambient pressure. In contrast, thehydrogenation at the GPa-order pressure of MgCo2 induces disproportionation into Co
phase and a Mg – Co hydride phase [74]. In Mg – Co – H ternary system, Mg2CoH5 andMg6Co2H11 have a hydrogen storage capacity of 4.5 mass% and 4.0 mass%, respectively.
Because Mg2Co is an unstable compound, Mg2CoH5 has been prepared by the sintering
process of Mg and Co under H2 pressure or ball milling of the MgH2 – Co mixture under
Ar atmosphere or the Mg – Co mixture under H2 atmosphere.
4 Mg + 2 Co + 5 H2 ↔

2 Mg2 CoH5

Eq. 1.24

14

Chapter 1 Introduction

4 MgH2 + 2 Co + H2 ↔

2 Mg2 CoH5

Eq. 1.25

Moreover, Mg6Co2H11 can be obtained by hydrogenation of ball-milled 2Mg – Co
mixture at 300 °C for 1 h [75].
Enthalpy of formation of Mg2CoH5 is approximately 86 kJ/mol H2 for desorption, and 60 kJ/mol H2 for absorption [76] and that of Mg6Co2H11 is approximately 75 kJ/mol H2 fordesorption and -62 kJ/mol H2 for absorption [77].

Furthermore, novel compounds between Mg and transition metals, which means there are
not exist under ambient pressure, have been synthesized to be Mg-based hydrides by highpressure synthesis technique. Kyoi et al. and Okada et al. reported the synthesis of Mg – TM
hydrides where TM was Ti, V, Cr, Mn, Zr, Hf, Nb, and Ta by high-pressure synthesis with aninternal H source [44,45,78–82]. Moreover, Goto et al. reported Mg – TM hydrides where
TM was Zr, Nb, and Mo by high-pressure synthesis without an additional H source [83,84].
The thermal stabilities of these hydrides are lower than that of MgH2, and the hydrides
decomposed around 100 – 180 °C.
Besides the transition metals (TM), rare earth (RE) metals have also been studied todecrease the absorption and desorption temperatures of the Mg-based alloys. Both Mg andrare earth metals can absorb hydrogen, therefore, hydrogen storage capacities of their alloys
have also been expected, and the Mg – RE alloys recombinated with H2 into MgH2 and REHx.
Except for the studies in Mg – RE – H systems (RE = Y, La, Ce, Pr, Sm, Gd, Tb, and Dy) by
high-pressure synthesis, they presented the hydrides of Mg3REH9 (RE = La, Ce, and Pr) withtetragonal structure and MgRE2Hx (RE = Y, Gd, Tb, and Dy) with an FCC structure [85–87].
In Mg – Y system, the nominal composition of Mg88Y12, which consists of Mg and Mg24Y5,
prepared by vacuum induction furnace has an efficient hydrogen storage capacity of 5.6
mass% at 380 °C [80]. Its enthalpy of formation was approximately -79.8 and 81.2 kJ/mol
H2 for hydrogenation and dehydrogenation, respectively [88]. The nominal composition of
Mg3Y, which consists of Mg24Y5 and Mg2Y, prepared by induction melting has a hydrogenstorage capacity of approximately 4.5 mass% at 375 °C, and the enthalpy of formation on
dehydrogenation process have been reported as 76.4 kJ/mol H2 [89].
Mg3RE (RE = La, Pr, Nd, and Mm) prepared by induction melting under an Ar atmospherehave hydrogen absorption capacities of 2.89, 2.58, 1.95, and 2.58 mass% and enthalpies of15

Chapter 1 Introduction

formation of those Mg3RE were reported to be 81.0, 79.9, 68.2, and 74.5 kJ/mol H2,
respectively, for dehydrogenation [59,61,62].
Mg3-REH9-2 prepared by high-pressure synthesis using MgH2 – x at% REH (x = 25 – 33
and REH = LaH3 and CeH2.5) presented the decreasing thermal stability of hydrides with anincreasing amount of rare earth element [85].

1.3 Synthesis of Mg-based materials by high-pressure synthesis
As mentioned in the previous sections, high-pressure synthesis technique has been used tosynthesize novel Mg-based alloys and hydrides. Many new structures, compositions, andhydrides have been successfully synthesized because of the effect of pressure on physical andchemical properties. At 6 GPa pressure, Mg has a change in atomic volume more than 10%,and its melting point increases to around 900 °C; therefore, the reaction between unusual
atomic sizes of Mg and other elements and the solid-state reaction at higher normal melting
points are practicable.
1.3.1 Mg – TM compounds
In Mg – TM binary systems, high-pressure phases of Mg – Cu and Mg – Ni compounds
were obtained by high-pressure synthesis. MgCu, Mg51Cu20, and Mg6Ni which belong to
CsCl-, Mg51Zn20-, and Mg6Pd-type structure, respectively, have been synthesized at 6 GPa[46,47,90]. The crystalline MgNi with a CuTi-type, prepared from amorphous MgNi, wasfirst reported by high-pressure synthesis at more than 5 GPa [46]. The crystalline MgNi hasreversible hydrogenation properties at 200 °C [91].
1.3.2 Mg – TM hydrides
MgH2 and transition metals which are immiscible with Mg were used as starting materials.
For high-pressure synthesis with internal H source, Mg7TiH16 [44,92], Mg6VHy [45] andFCC of Mg – Zr – H [93] were synthesized at 8 GPa pressure. Moreover, monoclinic
Mg3MnH5-6 (at 6 GPa)[79] and Mg3CrHx (at 6 GPa)[80], and FCC of Mg – Hf – H (at 4 GPa)[81] were also synthesized.
For high-pressure synthesis without internal H source, Mg2Zr3Hy was synthesized at
800 °C under a pressure of more than 2 GPa. Mg2Zr3Hy decomposed into Mg, ZrH2, and H2
with thermal stability up to around 450 °C. Mg4NbHy and Mg3MoH6 were synthesized under
5 GPa at 800 °C and 900 °C, respectively. Their thermal stabilities were reported up to 400– 420 °C. After dehydrogenation of these synthesized Mg – TM hydrides, stabilization of

16

Chapter 1 Introduction

Mg – TM alloys was not observed.
1.3.3 Mg – RE hydrides
In Mg – RE – H ternary systems, MgH2 and REH3 were used as starting materials. MgRE2
hydrides (RE = Y, Gd, Tb, and Dy) with an FCC-type structure were synthesized at 800 °C
under a pressure of more than 3 GPa except for MgTb2Hy which was obtained at 6 GPa.
Hydrogen capacity of MgY2 hydride was estimated to be 3.7 mass% [85], which partiallydesorbed approximately 1.4 mass% at 330 °C [83]. The reversible hydrogen absorption wasavailable under its synthesized condition with an internal H source.
Mg3REH~9 (RE = La, Ce, and Pr) with a primitive tetragonal-type structure weresynthesized at 800 °C under 3, 4, and 6 GPa, respectively. These Mg3REH~9 showed their
thermal stabilities up to around 330 °C. They decomposed into Mg phase and rare earth
hydride phases. Their hydrogen capacities were estimated to be approximately 4 mass%.

17

Chapter 1 Introduction

1.4 Purpose of this study
The same group's elements in the periodic table have similar chemical properties becauseof the same number of valence electrons. Therefore, the chemical composition and crystal
structure of A – X compounds where X elements are the elements in the same group tend to be
similar.
In the previous research studies of exploration of novel compounds by high-pressure
synthesis, MgCu with a CsCl-type, Mg51Cu20 with a Mg51Zn20-type and Mg6Ni with a Mg6Pdtype have been synthesized [90,94]. Those compounds have the same composition and crystal
structure with MgAg, Mg51Ag17, and Mg6Pd where Ag and Pd are the element in the samegroup with Cu and Ni. The effect of pressure on adjusting the atomic radius ratio between theconstituent elements has been postulated to be an important factor for obtaining thosecompounds. The atomic radius ratio between Mg and Cu, and between Mg and Ni under theeffect of pressure in GPa-order changed close to the atomic radius ratio between Mg and theother elements in their same group under ambient pressure due to the different compressibilityof each element. Furthermore, Cu and Ni also have the same crystal structures as Ag and Pd,
respectively. For the same crystal structure of the elements, it is related to the number of valence
electrons of the elements.
X elements are Co, Zn, and Y to clarify the effect of the atomic radius ratio and the crystal
structure of the constituent elements because Co, Zn, and Y have different crystal type structurewith the other elements in the same group under ambient pressure in this study.
Therefore, the purpose of this study is to investigate novel Mg – X compounds (X = Co, Zn,
and Y) using a cubic-anvil-type apparatus. The crystal structures, thermal stabilities, andhydrogenation properties of the newly found compounds were investigated. Furthermore, theeffect of pressure in GPa-order induces an increasing chemical potential of hydrogen in thesolid-state. Therefore, the hydrogenation of the compounds at high-pressure in GPa-order was
also conducted using cubic-anvil-type apparatus.
The hypothesis was that besides the ratio of the atomic radii of the constituent elements,
the crystal structures of the compounds would also influence the formation of somecompositions and types of structures of the compounds that were unable to be obtained underambient pressure conditions. It was thought that the effect of pressure would induce the change
in the valence electron configuration of the elements, which would then lead to changes in thechemical properties and crystal structures of the compounds. The changes in valence electron
configuration possibly lead to different chemical reactions occurring other than the one that
occurs under normal condition.
18

Chapter 1 Introduction

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https://doi.org/10.4028/www.scientific.net/MSF.419-422.983.
[87] Y. Goto, A. Kamegawa, H. Takamura, M. Okada, Synthesis of new hydrides in Mg–Y
systems by using high pressure, Mater. Trans. 43 (2002) 2717–2720.
https://doi.org/10.2320/matertrans.43.2717.
[88] T. Yang, Z. Yuan, W. Bu, Z. Jia, Y. Qi, Y. Zhang, Evolution of the phase structure andhydrogen storage thermodynamics and kinetics of Mg88Y12 binary alloy, Int. J.
Hydrogen Energy 41 (2016) 2689–2699.
https://doi.org/10.1016/j.ijhydene.2015.12.099.
[89] L.Z. Ouyang, Z.J. Cao, L. Yao, H. Wang, J.W. Liu, M. Zhu, Comparative investigationon the hydrogenation/dehydrogenation characteristics and hydrogen storage propertiesof Mg3Ag and Mg3Y, Int. J. Hydrogen Energy 39 (2014) 13616–13621.
https://doi.org/10.1016/j.ijhydene.2014.02.135.
[90] H. Watanabe, Y. Goto, H. Kakuta, A. Kamegawa, High Pressure Synthesis of Novel
Compounds in Mg–TM Systems (TM= Ti-Zn), Mater. Trans. 45 (2004) 1350–1354.
https://doi.org/10.2320/matertrans.45.1350.
[91] Y. Kamata, T. Kuriiwa, A. Kamegawa, M. Okada, Effect of Cu or Ti Substitution in
MgNi on Crystal Structure and Hydrogen Absorption-Desorption Properties, Mater.
Trans. 50 (2009) 2064–2068.

26

Chapter 1 Introduction

https://doi.org/10.2320/matertrans.M2009064.
[92] E. Rönnebro, D. Kyoi, A. Kitano, Y. Kitano, T. Sakai, Hydrogen sites analysed by X-ray
synchrotron diffraction in Mg7TiH13-16 made at gigapascal high-pressures, J. Alloys
Compd. 404–406 (2005) 68–72.
https://doi.org/10.1016/j.jallcom.2005.03.106.
[93] D. Kyoi, T. Sakai, N. Kitamura, A. Ueda, S. Tanase, Synthesis of FCC Mg–Zr and Mg–
Hf hydrides using GPa hydrogen pressure method and their hydrogen-desorption
properties, J. Alloys Compd. 463 (2008) 311–316.
https://doi.org/10.1016/j.jallcom.2007.09.004.
[94] Y. Goto, High pressure synthesis of novel compounds of Mg–X(–H) and their thermal
stability (X = Alkali metal, Transition metal, Rare earth) (in Japanese), Tohoku
University, 2006.

27

Chapter 2
Experimental method
2.1 Sample preparation
To perform high-pressure synthesis experiments in this study, sample precursors weremixed by grinding with an alumina mortar and a pestle. Then, they were pressed into a pellet
at a pressure of 20 MPa applied for 1 min by a hydraulic press machine. The sample pellets
were a diameter of 3.75 mm and a thickness within 2.0 mm. The sample pellet was assembled
into a cubic high-pressure cell, as described in section 2.3.
The sample preparation was performed in an argon gas-filled glove box to avoid oxidationof samples in air.

Fig. 2.1

Schematic of the sample pellet preparation for the high-pressure
synthesis.

28

Chapter 2 Experimental method

2.2 High-pressure synthesis
In this study, the solid pressure-transmitting medium of static-type high-pressure synthesis
was used to perform the experiments. 700-ton hydraulic press apparatus assembled with a
cubic-anvil-type high-pressure system (TRY ENGINEERING Co., Ltd.), as shown in Fig. 2.2,
was used to synthesize the samples. The cubic-anvil-type high-pressure system consists of 6anvils which are a top anvil, a bottom anvil, and 4 side anvils. The tungsten carbide (WC)
anvils were employed in the experiments. The specifications of the apparatus and available
experimental conditions are shown in Table 2.1.

Fig. 2.2700-ton hydraulic press of cubic high-pressure synthesis
apparatus. (TRY ENGINEERING Co., Ltd.)

29

Chapter 2 Experimental method

Table 2.1 Specifications of the 700-ton press apparatus and the cubic anvil-type high-pressure
systemMaximum Output (kg)7  105

Direction of Press<100>

Sample chamber size (mm3)12  12  12

Pyrophyllite volume (mm3)15  15  15

Maximum sample size (mm3)3.75  2.0

Generated pressure (GPa)2-6

The fundamental principle of high-pressure synthesis apparatus for generating pressure isexpressed as follows:

P =

F
A

Eq. 2.1

where,P= pressureF= applied forceA= areaThus, high-pressure is achieved by applying force to a small area. The force is transmitted
to a high-pressure cell in the <100> axis direction by a hydraulic press through the 6 anvils.
The anvils have a large surface area side that contacts a wedge block to apply the force from ahydraulic press and a small surface area side that contacts and transmits the force to a highpressure cell. The top view of the anvil system and schematic of the pressing system of a cubicanvil-type apparatus are shown in Figs. 2.3 and 2.4, respectively.
Electric power is converted to heat to generate heat in the synthesis process by the following
Joule's equation of electrical heating.
2

P = IV = I R

Eq. 2.230

Chapter 2 Experimental methodand2

H = Pt = I Rt

Eq. 2.3

whereH= heatP= electric powerI= electrical current flowR= electric resistancet= timeTherefore, heat can be increased by increasing electric power that electrical current is also
increased. The electrical current flows from the top anvil through the parts of the high-pressure
cell to the bottom anvil. The surface of side anvils between the top anvil and button anvil arecovered by Teflon sheets working as an electric insulator to control the electrical current
pathway. When the electrical current flows through the graphite, which is a high resistivity partin the high-pressure cell, it generates the heat to the samples.

Fig. 2.3

Top view of the anvil system.

31

Chapter 2 Experimental method

Fig. 2.4

Schematic of the pressing system of a cubic-anvil-type system.

The temperature calibration was performed by measuring the temperature at the center ofa cubic high-pressure cell by thermocouple under 105 kg of pressure. The relationship between
the amount of electrical current flow and temperature at the center of a cubic high-pressure cellis shown in Fig. 2.5.

Fig. 2.5

Calibration curve of sample cell temperature under 105 kg of pressure.

32

Chapter 2 Experimental method

2.3 High-pressure cell
2.3.1 High-pressure cell assembly without internal H source
The main parts of a cubic high-pressure cell consist of a pyrophyllite pressuretransmitting medium block preheated at 630 C for 8 – 15 h, molybdenum disks, and stainlesssteel rings (SUS rings) working as an electrode, carbon heater, and boron nitride (BN)container as a sample insulator. The schematic illustration of a high-pressure cell without a
H source is shown in Fig. 2.6(a).

2.3.2 High-pressure cell assembly with an internal H source
The high-pressure cell with an internal H source, which is available for synthesizing thesample with high hydrogen pressure, consists of the main parts of a cubic high-pressure celland the extra parts, which are hydrogen source and NaCl container for hydrogen gas sealing
during the synthesis process. The hydrogen source is a mixture of NaBH4 and Ca(OH)2 ,with a molar ratio of 1:2. When the hydrogen source is heated above 300 C, the reaction
occurs as follows [1,2]:

NaBH4 + 2 Ca(OH)2 → NaBH4 + 2 CaO + H2 O → NaBO2 + 2 CaO + 4 H2 ↑Eq. 2.4

The NaCl container is prepared by heating NaCl powder at 400 C for 8 h before using,then grinding with an alumina mortar and a pastel for obtaining fine NaCl powder. The fine
NaCl powder is pressed into a pellet and a sleeve with an inner diameter of 3.75 mm and anouter diameter of 5 mm at a pressure of 15 MPa by a hydraulic press machine. The schematic
illustration of the high-pressure cell with the internal H source is shown in Fig. 2.6(b).

33Chapter 2 Experimental method

Fig. 2.6

Schematic illustration of details of high-pressure cell used in this study,(a) without a hydrogen source, (b) with an internal hydrogen source.

34

Chapter 2 Experimental method

2.4 Characterization method
2.4.1 X-ray diffraction analysis (XRD)
X-ray diffraction analysis (XRD) is an essential technique for primary phase
identification and determination of phase amounts and lattice parameters and structure of
crystalline material.
The X-ray beam is generated by striking of electrons to an anode (target metal) from acathode (filament) of the filament X-ray tube in a vacuum. Electrons from the cathode arereleased by heating the filament cathode. The electrons are accelerated to move toward thetarget metal anode by the large voltage difference between both electrodes [3].
After the generated X-rays hit a family of crystal lattice planes, that is specified by Miller
indices (hkl) and has an interplanar spacing between closest lattice plane of dhkl with an angle
of incidence . The x-rays diffract from the lattice planes the same with the angle of incidence.
When the reflected x-rays reach to x-ray detector, if the reflected rays are in phase with theincident x-rays, peak intensity will be observed in the XRD pattern, as shown in Fig. 2.7.
The occurrence of in-phase x-rays conforms to the length of different travel paths
between the incident x-ray 1 and 2 (AB + BC) which is an integer (n) times of the wavelength

. Thus,n = AB + BC

Eq. 2.5

Since, from Fig. 2.7(c) the relation between the length of different paths between theincident x-rays and interplanar spacing is
AB + BC = 2d sinEq. 2.6

The condition for the occurrence of constructive interference must be
2dhkl sin = nEq. 2.7

where,
dhkl= interplanar spacing of the crystal planes of indices (hkl)= scattering x-ray angle

35

Chapter 2 Experimental methodn= order of the diffraction (integer)= wavelength for X-ray

This equation is known as Bragg’s law or Bragg’s equation. The x-ray diffraction resultshows the relation between diffraction angle (2) in degree and intensity (I) of the diffracted
x-ray beam in cps. Form the X-ray diffraction pattern, peak positions, peak intensities, andpeak shapes can be used to determine crystal structure, lattice parameters, quantitative phasefractions, and crystallite size [3,4].
In this work, the x-ray diffraction pattern of the samples was obtained by Rigaku
SmartLab instrument with Cu-k radiation of which wavelength is 1.54 Å at the applied
voltage of 45 kV and applied current of 200 mA, 2scanning from 10 to 90 with a speed

rate of 8/min and scan step size of 0.01.
The samples were ground to be powder by tungsten mortar and were packed into a Si
crystal sample holder for measurement. The obtained peak patterns were analyzed using the
PDXL software.

36

Chapter 2 Experimental method

Fig. 2.7

The reflection of an x-ray beam by the lattice planes (hkl) of a crystal. (a)Constructive interference of reflected rays (reflected rays are in phase).
(b) Destructive interference of reflected rays. (c) Schematic of Bragg’s
law condition.

37

Chapter 2 Experimental method

2.4.2 Scanning electron microscope with energy dispersive X-ray spectrometry (SEM/EDX)
Scanning electron microscope with energy dispersive X-ray spectrometry (SEM/EDX)
is utilized to studying the microstructure and observe the surface of a sample. Furthermore,
the identification and quantification of elements from atomic number 4 to 92 (Be to U) [5]in a sample can be understood by EDX analysis. SEM utilizes electrons to form highresolution SEM images by producing primary electrons to interact with atoms in a sample.
Signals of emitted electrons from a sample are detected, amplified, and converted to be animage. The primary electrons are produced and are emitted from electron gun move througha condenser lens, a scanning coil, and an objective lens to a sample in a vacuum chamber, as
shown in Fig. 2.8. A vacuum environment in the chamber is necessary for using SEM because
air is a barrier to the movability of primary electrons to a sample.
For SEM imaging, 2 types of emitted electrons are detected.
1. Secondary electrons
Secondary electrons are electrons in atoms of a sample displaced by the primaryelectrons as a result of inelastic collision [6]. The displaced electrons (secondary
electrons) that move out from atoms are detected by a secondary electron detector andamplified to form a secondary electron image. The secondary electron image is utilized
for the sample's surface observation.
2. Backscattered electrons
Backscattered electrons are the primary electrons reflected to a backscattered electron
detector as a result of an elastic collision with atoms of a sample. The number ofbackscattered electrons depends on the atomic number (Z) of the collided atom [7]. The
collided atom, which has a larger atomic number, can reflect backscattered electrons morethan the lighter atom. These backscattered electrons are detected by a backscattereddetector and are amplified to form a backscattered electron image [8]. In the backscatteredelectron image, a brighter area identifies the larger atomic number. The difference in thenumber of backscattered electrons of each atom is used to separate phases and element
composition in a sample by EDX analysis. Therefore, the backscattered electron image isuseful for the identification and qualification of phases and elements in a sample.
In this work, SEM/EDX analysis was performed to identify the element compositions inthe synthesized samples by JEOL (JSM-6510) instrument with a spot size of 50 nm in diameter,
beam accelerating voltage of 15 kV. For the EDX analysis condition, 60 seconds/point and 10
points/phase.

38

Chapter 2 Experimental method

Fig. 2.8

Schematic diagram of the basic design of a scanning electron microscopewith energy dispersive X-ray spectrometry [3].

39Chapter 2 Experimental method

2.4.3 Differential scanning calorimetry (DSC)
Differential scanning calorimetry (DSC) is a technique for investigating the thermal
stability of a sample. DSC analysis is used to study changes in a sample such as phasetransformation, crystallization, and melting by measuring heat flow difference between asample and a reference for maintaining both at the same temperature as a function of time ortemperature. The reference is an empty pan, which is the same material as the sample pan.
In general, during heating, while no reaction occurs, the temperature of a sample and areference are increased at a constant heating rate, whereas the heating rate of sample side isslower than that of reference side due to the heat capacity (Cp) of a sample which is definedas [9,10]

Cp = (dH⁄
dH
dt = Heat flow
) =
dT⁄
dT P
Heat ratedt

Eq. 2.8

When a sample undergoes a reaction that absorbs heat (endothermic) from or releases
heat (exothermic) to the surroundings, the temperature of a sample becomes different from areference. Therefore, the heat flow of the sample side is increased or decreased to maintainthe temperature difference between a sample and a reference at nearly zero, as shown in Fig.
2.9. This change in heat flow is detected and appears to be a curve in the DSC result.
The DSC result can be used to calculate the enthalpy change of the changes in a sample
by integrating the area under the curve given by Eq. 2.9.
T2

T2
dH
∆H = ∫ ( ) dT = ∫ Cp dT
dT P
T1
T1

Eq. 2.9In this work, DSC analysis was used to determine the thermal stability of the synthesized
samples, such as phase transformation temperature and kinds of phase transformation
reaction. The samples were measured in Tzero aluminum pans with Tzero aluminum lids by
TA Instruments DSC Q2000 with a heating rate of 10 C/min at 50 ml/min of constant Ar
flow rate to drive out any volatiles emitted during the measurement.

40

Chapter 2 Experimental method

Fig. 2.9

Measurement principles of DSC (from Mettle Toledo).

41

Chapter 2 Experimental method

2.4.4 Thermogravimetry-differential thermal analysis-mass spectrometry (TG-DTA-MS)
Thermogravimetry-differential thermal analysis-mass spectrometry (TG-DTA-MS) is
also a fundamental technique for investigating the thermal stability of samples. It is acombination of 3 techniques that are helpful for understanding change in mass, energychange, and kinds of reactions [11].
Thermogravimetry (TG) is used to measure a change in mass of a sample as a functionof time or temperature with a precision balance.
Differential thermal analysis (DTA) is a technique combined with TG for measuring thetemperature difference between a sample and a reference as a function of time or temperature
due to endothermic and exothermic phenomena of a sample [12].
Furthermore, mass spectrometer (MS) is used to identify the amount and type of gasphase ion of a sample by measuring mass to charge ratio(m/z) of the ions and relative
abundance of each gas phase ion type. MS instrument consists of 3 main components: anionization source, a mass analyzer, and a detector system [13].
The gas-phase ions of a sample are produced by electron ionization in ionization source.
In the mass analyzer, the ions are separated according to their mass to charge ratio(m/z), andthey are detected in proportion to their abundance.

Fig. 2.10

Components of a mass spectrometer.

In this work, TG-DTA-MS were utilized for the analysis of the thermal decomposition ofhydrogen gas. The samples were measured in aluminum pans with a heating rate of 10 C/min with a dynamic thermogravimetry method at 300 ml/min of constant He flow rate by
Rigaku ThermoMass.

42

Chapter 2 Experimental method

2.4.5 PCT measurement
Pressure-composition-temperature (PCT) measurement is a gas sorption analysis
measurement by gravimetric and volumetric techniques. Generally, PCT is performed forstudying gas absorption-desorption behaviors of a sample as a function of pressure at
isothermal conditions. In the gravimetric technique, the amount of sorbed gas is determinedby the mass change of a sample. In the volumetric technique, the gaseous pressure change ina sample cell's calibrated volume is measured to determine the amount of sorbed gas of thesample. The PCT volumetric technique, also known as Sievert’s method. The schematic of
PCT Sievert’s type apparatus system is shown in Fig. 2.11. The PCT apparatus consists of 5
systems: control system, measurement system, vacuum system, gas supply system andtemperature control system
The volumes of a sample cell and the gas reservoir must be measured in advance toperform the PCT measurement because the Sievert's method measures the change in pressurein the volume of a sample cell. For starting the measurement, the test gas is filled in / isreleased from the pressure reservoir to initial test gas pressure. When the valve between thepressure reservoir and the sample cell is opened, the test gas pressure between the pressure
reservoir and the sample cell is brought into equilibrium. The amount of absorbed/desorbed
gas is estimated by knowing the initial test gas pressure and the volumes of the apparatus
system
In this work, PCT measurement was utilized for studying the hydrogen absorptiondesorption behavior of the synthesized samples. Nitrogen gas (N2) and Hydrogen gas (H2)
are used as the valve controlling gas and test gas, respectively. The hydrogen pressure isavailable in the range of 0.01 MPa - 9.8 MPa. The amount of absorbed/desorbed gas is
estimated by using the software program of SUZUKI SHOKAN Co., Ltd.

43

Chapter 2 Experimental method

Fig. 2.11

Schematic of PCT Sievert’s type apparatus system

44

Chapter 2 Experimental method

2.5 Crystal structure analysis
2.5.1 Rietveld refinement analysis
Rietveld refinement analysis is a structure refinement technique to obtain the crystal
structure information of a material such as lattice parameters, atomic coordination, and site
fractions by utilizing X-ray diffraction data. This technique uses a non-linear least-squares
fitting method to refine the model of a structure. The least-squares fitting method minimizes
the sum of the squared difference between the observed X-ray diffraction data profile andthe calculated profile [14].
The profile refinement (S) which is the sum of squared residuals is defined as

S = ∑ wi (yobs− ycal)ii2

Eq. 2.10iwhere,wi= the statistical weight of each observation stepyobsi

= the observed intensity at the ith stepycali

= the calculated intensity at the ith step

The ycalvalues are calculated from the model by the summation of Bragg reflectioniintensity and background function.

2

bkg

ycal
= SF ∑ Lk |Fk,j | Sj (2θi − 2θk )Pk,j Aj + yii

Eq. 2.11kwhere,
SF= scale factor

Lk= Lorentz-polarization factor

Fk,j= structure factor

Sj (2θi − 2θk )= reflection profile function

Pk,j= preferred orientation function

Aj= absorption function

45

Chapter 2 Experimental methodbkg= background intensity at the ith stepyi

The summation in Eq. 2.11 is performed over all reflections in the X-ray diffraction data.
The quality of the refinement is measured by agreement factors (R-factors). The profile
R-factor (Rp) and the weighted profile R-factor (Rwp) which indicate agreement between theobserved and calculated profiles are defined as

Rp =Rwp = [

∑i|yobs− ycal
|
i
i
obs
∑i yi

∑i wi (yobs− ycal)ii∑i wi yobsi2

Eq. 2.122 1/2

Eq. 2.13]

The expected R factor (Rexp) refers to the quality of the data. This factor value shows thebest possible fit between the observed and the calculated profiles. In other words, Rexp is theminimum possible value of Rwp.

Rexp = [

1/2

N − P
∑i wi yobsi2

Eq. 2.14]

where,N= the number of independent observations

P= the number of refined parameters

Therefore, the goodness of fit, which is the ratio between Rwp and Rexp, should be close
to 1. The small values of Rwp and Rexp are expected to obtain a good quality of refinement[15].

G of F =∑i wi (yobs− ycal
)
i
i
N − P

2

2

Rwp
= (
) = 2
Rexp

Eq. 2.15

46

Chapter 2 Experimental method

For single-crystal refinements, RF and RB factors refer to agreement between the observed
and the calculated structure factors which indicates the reliability of the structure model andthe fit of observed and calculated integrated intensities, respectively [14,15]. RF and RB are
defined as

RF =

RB =

∑k|Fkobs − Fkcal |∑k|Fobsk |∑k|Ikobs − Ikcal |∑k|Ikobs |Eq. 2.16

Eq. 2.17

where,
Ik= the integrated intensity of the kth reflection

In this study, the Rietveld refinement analysis was performed to refine crystal structure
information of synthesized compounds using the RIETAN-VENUS program. The refined
crystal structure model of the materials was drawn using the Vesta program.

47

Chapter 2 Experimental method

References[1] Y. Fukai, Y. Ishii, Y. Goto, K. Watanabe, Formation of superabundant vacancies in Pd–
H alloys, J. Alloys Compd. 313 (2000) 121–132.
https://doi.org/10.1016/S0925-8388(00)01195-6.
[2] Y. Fukai, S. Yokota, J. Yanagawa, The phase diagram and superabundant vacancy
formation in Co–H alloys, J. Alloys Compd. 407 (2006) 16–24.
https://doi.org/10.1016/j.jallcom.2005.06.016.
[3] W.F. Smith, J. Hashemi, F. Presuel-Moreno, Foundations of materials science andengineering, 5th editio, Mcgraw-Hill Publishing, 2009.
[4] P. Atkins, T. Overton, J. Rourke, M. Weller, F. Armstrong, M. Hagerman, Shriver andAtkins’ Inorganic Chemistry, Fifth Edit, W. H. Freeman and Company, 2009.
[5]

[6]
[7]

[8]

S.J.B. Reed, Electron Probe Microanalysis, in: P.J. Potts, J.F.W. Bowles, S.J.B. Reed,
M.R. Cave (Eds.), Microprobe Tech. Earth Sci., Springer, Boston, MA., 1995: pp. 49–
89.
https://doi.org/10.1007/978-1-4615-2053-5_2.
Materials Evaluation and Engineering Inc., Handbook of Analytical Methods forMaterials, 2014.
J.I. Goldstein, D.E. Newbury, J.R. Michael, N.W.M. Ritchie, J.H.J. Scott, D.C. Joy,
Scanning electron microscopy and x-ray microanalysis, 2017.
https://doi.org/10.1007/978-1-4939-6676-9.
P.A. Scherer, H.P. Bochem, Energy-dispersive X-ray microanalysis of the methanogen
Methanosarcina barkeri “Fusaro” grown on methanol and in the presence of heavy

metals, Curr. Microbiol. 9 (1983) 187–193.
https://doi.org/10.1007/BF01567579.
[9] L.C. Thomas, Practical Benefits of using Heat Capacity Versus Heat Flow Signals, in:
TA Instruments, 2001: pp. 1–6.
[10] P. Simon, Z. Cibulkova, Measurement of heat capacity by Differential Scanning
Calorimetry, Thermophysics (2002) 77–80.
[11] M. Kamruddin, P.K. Ajikumar, S. Dash, A.K. Tyagi, B. Raj, Thermogravimetry-evolved
gas analysis-mass spectrometry system for materials research, Bull. Mater. Sci. 26(2003) 449–460.
https://doi.org/10.1007/BF02711191.
[12] R. Sillen, Introduction to thermal analysis of metals, Foundryman 96 (2003) 245–246.
[13] V.S. Edmond de Hoffmann, Mass Spectrometry: Principles and Applications, Third Edit,
John Wiley & Sons Ltd, 2007.
[14] I. Nagai, F. Izumi, Practical X-ray Diffractometry - Introduction to Rietveld method,
Asakura Pulishing Co., Ltd, 2002.
[15] L.B. Mccusker, R.B. Von Dreele, D.E. Cox, D. Louër, P. Scardi, Rietveld refinement

48Chapter 2 Experimental method

guidelines, J. Appl. Crystallogr. 32 (1999) 36–50.
https://doi.org/10.1107/S0021889898009856.

49

Chapter 3
High-pressure synthesis of a novel compound in the Mg – Co binary system
3.1 Introduction
Although MgCo2 is a stable phase in the Mg–Co binary phase diagram (Fig. 3.1), there
have been no reports on MgCo2 hydrides. However, Mg2CoH5 [1,2] and Mg6Co2H11 [3] thathave high hydrogen storage capacities of up to 4.5 and 4.0 mass%, respectively, weresynthesized in the Mg – Co – H ternary system even though their parent compounds are notreported in the phase diagram. After dehydrogenation of Mg2CoH5 and Mg6Co2H11, MgCo,
which was identified as Mg2Co in some previous studies [4,5], was observed [4–9] as ametastable phase. Based on theoretical density functional theory (DFT) calculations onhydrogen storage in MgnCo clusters (n = 1-10), it was concluded that Mg4Co and Mg6Co are
possible to stabilize, and it was found that it is possible to use Mg5Co for hydrogen storage[10]. However, Mg4Co, Mg5Co, and Mg6Co have still not been experimentally reported.
In a previous study, novel Mg – TM (TM = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) binary
system compounds were explored at 6 GPa by high-pressure synthesis [11]. In Mg – Ni and
Mg – Cu binary systems, novel compounds of Mg6Ni, MgCu and Mg51Cu20 compounds weresynthesized. In the Mg – Co system, a novel compound was observed only in a composition
ratio of 1:1 at 950 C and 6 GPa, however, the novel compound was not synthesized in thatstudy.
Considering the elements in the same group as Co, which are Rh and Ir, many Mg – Rh,
and Mg – Ir stable phases have been reported in the Mg – Rh and Mg – Ir binary phase diagrams,
as summarized in Fig. 3.2. Therefore, it should be possible to obtain a novel compound in the
Mg – Co binary system by high-pressure synthesis in the same way as the novel compounds in
the Mg – Cu and Mg – Ni systems.
Therefore, the purpose of the work in this chapter was to explore a novel compound in the
Mg – Co binary system using a cubic-anvil-type apparatus and to investigate the crystal
structure, thermal stability, and hydrogenation properties of the new Mg – Co compound.

50Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system

Fig. 3.1

Fig. 3.2

Binary phase diagram of Mg – Co [12].

Schematic of phase stability at composition in Mg – X systems(X = Co, Rh, and Ir). (-) denotes no report of the crystal structureof that compound.

3.2 Sample preparation
To synthesize the Mg – Co compounds, fine Mg and Co powders (high purity chemicals,
up to 99%, 5 microns in size) were used as raw materials. The fine Mg powder was obtained
via the dehydrogenation of MgH2 powder (Alfa Aesar, 98%) at 420 C under vacuum for 2 h
in a stainless-steel cell. The Mg and Co powders were mixed by hand-milling them in a high51

Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system

purity alumina mortar for 20 min. Mg – x at% Co (x = 10, 15, 16.67, 20, 25, 33.3, and 50)
mixtures were prepared, pressed into pellets and then packed into high-pressure cells without
internal hydrogen source. These mixtures were synthesized under various conditions, as shownin Table 3.1. After being synthesized, all the samples were quenched to room temperature andthe pressure was released to ambient pressure.
Table 3.1 The synthesized conditions of Mg – x at% Co (x = 15, 16.67, 20, 25, 33.3, and 50)
mixtures.
SamplePressure (GPa)Temperature (C)Time (h)

6

700

8

Mg – 16.67 at% Co

5

700

8

Mg – 16.67 at% Co

4

700

8

Mg – 20 at% Co

4

700

8

Mg – 10 at% Co
Mg – 15 at% Co
Mg – 16.67 at% Co
Mg – 20 at% Co
Mg – 25 at% Co
Mg – 33.3 at% Co
Mg – 50 at% Co

3.3 High-pressure synthesis of novel Mg – Co compounds
To investigate novel compounds in the Mg – Co binary system, various compositions of
Mg – Co mixtures were prepared and subjected to heating under high-pressure. Fig. 3.3 shows
the X-ray diffraction (XRD) patterns of the high-pressure synthesized Mg – x at% Co (x = 10,
15, 16.67, 20, 25, 33.3 and 50) at 700 C and 6 GPa for 8 h. Mg (▽), Mg1+yCo (y = 0 or 1) (),and unidentified (○) peaks were observed in the patterns. For the Mg – 16.67 at% Co and Mg– 50 at% Co samples, unidentified and Mg1+yCo (y = 0 or 1) peak patterns were observed as
single phases, respectively.
In the powder pattern of Mg – 50 at% Co which showed a Mg1+yCo peak pattern as a single
phase, in many previous studies, this peak pattern has been attributed to MgCo or Mg2Co

52

Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary systembecause they possess similar XRD patterns. In this study, the Mg1+yCo peak pattern, which wasobserved in the results of samples subjected to high-pressure treatment, was judged byconsidering the nominal composition of the sample, which in this case was Mg – 50 at% Co.
Furthermore, elemental composition analysis by energy-dispersive X-ray spectroscopy (EDX)
was also used to identify that the synthesized Mg1+yCo phase had the atomic percentage
composition of 52.0 at% Mg and 48.0 at% Co. Therefore, the Mg1+yCo peak pattern in thisstudy was assigned as MgCo.
As mentioned above, the Mg – 16.67 at% Co showed only an unidentified peak pattern that
does not appear in the Mg – Co binary phase diagram, while the powder patterns of its
neighboring compositions samples, in which x = 15 and 20 also showed peak patterns for Mg
and MgCo, respectively. Elemental composition analysis showed that the unidentified phase
had an atomic percentage composition containing 83.2 at% Mg and 16.8 at% Co. Therefore,
the unidentified phase with the composition Mg – 16.8 at% Co was identified as a novel phasecompound in the Mg – Co system.
In order to clarify the effect that pressure has on the appearance of the novel phasecompound, the Mg – 16.67 at% Co sample was also synthesized under 4 and 5 GPa pressuresat 700 °C for 8 h. The results of varying the pressure during synthesis on Mg – 16.67 at% Co
are shown in Fig. 3.4. At 5 GPa, the novel phase was observed as the main phase with a minor
MgCo phase. Elemental composition analysis identified that the composition of the novel phaseat 5 GPa was Mg – 16.1 at% Co. At 4 GPa, the novel phase peak pattern was not observed. The
XRD result showed only Mg and MgCo peak patterns, whereas, in EDX results, phases being
identified, a Mg – 19.2 at% Co phase was also observed.

53

Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system

Fig. 3.3

XRD patterns of Mg – x at% Co (x = 10, 15, 16.67, 20, 25, 33.33 and 50)
prepared at 6 GPa and 700 °C for 8 h.

54Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system

Fig. 3.4

XRD patterns of Mg – 16.67 at% Co prepared at 700 °C and 4 – 6 GPafor 8 h.

3.4 Crystal structure refinement
The crystal structure of the synthesized novel Mg – Co phase compounds were refined by
Rietveld refinement. The novel phase peak pattern is similar to the peak pattern of Mg44Rh7,
which is a stable phase in the Mg – Rh binary system, and a Mg6Pd-type structure of Mg6Ni,
which was synthesized by high-pressure synthesis [11,13,14]. The unidentified peak pattern
was refined by fitting the Mg44Rh7-type structure as Mg44Co7, space group with the F–43m,
because Rh is an element that is in the same group as Co. The results of the refinement and the
crystal structure of Mg44Co7 are shown in Figs. 3.5 and 3.6, respectively. The final refinement(Fig. 3.5) converged to Rwp = 0.733, Re = 0.570, and RB = 12.102, GofF = 1.650. The lattice
parameter was estimated to be a = 2.0126(7) nm. The atomic sites and positions are presented
in Table 3.2.

55

Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system

Fig. 3.5

Rietveld refinement of the Mg44Co7 with a Mg44Rh7-type structure
prepared by high-pressure synthesis.

Fig. 3.6

Crystal structure of the refined Mg44Co7 with a Mg44Rh7-type structure.

56

Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system

Table 3.2 Atom positions of the Mg – 16.67 at% Co sample and their site parameters estimatedby Rietveld analysis.
Atom

Site

x

y

z

Co1

24g

1/4

1/4

0.5854Co2

16e

0.0811

x

x

Co3

16e

0.3484

x

x

Mg1

48h

0.0480x

0.3360Mg2

48h

0.1075x

0.2132

Mg3

48h

0.1941x

0.4824Mg448h0.1050x

0.7233Mg5

48h0.1539x0.9757

Mg6

24g

1/4

1/4

0.3585

Mg7

24f

0

0

0.1855Mg8

16e0.7052

x

x

Mg9

16e

0.8334

x

x

Mg10

16e

0.5616

x

x

Mg11

16e

0.9491xx

3.5 Effect of pressure on the presence of the Mg44Co7 phase
1. Effect of pressure on the atomic radius ratio between the constituent elements
Under ambient pressure, the atomic radius ratios (rB/rA) of complex A7B44-type orAB6-type structure compounds such as Mg44Rh7 [15], Mg44Ir7 [16], Mg44Ru7 [17], Sc44Ir7[18], Sc44Ru7 [19], Sc44Os7 [18], Mg6Pd [20], Mg6Pt [21] and metastable Mg6Ni preparedby melt-spinning technique [22] are 1.15 – 1.21 except for that of Mg and Ni which has avalue of 1.28. In the case of Mg and Co, the complex A7B44-type structure does not appear
in their binary phase diagram, in which the atomic radius ratio of Mg and Co (rMg/rCo) is
1.28. In terms of Mg6Ni, it was reported as a metastable phase that was synthesized at a

57

Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system

high-pressure of more than 3 GPa [13]. The atomic radius ratios of A7B44- and AB6-type
structures are shown in Fig. 3.7, in which the closed and open marks indicate the presence
and absence of the phases at that pressure, respectively, where it can be seen that Mg6Ni
not formed under ambient pressure.

Fig. 3.7

Atomic radius ratios of compounds that have A7B44- or AB6-type
structures under ambient pressure (blue circles), in which the atomic
radius ratios of Mg and Co and Mg and Ni are shown at pressures on the
GPa-order (red triangles). The closed and open marks indicate thepresence and absence of the A7B44- or AB6-type structures under ambientpressure.

Due to the different compressibilities of the elements with increasing pressure, theatomic radius ratios of the constituent elements at a pressure in the order of GPa changes
from that at ambient pressure. The rMg/rNi ratio changes from 1.28 (at ambient pressure, ○)to 1.262 (▽), 1.255 (), 1.248 (), 1.243 () and 1.240 () at 2, 3, 4, 5 and 6 GPa, andthe rMg/rCo ratio changes from 1.28 (at ambient pressure, ○) to 1.248 (▽), 1.243 () and
1.240 () at 4, 5 and 6 GPa, respectively. Based on the calculation of the atomic radiusratios at high-pressure in GPa-order, the rMg/rNi and rMg/rCo ratios are closer to the rB/rA
ratios of the other A7B44-type compounds than the rMg/rNi and rMg/rCo ratios under ambientpressure. Assuming that the atomic radius ratio of the elements is an essential factor in AB6and A7B44-type compounds, the upper limit of the rMg/rNi ratio for the AB6-type structure islower than 1.262, which is the rMg/rNi ratio at 2 GPa in the range of 700 – 950 °C [13]. The
58

Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system

upper limit of the rMg/rCo ratio for the A7B44-type structure is lower than 1.248, which is therMg/rCo ratio at 4 GPa and 700 °C.

2. Effect of pressure on the crystal structures of the constituent elements
Under ambient pressure, both Mg and Co exhibit hexagonal close-packed (HCP)
structures, while Rh, Ir, Ni, Pd and Pt which have A7B44- or AB6-type structure compoundswith Mg in the form of Mg44Rh7 [15], Mg44Ir7 [16], Mg6Ni [11,22] Mg6Pd [20], and Mg6Pt[21] exhibit face-centered cubic (FCC) structures. As shown in Fig. 3.8, the crystal structureof Mg maintains an HCP structure at below 730 °C in a range of pressures between
approximately 1 and 8 GPa [23–25]. Although the crystal structure of Mg does not changeover that temperature and pressure range, the 3s2 (ground-state) electron configuration of
Mg may change to 3s1 3p1 (excited-state), maintaining as HCP structure following theEngel-Brewer theory, with a promotional energy of 263.6 kJ/mol [26,27]. For the crystal
structure of Co, as shown in Fig. 3.9, at 5 GPa and 700 °C, the HCP structure of the Cotransforms into an FCC structure, which is the same structure as those of the Rh, Ir, Ni, Pd,
and Pt.

Fig. 3.8

The pressure-temperature phase diagram of magnesium from Ref. [25]59Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system

Fig. 3.9

The pressure-temperature phase diagram of cobalt from Ref. [28]As described by the Engel-Brewer theory, the change in the crystal structure of Co isrelated to the valence electron configuration. The electron configuration of Co in the
ground-state is [Ar] 3d7 4s2. However, when energy in the form of temperature and pressure
is introduced, this promotes the transformation of the electron configuration of Co from theground-state to the excited-state electron configuration as [Ar] 3d7 4s2  [Ar] 3d7 4s1 4p1(HCP)  [Ar] 3d6 4s1 4p2 (FCC)  [Ar] 3d8 4s1, where the promotional energy forachieving each state was 280, 498, and 42 kJ/mol, respectively [26,27]. The total numbersof valence electrons of Mg and Co do not change, meaning that there is no change in thevalence electron concentration (VEC), which is one of the influencing factors forcompound formation [29,30]. However, the changes in the numbers of unpaired electrons
and the energy levels of the electron orbitals induce a change in the electronic properties
and reactivity of the elements [31].
The increasing pressure, which induces the change in electron configuration,

results

in an increase in the electronegativity of the constituent elements [32], which is related tothe compound formation as an electrochemical factor [29,30], in the form of the differencein electronegativity of the compound according to the Martynov–Batsanov scale ofelectronegativity [33] for AxBy :∆ χAxBy = [2x] [χ − χB ](x + y) A

Eq. 3.1

60

Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system

where, x < y [34]. The difference in electronegativity is used to categorize compounds as
AB, A3B5, AB2, AB3, and AB5 structures [34–40]. For Mg44X7 and Mg6X, the Mg and X
elements can be considered as B and A, respectively. The differences in theelectronegativities of the Mg44X7 and Mg6X compounds were calculated using the
Martynov–Batsanov scale of electronegativity [34], the results of which are listed in Table
3.3, where it was found that the difference in the electronegativity of Mg44Co7 was smaller
than the values calculated for all the other compounds.

Table 3.3 The differences in the electronegativity of the Mg44X7 and Mg6X compounds underambient pressure.
Mg44X7 and Mg6X compounds∆ χAxBy

Mg44Rh7

0.187

Mg44Ir7

0.154

Mg44Ru7

0.181

Mg6Pd

0.220

Mg6Pt

0.171

Mg44Co7

0.112

Based on the calculation of the electronegativity of the element as a function ofpressure, the trend of increasing electronegativity of Co was higher than that of Mg when
the pressure increased from 0 to 6 GPa ( ∆ χCo > ∆ χMg ) [41,42]. At -273 °C andapproximately 5 GPa, the electronegativity of Co sharply increased. indicating electrontransition from the ground-state 3d7 4s2 to the excited-state 3d8 4s1 [42]. As theelectronegativity of Co is higher than that of Mg under ambient pressure, the term χCo −χMg at high-pressure is greater than that under ambient pressure. As a result, the differencein the electronegativity of Mg44Co7 (∆ χMg44Co7 ) at high-pressure is also greater than that
under ambient pressure, meaning that the value of ∆ χMg44Co7 might be close to that of ∆
χMg44X7 . The change in the value of ∆ χMg44Co7 with increasing pressure is one of the reasons
why Mg44Co7 is obtained via high-pressure synthesis.

61

Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system

3.6 Thermal stability measurement
Thermal stability measurements were carried using differential scanning calorimetry (DSC)
under a flow of Ar in the temperature range between room temperature and 450 °C at a rate of
10 °C/min. The DSC analysis results of the Mg44Co7 prepared at 700 °C and 6 GPa for 8 h are
shown in Fig. 3.10. The XRD patterns of the Mg44Co7 before and after the DSC measurements
are shown in Fig. 3.11, in which it can be observed that the DSC curve exhibits broad
exothermic peaks (B to C) and a sharp exothermic peak (C to D). The Mg44Co7 disappeared at
the sample heated up to 450 °C (D). Mg and MgCo became observed. Judging from the DSC
and the XRD analysis, the decomposition of Mg44Co7 into Mg and MgCo at 390 °C is an
exothermic reaction, where the decomposition reaction of Mg44Co7 can be expressed asfollows:

Mg44 Co7 →390 °C37 Mg + 7 MgCo

Eq. 3.2

As a result, this showed that Mg44Co7 is less stable than Mg and MgCo because of
decomposition of the Mg44Co7 in the form of the aforementioned exothermic reaction.
Mg44Co7 was synthesized and quenched at the high-pressure conditions to preserve thehigh-pressure phase under ambient pressure. Under ambient pressure, Mg44Co7 can be regarded
as a metastable phase, but when it is heated above 390 °C, Mg44Co7 rearranges to the morestable phases, Mg and MgCo, by releasing excess energy.

Fig. 3.10

DSC curve of Mg44Co7 prepared from Mg – 16.67 at% Co at 700 °C and6 GPa for 8 h.

62Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system

Fig. 3.11

XRD patterns before and after the DSC analysis of Mg44Co7 prepared at
700 °C and 6 GPa for 8 h.

3.7 Hydrogenation properties
To form metallic bonds between hydrogen and metals, the hydrogen atoms occupy
tetrahedral (T-site) and octahedral (O-site) sites. Empirical rules for stable hydrides proposethat the minimum size of the spaces of the T- and O-sites for hydrogen atoms is 0.04 nm (inradius), where the H–H distance is no less than 0.21 nm [43]. Taken from its refined crystal
structure, the T- and O-sites of Mg44Co7 are given in Table 3.3. Assuming that the hydride of
Mg44Co7 has the same structure as its parent intermetallic compound, the size of the spaces ofT- and O-sites were calculated from the metallic radii of each element according toGoldschmidt theory[44], as presented in Table 3.3. The results of the calculated size of thespaces show that hydrogen atoms can occupy some T- and O-sites in Mg44Co7.

63

Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system

Table 3.3 T- and O-sites in Mg44Co7 and the calculated maximum space sizes of each site.
Site

Distance between atoms *(*measured from center to center)

Mg1 – Co3

0.020

Mg – Co: 0.279 nm
Mg – Mg: 0.299 nm

Mg6

0.058

Mg8 – Mg9: 0.293 nm
Mg8 – Mg8: 0.261 nm

Mg6 – Co3

0.077

Mg – Co: 0.281 nm
Mg – Mg: 0.307 nm

Mg8 – Mg9

size (nm)Mg – Co: 0.297 nm
Mg – Mg: 0.324 nm

Mg2 – Co2

Maximum space

0.05

Mg – Mg: 0.299 nm
0.102

The hydrogenation of Mg44Co7 was carried out in a stainless steel(SUS) cell in the rangeof 300 – 350 °C, at 8 MPa of H2. The XRD patterns of the sample after hydrogenation undervarious conditions are shown in Fig. 3.12. From the hydrogenation results, it was determinedthat the hydride phase. as Mg44Co7Hx, was not observed. Instead, after the hydrogenation of
Mg44Co7 at 300 °C, presented MgH2 and Mg6Co2H11 were present as the main phases, alongsidea small amount of Mg2CoH5 and Mg. As the hydrogenation progressed to 350 °C, Mg2CoH5
was observed as one of the main phases. Under these hydrogenation conditions, more of the
Mg2CoH5 phase formed than that of Mg6Co2H11. In previous research studies, the formation of
Mg6Co2H11 and Mg2CoH5 occurred at lower and higher plateau pressures of hydrogenation of64

Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system

the MgCo phase, respectively, at various temperatures [8,45], while the results of this study
indicate that the hydrogenation temperature also affects the formation of Mg6Co2H11 andMg2CoH5. The results in this study are in agreement with those of the previous study, in which
it was reported that the formation of Mg6Co2H11 occurred below 300 °C, and that of Mg2CoH5
occurred above 300 °C [9].
The presence of a small amount of Mg2CoH5 phase in the sample hydrogenated at 300 °C
indicates that both the decomposition reaction of Mg44Co7 and the hydride-forming reaction
are exothermic reactions. Although the hydrogenation temperature was set to 300 °C, theprocess might actually have occurred at a slightly higher temperature than the set temperature,
thus, explaining the presence of the Mg2CoH5 phase.
Thermogravimetric (TG) analysis was carried out in the temperature range between room
temperature and 450 °C at a rate of 5 °C/min under a 300 ml/min flow of He to investigate thehydrogen desorption temperature, the results of which are shown in Fig. 3.13. For Mg44Co7hydrogenated at 300 °C, the weight loss due to hydrogen desorption was observed at 377 °C,
where the total amount of hydrogen content was estimated to be 3.5 mass%. The total amountof hydrogen content of Mg44Co7 hydrogenated at 350 °C was estimated to be 4.2 mass%. The
XRD results of the dehydrogenated samples revealed that all of the hydrides decomposed into
Mg and MgCo, the dehydrogenation reactions can be expressed as follows:

MgH2 →

Mg2CoH5 →Mg6Co2H11 →Mg + H2

Eq. 3.3

Mg + MgCo + 5/2 H2

Eq. 3.4

4 Mg + 2 MgCo + 11/2 H2

Eq. 3.5

Furthermore, the reversibility between the hydrogenation and dehydrogenation reactions of
Mg44Co7 was also investigated. Based on the XRD results of a sample re-hydrogenated at
350 °C and 8 MPa of H2, the formation of MgH2, Mg2CoH5, and a small amount of Mg6Co2H11
was observed. Compared to the hydrogenation results carried out at the same temperature, there
was an obvious decrease in the amount of Mg6Co2H11 formed. The decrease in the amount of
Mg6Co2H11 thus affects the amounts of MgH2 and Mg2CoH5 that also affect the hydrogencontent of the sample. It was observed that the weight loss due to hydrogen desorption began
at 337 °C, and the total amount of hydrogen lost from the sample was estimated to be 5.065

Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system

mass%.
Based on the TG results, for the (re-)hydrogenation at 350 °C of samples that contained
Mg2CoH5 hydrogen desorbed at a temperature lower than the 300 °C required to hydrogenate
the sample. This showed that the weight losses at 337 °C and 377 °C were due to the desorptionof Mg2CoH5 and Mg6Co2H11, respectively. It was also observed that the Mg that was presented
in the sample as a result of the decomposition of Mg2CoH5 indicated the simultaneousdesorption of hydrogen from MgH2 alongside the decomposition of Mg2CoH5 [46].
The re-hydrogenated sample was dehydrogenated during the TG analysis between room
temperature and 350 °C, wherein the weight loss due to H desorption was clearly observed,
confirming simultaneous hydrogen desorption from MgH2 and Mg2CoH5. The XRD results
revealed that the intensities of the MgH2 and Mg2CoH5 peaks became weak, and Mg and MgCopeaks were observed, as shown in (F) in Fig. 3.12.
The experimental hydrogenation results determined that Mg44Co7 decomposed into MgH2,
Mg2CoH5, and Mg6Co2H11. Theoretically, it can be assumed that the minimum and maximum
hydrogen contents of Mg44Co7 were obtained from the reactions shown in Eqs. 3.6 and 3.7,
respectively. The minimum and maximum hydrogen content were calculated to be 5.4 and 6.0
mass%, respectively:

Mg44 Co7 + 169/4 H2 →Mg44 Co7 + 95/2 H2 →7/2 Mg6Co2H11 + 23 MgH2

Eq. 3.6

7 Mg2CoH5 + 30 MgH2

Eq. 3.7

High-pressure hydrogenation of Mg44Co7 was conducted at 700 °C and 6 GPa for 8 h. After
high-pressure hydrogenation, the results showed that the three hydride phases obtained via
hydrogenation using conventional methods were not detected, with only unidentified peaks
observed, as shown in Fig. 3.14. It is possible that a novel Mg – Co hydride was obtained byhigh-pressure hydrogenation under these conditions. The hydrogen content in high-pressurehydrogenated Mg44Co7 was investigated by TG analysis over the range of room temperature to
450 °C and was estimated to be 1.5 mass%. The weight loss due to hydrogen desorption wasobserved at 365 °C, as shown in Fig. 3.15.

66

Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system

Fig. 3.12

XRD patterns of Mg44Co7 obtained (A) before hydrogenation, (B) afterhydrogenation at 300 °C, (C) after hydrogenation at 350 °C, (D) after
dehydrogenation via TG-TDS analysis, (E) after re-hydrogenation at
350 °C, and (F) after re-hydrogenation and carrying out TG-TDS
analysis up to 350 °C.

67

Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system

Fig. 3.13

TG curves and intensity of the emitted H2 gas curves of the hydrogenated
Mg44Co7.

68Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system

Fig. 3.14

XRD patterns of Mg44Co7 before and after high-pressure hydrogenation.

69

Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system

Fig. 3.15

TG curve and the intensity of the emitted H2 gas curve of high-pressurehydrogenated Mg44Co.

3.8 Summary
In this chapter, a novel Mg – Co compound was successfully synthesized by high-pressure
synthesis, in which a newly found Mg44Co7 phase was synthesized under a pressure of morethan 5 GPa pressure at 700 °C for 8 h. Mg44Co7 was found to be a cubic Mg44Rh7-type structure
belonging to the space group F–43m (No. 216) with a lattice parameter of a = 2.0130(8) nm,
which is stable from room temperature up to 390 °C, where after it decomposes exothermicallyinto Mg and MgCo. Under 8 MPa of H2 pressure in the range of 300 – 350 °C, hydride in theform of Mg44Co7Hx was not observed, and Mg44Co7 instead decomposed into MgH2, Mg2CoH5,
and Mg6Co2H11. The lowest dehydrogenation temperature of hydrogenated Mg44Co7 was
337 °C. The maximum experimental amount of hydrogen desorption of Mg44Co7 was estimatedto be 4.2 mass%. High-pressure hydrogenation of Mg44Co7 showed an unidentified phase,
which might be a novel Mg – Co hydride phase with a hydrogen content of 1.5 mass%.

70

Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system

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[20] S. Samson, Complex cubic A6B compounds. II. The crystal structure of Mg6Pd, Acta
Crystallogr. Sect. B-Structural Crystallogr. Cryst. Chem. 28 (1972) 936–945.
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electropositive elements, J. Less Common Met. 2 (1960) 383–391.
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[22] A. Teresiak, M. Uhlemann, J. Thomas, A. Gebert, The metastable Mg∼6Ni phaseThermal behaviour, crystal structure and hydrogen reactivity of the rapidly quenchedalloy, J. Alloys Compd. 475 (2009) 191–197.
https://doi.org/10.1016/j.jallcom.2008.07.046.
[23] E.Y. Tonkov, E.G. Ponyatovsky, Phase Transformations of Elements Under High
Pressure, 1st Editio, CRC Press, Boca Raton, 2004.
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M.I. McMahon, Equation of state and high-pressure/high-temperature phase diagram ofmagnesium, Phys. Rev. B - Condens. Matter Mater. Phys. 90 (2014) 1–7.
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Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system[25] P. Li, G. Gao, Y. Wang, Y. Ma, Crystal structures and exotic behavior of magnesium
under pressure, J. Phys. Chem. C 114 (2010) 21745–21749.
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[26] W. Hume-Rothery, The Engel-Brewer theories of metals and alloys, Prog. Mater. Sci. 13(1968) 229–265.
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[27] S. Raju, E. Mohandas, V.S. Raghunathan, Engel-Brewer Electron Correlation Model: A
Critical Discussion and Revision of Concepts, Mater. Trans. 37 (1996) 195–202.
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[28] C.-S. Yoo, P. Söderlind, H. Cynn, The phase diagram of cobalt at high pressure andtemperature: the stability of -cobalt and new -cobalt, J. Phys. Condens. Matter 10 (1998)
L311–L318.
https://doi.org/10.1088/0953-8984/10/20/001.
[29] W. Pfeiler, Alloy physics: a comprehensive reference, John Wiley & Sons, 2007.
[30] R. Ferro, A. Saccone, Intermetallic chemistry, illustrate, Elsevier, 2008.
[31] H.J. Himmel, A.J. Downs, T.M. Greene, Reactions of ground state and electronically
excited atoms of main group elements: A matrix perspective, 2002.
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[33] A.I. Martynov, S.S. Batsanov, A new approach to the determination of theelectronegativity of atoms, Russ. J. Inorg. Chem. 25 (1980) 1737–1739.
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binary A3B5 intermetallic compounds: III, J. Less-Common Met. 102 (1984) 199–211.
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73Chapter 3 High-pressure synthesis of a novel compound in the Mg – Co binary system[39] P. Villars, K. Mathis, F. Hulliger, Environment Classification and Structural Stability
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http://arxiv.org/abs/1503.00230.
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[44] V.M. Goldschmidt, Crystal structure and chemical constitution, Trans. Faraday Soc. 25(1929) 253–283.
[45] M.G. Verón, F.C. Gennari, Thermodynamic behavior of the Mg–Co–H system: The
effect of hydrogen cycling, J. Alloys Compd. 614 (2014) 317–322.
https://doi.org/10.1016/j.jallcom.2014.06.092.
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Mg2CoH5 nanocomposites, Int. J. Hydrogen Energy 42 (2017) 14593–14601.
https://doi.org/10.1016/j.ijhydene.2017.04.237.

74

Chapter 4
High-pressure synthesis of a novel compound in the Mg – Zn binary system
4.1 Introduction
In Mg – 3d-transition metal (TM) systems, a novel compound in the Mg – Zn binary system
synthesized via high-pressure synthesis similar to that of the Mg – Co binary system studied in
Chapter 3 has not been reported.
Considering the development of hydrogen storage materials, addition of Zn in an Mg has
been studied both experimentally and theoretically in an attempt to enhance the sorption
kinetics of hydrogen storage and to reduce the stability of MgH2 [1–3]. Liu et al. [3] reported
that Mg – 6.9 at% Zn prepared via a hydrogen plasma-metal reaction method enhanced theabsorption kinetic properties of Mg to better than that of micro-sized Mg particles at 200 °C.
The reversible hydrogen capacity of the Mg – 6.9 at% Zn was reported to be in the range of 4.6– 5.2 mass% depending on the operation temperature being in the range of 300 – 375 °C.
However, it seems that Mg – 6.9 at% Zn decomposes into MgH2 and MgZn2 at a temperature
higher than 300 °C after hydrogenation.
For the Mg – Zn – H system, the hydride of Mg51Zn20 was obtained at 330 °C and 10 MPa
of H2 with a maximum hydrogen capacity of 3.62 mass% corresponding to Mg51Zn20H95 [4].
Theoretical DFT calculations suggested the formation of Mg7ZnH16 with a Ca7Ge-type
structure [5], which was studied for hydrogen storage applications as it has lower
thermodynamic stability than MgH2.
Considering the elements in the same group as Zn, which are Cd and Hg, Zn and Cd exhibit
HCP structures, while Hg has a rhombohedral structure. The stable phases with the same
compositions in the Mg – X systems (X = Zn, Cd, and Hg) have different types of structures,as shown in Fig. 4.1. Therefore, there is the possibility that a novel Mg – Zn compound or astable phase with a new-type of structure may be obtained via high-pressure technique.
In this chapter, the purpose is to explore a novel compound in the Mg – Zn binary systemusing cubic-anvil-type apparatus and to investigate its crystal structure, thermal stability, andhydrogenation properties.

75

Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn binary system

Fig. 4.1

Schematic diagram of the phase stability of compositions of Mg – X systems(X = Zn, Cd, and Hg) [6], where (-) denotes that there has been no report ofthe crystal structure of that compound76

Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn binary system

4.2 Sample preparation
Mg powder (Alfa Aesar, 99.8%, 325 mesh) and Zn powder (High purity chemical, 99.9%,200 mesh) were used as raw materials to synthesize the Mg – Zn compounds. The Mg and Zn
powders were mixed by hand-milling in a high purity alumina mortar for 20 min. Mg – x at%
Zn mixtures with a nominal composition of x = 0, 7, 10, 12, 15, and 40 were prepared andpressed into pellets and packed into high-pressure cells without internal hydrogen source. The
conditions used to synthesize the Mg – x at% Zn mixtures are shown in Table 4.1.

Table 4.1 The synthesis conditions of Mg – x at% Zn mixtures with nominal compositions of x
= 0, 7, 10, 12, 15, and 40.
SamplePressure (GPa)Temperature(C)

Time(h)6500

8

Mg – 10 at% Zn

5

500

8

Mg – 10 at% Zn

4

500

8

Mg – 10 at% Zn

3

500

8

Mg – 10 at% Zn

2

500

8

Mg – 0 at% Zn
Mg – 7 at% Zn
Mg – 10 at% Zn
Mg – 12 at% Zn
Mg – 15 at% Zn
Mg – 40 at% Zn

4.3 High-pressure synthesis of novel Mg – Zn compounds
The XRD patterns of Mg – x at% Zn (x = 0, 7, 10, 12, 15, and 40) prepared at 500 °C and6 GPa for 8 h are shown in Fig. 4.2. At x = 0, 7, and 10, the XRD patterns revealed a phasewith an HCP structure as a single phase. When x = 12 and 15, Mg51Zn20 peaks were observed
alongside the HCP peaks, and at x = 40, Mg51Zn20 and Mg4Zn7 peaks were observed without
any HCP peaks.

77

Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn binary system

Although a novel compound in the Mg – Zn binary system was not synthesized in this study,
the solid solubility limit of Zn in Mg was expanded based on the presence of the HCP phases
as a single phase in the x = 7 and 10 samples. Moreover, it was indicated from the EDX analysisof the compositions of the HCP phases of the x > 10 samples that the amount of Zn in Mg was
10.3 at% Zn. In the Mg – Zn binary phase diagram, as shown in Fig. 4.3, the maximum solid
solubility of Zn in Mg under ambient pressure was 2.4 at% Zn. From the results in this study,
it was found that the solid solubility of Zn was expanded from 2.4 at% Zn to 10 at% Zn at
500 °C and 6 GPa.

Fig. 4.2

XRD patterns of Mg – x at% Zn (x = 0, 7, 10, 13, 15, and 40) prepared at
500 °C and 6 GPa for 8 h and that of Mg – 0 at% Zn before the high-pressure
synthesis.

78

Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn binary system

Fig. 4.3

Binary phase diagram in Mg – Zn system [7].

From the XRD patterns of the Mg – x at% Zn (x = 0, 7, and 10) compounds shown in Fig.
4.4, it can be seen that when the amount of Zn in the Mg phase increased, the HCP patterns
shifted to higher angles. That indicates that the lattice parameters of the HCP phases are lower
than those of pure Mg. As the atomic size of Zn is smaller than that of Mg, the increasing solid
solution of Zn in Mg causes a decrease in the lattice parameters a and c of the HCP phases(Figs. 4.5 and 4.6). These lattice parameters were estimated by Rietveld analysis and for the
HCP structure, the ideal c/a ratio was found to be 1.633. The c/a ratios of the HCP the phasesof Mg – x at% Zn (x = 7 and 10) compounds in this study were found to be 1.621, close to thepreviously reported c/a ratios of the Mg – x at% Zn (x = 0 – 2.4) compounds in their binary
phase diagram [8–10], as shown in Fig. 4.7. It is possible that when Zn is substituted in Mg at
more than 10 at% at 6 GPa, the c/a ratio of the HCP structure cannot be maintained, therefore,
the Mg and Zn form to be other phases.

79

Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn binary system

Fig. 4.4

XRD patterns of the Mg – x at% Zn (x = 0, 7 and 10) compounds preparedat 500 °C and 6 GPa for 8 h.

80

Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn binary system

Fig. 4.5

The lattice parameters c of the Mg – x at% Zn (x = 0 – 10).

Fig. 4.6

The lattice parameters a of the Mg – x at% Zn (x = 0 – 10).

Fig. 4.7

The c/a ratios of the Mg – x at% Zn (x = 0 – 10).

81

Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn binary system

4.4 Crystal structure refinement
Crystal structure refinement of the HCP phase of Zn substituted in Mg was achieved using
the Rietveld method. The HCP pattern of the Mg – 10 at% Zn compound prepared at 500 °C
and 6 GPa for 8 h was refined by fitting it to a Mg-type Mg – 10 at% Zn structure, in the spacegroup P63/mmc. The results of the refinement and the crystal structure of Mg – 10 at% Zn are
shown in Figs. 4.8, and 4.9, respectively. The final refinement (Fig. 4.8) converged to Rwp =10.638, Rp = 6.181, Re = 2.413, and GofF = 19.4372, the lattice parameters were estimated to
be a = 3.1791(1) nm and c = 5.1555(4) nm.

Fig. 4.8

Rietveld refinement of the Mg – 10 at% Zn prepared at 500 °C
and 6 GPa for 8 h.

Fig. 4.9

Crystal structure of the refined Mg – 10 at% Zn.

Table 4.2 The atom positions and their sites in the Mg–10 at% Zn compound synthesized underhigh-pressure.
Atom

Site

g

x

y

z

Mg

2c

0.9

1/3

2/3

1/4

Zn

2c

0.1

1/3

2/3

1/4

82

Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn binary system

4.5 Effect of pressure on the solid solubility limit of Zn in Mg
To investigate the effect of pressure in GPa-order on the solid solubility of Zn in Mg, theMg – 10 at% Zn samples were synthesized at 500 °C and 2 – 6 GPa, for 8 h. The XRD patterns
and the amount of Zn in the HCP phase of each sample determined by EDX analysis are shownin Figs. 4.10 and 4.11, respectively. At 5 GPa and lower, Mg51Zn20 peaks were observed
alongside the peaks for the HCP phase. When the synthesis pressure was decreased, the amountof Zn in the HCP phase decreased and the Mg51Zn20 peaks were clearly observed. Therefore, it
can be concluded that the solid solubility limit of Zn in Mg increased with increasing pressure.

Fig. 4.10

XRD patterns of the Mg – 10 at% Zn prepared at 500 °C
and 2 – 6 GPa for 8 h.

83

Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn binary system

Fig. 4.11

Solubility of the Zn in Mg in the Mg – 10 at% Zn preparedat 500 °C and 2 – 6 GPa for 8 h.

The conditions required for complete solid solution formation are outlined the HumeRothery rules, which are:
1. Atomic size factor: The atomic size difference of the elements is less than 15%.
rsolvent − rsolute
Atomic size factor = || × 100%
rsolvent
2. Crystal structure: The elements have the same crystal structure.
3. Electronegativity: The elements have approximately the similar electronegativity(according to the Pauling scale).
4. Valency: The elements have the same valency.
In the Mg – Zn binary system, Zn has limited solid solubility in Mg. Mg and Zn have thesame crystal structure element and same valency, which experience no changes under the effectof pressure. The other two factors, the atomic size and electronegativity, can be considered asfollows.
1. Factor of the atomic size of the constituent elements
Under ambient pressure, the atomic radii of Mg and Zn are 0.160 and 0.133 nm,
respectively, and the atomic radius ratio of Mg and Zn (rMg/rZn) is 1.203. Cd, which is in
the same group as Zn, has an HCP structure the same as that Zn and exhibits complete solid84

Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn binary systemsolubility with Mg. The atomic radius of Cd is 0.157 nm which is almost the same size asthat of Mg. The atomic radius ratio of Mg and Cd (rMg/rCd) is 1.07. Considering the HumeRothery rules, the atomic size factor of Mg as a solvent and Cd as a solute is 6.875%,
whereas that of Mg and Zn is 16.875%. As atomic size factor of Mg and Zn is greater than
15%, the solid solubility of Zn in Mg is limited.
Under GPa scale pressure, Mg and Zn exhibit different volume compressibilities [11].
The volume compressibility of Mg is greater than that of Zn, which leads to a decrease in
the atomic radius ratio of Mg and Zn (rMg/rZn) upon increasing pressure, as shown in Fig.
4.12. The atomic radius ratio of Mg and Zn at 6 GPa (rMg/rZn, 6 GPa) pressure can beextrapolated to 1.183, and the atomic size factor of Mg and Zn at the same pressure can be
estimated as 15.462%. It is possible that the solid solubility limit of Zn in Mg increases dueto the decrease in the atomic sizes of both elements. As the atomic size factor under 6 GPapressure is greater than 15%, the solid solubility of Zn in Mg is still limited.

Fig. 4.12

Atomic radius ratio of Mg and Zn under ambient pressure and at 2 – 6 GPa.

The fitting line in Fig. 4.12 shows that the atomic radius ratio of Mg and Zn (rMg/rZn)converges to 1.167, therefore, it is expected that the solid solubility of Zn in Mg can beincreased with a higher pressure. When the atomic radius ratio of Mg and Zn (rMg/rZn) was
1.167, the atomic size factor was estimated to be 14.369%, under a pressure of
approximately 10 GPa. The complete solid solubility of Zn in Mg might be exhibited at apressure of above 10 GPa. However, in this case, the electronegativity of both the elements
under the effect of pressure must be considered.

85

Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn binary system

2. Electronegativity of the constituent elements
According to the Pauling scale, the electronegativities of Mg and Zn are 1.31 and 1.65,
respectively. Based on the calculation of the electronegativies of each of the elements as afunction of pressure, it was shown that the trend in the increase in the electronegativity of
Mg with increasing pressure was higher than that of Zn [12]. The difference in theelectronegativity between Mg and Zn at any pressure became lower than under ambientpressure. Therefore, the electronegativities of Mg and Zn should not be an obstacle in thecomplete solid solubility of Zn in Mg.

The influencing factor on the increasing solid solubility of Zn in Mg is the atomic sizes ofMg and Zn, which are adjusted under the effect of pressure. However, besides the atomic size
factor, selecting an appropriate temperature is also an important factor because Mg and Cd,
which have complete solid solubility, form compounds as Mg3Cd, MgCd and MgCd3 attemperatures below 253 °C [13].
In the previous chapter, the atomic radius ratio of the constituent elements and the crystal
structures of the constituent elements and how they correlate with electron configuration andelectronegativity were discussed as the factors underpinning compound formation.
In this chapter, a novel compound in the Mg – Zn system was not obtained. Considering
the types of structures of the compounds in the Mg – Cd system, where Zn has an HCP crystal
structure the same as that of Cd and both elements are in the same group, below 253°C, Mg
and Cd form Mg3Cd and MgCd3 compounds with a Ni3Sn-type structure, and MgCd with a
AuCd-type structure [13]. The atomic radius ratio and the electronegativity difference (∆χAxBy ; x < y) calculated using the Martynov–Batsanov scale of electronegativity using equationin Eq. 3.1 for each of the Ni3Sn- and the AuCd-type structures are given in Tables 4.3 and 4.4,
respectively. The atomic radius ratio of the compound with the AuCd-type structure wascalculated using the atomic radius of the element occupying the 2f site divided by that of theelement occupying the 2e site (r2f/r2e).
Based on the calculation of the differences in the electronegativities of the compounds with
the Ni3Sn- and the AuCd-type structures, the differences in the electronegativities of the Mg –Zn compounds with those type structures are in the range of the difference in theelectronegativity of each type of structure. However, the atomic radius ratio of Mg and Zn(rMg/rZn) is out of range of the atomic radius ratios of the constituent elements (rB/rA) of bothtype structures. Although, the rMg/rZn decreases under the effect of pressure, it seems that the86Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn binary systemrMg/rZn value is still out of the range of each type of structure. It is possible that the rMg/rZnvalue being out of range results in the inability to synthesize a novel compound in the Mg – Zn
binary system.

Table 4.3 Calculated atomic radius ratios and differences in electronegativity of compoundswith the Ni3Sn-type structure
Compounds with the Ni3Sn-type

Atomic radius ratios(AB3)(rB/rA)∆ χAB3

CdMg3

1.074

0.045

MgCd3

0.931

-0.045

SnNi3

0.887

0.06

GeFe3

0.992

0.16

AlTi3

1.014

-0.11

ScCd3

0.925

0.05

YCd3

0.823

0.005

1.203 (ambient)

0.065 (ambient)

ZnMg3 (assumed)

1.183 (6 GPa)

Table 4.4 Calculated atomic radius ratios and differences in electronegativity of compoundswith the AuCd-type structure
Compounds with the AuCd-type

Atomic radius ratios(AB)(rB/rA = r2f/r2e)

∆ χAB

CdMg

1.074

0.09

CdAu

0.966

0.21

MoIr

1.000

0.07

WIr

0.993

-0.08

NbRh

0.951

0.04

1.203 (ambient)0.13 (ambient)

ZnMg (assumed)

1.183 (6 GPa)

4.6 Thermal stability measurement
Thermal stability measurements of Mg – 10 at% Zn prepared at 700 °C and 6 GPa for 8 h
were carried out by DSC analysis from room temperature to 300 °C at a heating rate of87

Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn binary system

10 °C/min under an Ar flow. The thermal stability results and XRD patterns before and after
the DSC analysis of Mg – 10 at% Zn are shown in Figs. 4.13. and 4.14, respectively. A phasetransition peak was not observed from the DSC curve (Fig. 4.13), however, the phase transitionof Mg – 10 at% Zn was observed in the XRD pattern after DSC analysis at 100 and 150 °C.
The phase transformation of Mg – 10 at% Zn was observed from the change in the XRD
pattern after DSC analysis at 150 °C at 2  40° – 43°. After DSC analysis at 300 °C, it was
clear that the peaks in the range of 2  40° – 43° could be assigned to Mg21Zn25. The HCP
pattern in the result after DSC analysis at 300 °C shifted to higher angles than those of pure
Mg. Therefore, the HCP peaks that are present in the powder pattern after the decompositionof Mg – 10 at% Zn should be attributable to Mg – 2.4 at% Zn, which is the compound thatexhibits the maximum solubility of Zn in Mg in their binary phase diagram.
Therefore, it was determined that Mg – 10 at% Zn decomposes into Mg – 2.4 at% Zn and
Mg21Zn25 at 100 – 150 °C, where the decomposition reaction of Mg – 10 at% Zn can be
expressed as follows:

100 - 150 °C

314.42 (Mg – 10 at% Zn) →Fig. 4.13

268.42 (Mg – 2.4 at% Zn) + Mg21Zn25

Eq. 4.1

DSC curve of Mg – 10 at% Zn prepared at 500 °C and 6 GPa for 8 h.

88

Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn binary system

Fig. 4.14

XRD patterns of Mg – 10 at% Zn prepared at 500 °C and 6 GPa for 8 h
before and after DSC analysis.

4.7 Hydrogenation properties
Hydrogenation of Mg – 10 at% Zn was carried out in a SUS cell filled with 5 MPa of H2 at
300 °C. The XRD pattern of Mg – 10 at% Zn after hydrogenation is shown in Fig. 4.15,
showing its decomposition into Mg21Zn25, HCP-like Mg, and MgH2 phases. The HCP peaks
after hydrogenation shifted to lower angles than before hydrogenation. The observed Mg21Zn25
peaks and the HCP peaks shifted to lower angles indicated that the Mg – 10 at% Zn phase could
not absorb hydrogen; it decomposed into stable phases and then absorbed hydrogen. However,the positions of the MgH2 peaks shifted to slightly higher angles than those of the MgH2
prepared from pure Mg. Therefore, it is thought that the obtained MgH2 is in the form of (Mg– x at% Zn)H2.
Dehydrogenation was carried out by TG analysis from room temperature to 420 °C at aheating rate of 5 °C/min under 300 ml/min of flowing He to investigate the weight loss due to89

Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn binary systemhydrogen desorption and the desorption temperature, the results of which are shown in Fig.
4.16. The weight loss due to the hydrogen desorption of hydrogenated Mg – 10 at% Zn wasobserved at 392 °C, lower than the desorption temperature of pure MgH2. Zn substitution
affects the decrease in the thermal stability of MgH2 due to a decrease in the ionic bonding
interaction between Mg and H upon Zn substitution in the Mg – H system [14]. The weightloss due to hydrogen desorption was estimated to be 2.8 mass%.
Based on the DTA curve, before the dehydrogenation reaction occurred, a small heat flow
peak was observed at between 331 – 350 °C. Based on the Mg – Zn binary phase diagram, thisrange of temperature is the phase transformation temperature of Mg21Zn25. The XRD patternof hydrogenated Mg – 10 at% Zn after TG analysis exhibited peaks for an HCP phase, Mg51Zn20,
and Mg4Zn7. The HCP pattern was observed at a lower angle than in the HCP pattern appeared
after hydrogenation. The HCP pattern appeared after dehydrogenation was the same as thatappeared after DSC analysis at 300 °C, attributed to Mg – 2.4 at% Zn.
High-pressure hydrogenation of Mg – 10 at% Zn was conducted at 600 °C and 6 GPa for 8
h. The XRD pattern of high-pressure hydrogenated Mg – 10 at% Zn exhibited peaks for an
HCP phase, Mg102.08Zn39.4, MgH2, and -MgH2 , as shown in Fig. 4.17. Since the HCP peaks
remained after hydrogenation, this showed that the HCP phase decomposed into MgH2 and
Mg51Zn20 phases, and then MgH2 transformed into -MgH2, which has been reported to be thehigh-pressure phase of MgH2 obtained at more than 2 GPa [15].

90Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn binary system

Fig. 4.15

XRD patterns of the Mg – 10 at% Zn prepared at 500 °C and 6 GPa for 8 h
before and after hydrogenation, and after dehydrogenation.

91

Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn binary system

Fig. 4.16

TG-DTA and H2 emission curves of the hydrogenated Mg – 10 at% Zn.

92

Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn binary system

Fig. 4.17

XRD patterns of the Mg – 10 at% Zn prepared at 500 °C and 6 GPa for 8 h
before and after high-pressure hydrogenation at 500 °C and 6 GPa for 8 h.

4.8 Summary
In this chapter, no novel Mg – Zn compound was obtained by high-pressure synthesis.
An increase in the solid solubility limit of Zn in Mg was observed upon an increased
pressure. The solid solubility of Zn in Mg increased from 2.4 at% Zn to 10 at% Zn in thesample obtained at 500 °C and 6 GPa for 8 h. It was found that the Mg – 10 at% Zn belongs
to the space group P63/mmc (No.194). The increase in the solid solubility of Zn in Mg
results in a decrease in the lattice parameters a and c, where the lattice parameters of Mg –10 at% Zn were estimated to be a = 0.3174(2) nm and c = 0.5148(3) nm. The Mg – 10 at%
Zn decomposed into Mg – 2.4 at% Zn and Mg21Zn25 in the temperature range of 100 –150 °C. After hydrogenation of the Mg – 10 at% Zn, it was found that it decomposed into
(Mg – x at% Zn)H2 and Mg21Zn25. The dehydrogenation temperature of the hydrogenatedMg – 10 at% Zn was determined as 392 °C. Substitution of Zn in MgH2 was thought topossibly decrease its thermal stability.

93Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn binary system

References[1] D. Chen, Y.M. Wang, L. Chen, S. Liu, C.X. Ma, L.B. Wang, Alloying effects of transition
metals on chemical bonding in magnesium hydride MgH2, Acta Mater. 52 (2004) 521–
528.
https://doi.org/10.1016/j.actamat.2003.09.037.
[2] G. Liang, Synthesis and hydrogen storage properties of Mg-based alloys, J. Alloys
Compd. 370 (2004) 123–128.
https://doi.org/10.1016/j.jallcom.2003.09.031.
[3] T. Liu, T. Zhang, X. Zhang, X. Li, Synthesis and hydrogen storage properties of ultrafine
Mg–Zn particles, Int. J. Hydrogen Energy 36 (2011) 3515–3520.
https://doi.org/10.1016/j.ijhydene.2010.12.049.
[4]

G. Bruzzone, G. Costa, M. Ferretti, G.L. Olcese, Hydrogen storage in Mg51Zn20, Int. J.
Hydrogen Energy 8 (1983) 459–461.
https://doi.org/10.1016/0360-3199(83)90167-2.

[5]

A. Erkisi, G. Gökoglu, First principles investigation of Mg7XH16 (X = Ti, Zn, Pd, andCd) ternary hydrides for hydrogen storage applications, Mater. Res. Express 5 (2018).
https://doi.org/10.1088/2053-1591/aacbb6.
A. Nayeb-Hashemi, Phase diagrams of binary magnesium alloys, ASM Int. Met. Park.
Ohio 44073, USA, 1988. 370 (1988).
H. Okamoto, Supplemental literature review of binary phase diagrams: Cs–In, Cs–K,
Cs–Rb, Eu–In, Ho–Mn, K–Rb, Li–Mg, Mg–Nd, Mg–Zn, Mn–Sm, O–Sb, and Si–Sr, J.
Phase Equilibria Diffus. 34 (2013) 251–263.

[6]
[7]

https://doi.org/10.1007/s11669-013-0233-2.
[8] H.W. King, Crystal Structures of the Elements at 25° C, J. Phase Equilibria 2 (1981)
401–402.
[9] D. Hardie, R.N. Parkins, Lattice spacing relationships in magnesium solid solutions,
Philos. Mag. 4 (1959) 815–825.
https://doi.org/10.1080/14786435908238237.
[10] R.S. Busk, Lattice parameters of magnesium alloys, Jom 2 (1950) 1460–1464.
https://doi.org/10.1007/bf03399173.
[11] K.A. Gschneidner, Physical Properties and Interrelationships of Metallic and
Semimetallic Elements, Solid State Phys. - Adv. Res. Appl. 16 (1964) 275–426.
https://doi.org/10.1016/S0081-1947(08)60518-4.
[12] M. Rahm, R. Cammi, N.W. Ashcroft, R. Hoffmann, Squeezing All Elements in the
Periodic Table: Electron Configuration and Electronegativity of the Atoms underCompression, J. Am. Chem. Soc. 141 (2019) 10253–10271.
https://doi.org/10.1021/jacs.9b02634.
[13] Z. Moser, W. Gasior, J. Wypartowicz, L. Zabdyr, The Cd–Mg (Cadmium-Magnesium)

94

Chapter 4 High-pressure synthesis of a novel compound in the Mg – Zn binary systemsystem, Bull. Alloy Phase Diagrams 5 (1984) 23–30.
https://doi.org/10.1007/BF02868720.
[14] R. Varunaa, H. Fjellvåg, P. Ravindran, Zinc substituted MgH2 - a potential material forhydrogen storage applications, Int. J. Hydrogen Energy 44 (2019) 13632–13646.
https://doi.org/10.1016/j.ijhydene.2019.04.016.
[15] M. Bortz, B. Bertheville, G. Böttger, K. Yvon, Structure of the high pressure phase λMgH2 by neutron powder diffraction, J. Alloys Compd. 287 (1999) 4–6.
https://doi.org/10.1016/S0925-8388(99)00028-6.

95

Chapter 5
High-pressure synthesis of a novel compound in the Mg – Y binary system
5.1 Introduction
Generally, hydrogen storage materials are hydride-forming and non-hydride-forming
compounds such as Mg – TM and RE – TM alloys, while in the case of Mg – RE alloys, both
Mg and RE are hydride-forming elements. Ouyang et al. studied the hydrogen storage
properties of Mg3RE (RE = La, Nd, Pr, and Mm) at room temperature and temperatures in therange of 270 – 340 °C [1–3]. They reported that the reversible hydrogen capacities of Mg3RE(RE = La, Nd, Pr, and Mm) were 2.89, 1.95, 2.58, and 2.58 mass%, respectively, and theirhydrogenation properties at room temperature were also determined. Furthermore, thehydrogen storage properties of nominal Mg3Y, which presented as Mg24Y5 and Mg2Y phases,
were studied in the range of 350 – 380 °C [4]. During the hydrogenation process, a
disproportionation reaction took place in which Mg24Y5 and Mg2Y decomposed into MgH2,
YH2, and YH3. It has been reported that the maximum hydrogen capacity of nominal Mg3Y is
5.4 mass%. Furthermore, the hydrogenation properties of the other Mg – Y-based alloys havebeen investigated [5–8]. The previous reports indicated that the alloys decompose into MgH2,
YH2 [4,6,8], and Mg-based alloys or Y-based alloys depending on the materials used [5,7].
In the previous research studies on the high-pressure synthesis, new hydrides in Mg – RE
– H systems (RE = Y, La, Ce, Pr, Sm, Gd, Tb, and Dy) were investigated [9–11]. Mg3REH9(RE= La, Ce, and Pr), which have hydrogen capacities of approximately 4 mass%, andMghRE2Hx (hRE = Y, Dy, Tb, and Gd) were reported. After dehydrogenation, it has beenreported that Mg3REH9 decomposes into Mg and REH2 [9], and MghRE2Hx decomposes into
hREH2 with no Mg [10].
Tao et al. carried out a theoretical study on the phase stability of Mg – RE binary systems[12], in which it was suggested that some Mg – Y phases that are known in their binary system
exist.
In this chapter, the purpose of the study was to explore a novel compound in the Mg – Ybinary system using cubic-anvil-type apparatus and to investigate the crystal structure, thermal
stability, and hydrogenation properties of the new Mg – Y compound.

96Chapter 5 High-pressure synthesis of a novel compound in the Mg – Y binary system

5.2 Sample preparation
To investigate a novel compound in the Mg – Y binary system, Mg powder (Alfa Aesar,
99.8%, 325 mesh) and Y powder, which was prepared by filing a Y rod to produce particles
that were able to pass through a 150 m sieve, were used as starting materials. Mg and Y
powders were mixed by hand – milling in a high purity alumina mortar for 20 min. Mg – x at%
Y mixtures with nominal compositions of x = 10, 20, 25, 40, 66.7, and 85 were prepared andpressed into pellets, before being packed into high-pressure cells without internal hydrogensource. The conditions used for synthesizing the Mg – x at% Zn mixtures are shown in Table
5.1.

Table 5.1 The conditions used to synthesize the Mg – x at% Y mixtures with nominal
compositions of x = 10, 20, 25, 40, 66.7, and 85.
SamplePressure (GPa)Temperature (C)Time (h)7002Mg – 10 at% Y
Mg – 20 at% Y
Mg – 25 at% Y
6Mg – 40 at% Y
Mg – 66.7 at% Y
Mg – 85 at% Y
Mg – 25 at% Y

4

Mg – 25 at% Y

3Mg – 25 at% Y

2

5.3 High-pressure synthesis of novel Mg – Y compounds
To investigate a novel compound in the Mg – Y binary system, Mg – x at% Y (x = 10, 20,
25, 40, 66.7, and 85) mixtures were prepared and synthesized by high-pressure synthesis at
700 °C and 6 GPa for 2 h. Figure 5.1 shows the XRD patterns of the high pressure synthesized
Mg – x at% Y (x = 10, 20, 25, 40, 66.7 and 85). In all of the samples, except for the x = 85
sample, FCC peak patterns (BiF3-type structure) were observed along with Mg and monoclinic-

97

Chapter 5 High-pressure synthesis of a novel compound in the Mg – Y binary system

Y2O3 (-Y2O3) peak patterns for the x = 10, 20 and 25 samples, alongside Mg, MgY, Y, and Y2O3 peak patterns for the x = 40 and 66.7 samples. The synthesis of the FCC single phase was
not obtained in these experiments. As there is high reactivity between Y with O, some of the Yis oxidized during the synthesis process, therefore, some Mg remains in the sample. SEM/EDX
analysis was carried out and was used to identify the elemental composition of the FCC phasecorresponding to Mg – 25 at % Y.
To investigate the effects of the pressure used in the synthesis on the FCC phase of the Mg– Y compound, Mg – 25 at% Y samples were synthesized at 700 °C under a pressure of 2 – 6
GPa for 2 h. According to the XRD results shown in Fig. 5.2, at 2 – 3 GPa, FCC peaks wereobserved alongside those of Mg24Y5 and Mg2Y. At 4 GPa pressure, the Mg24Y5 and Mg2Y peaks
disappeared and the pattern was identical to the result at 6 GPa.

Fig. 5.1

XRD patterns of Mg – x at% Y (x = 10, 20, 25, 40, 66.7, and 85) preparedat 700 °C and 6 GPa for 2 h.

98

Chapter 5 High-pressure synthesis of a novel compound in the Mg – Y binary system

Fig. 5.2

XRD patterns of Mg – 25 at% Y prepared at 700 °C and2 – 6 GPa for 2 h.

5.4 Crystal structure refinement
The crystal structure of the high-pressure synthesized novel Mg – Y phase was refinedusing by Rietveld refinement. The XRD pattern of Mg – 25 at% Y was refined by fitting it to a
BiF3-type structure belonging to the space group Fm–3m with FCC peaks, where Mg belongs
to the space group P63/mmc and -Y2O3 belongs to the space group C2/m. The chemical
formula of the FCC was expressed as Mg3Y. The results of the refinement and the crystal
structure of Mg3Y are shown in Figs. 5.3 and 5.4, respectively. The final refinement converged
to Rwp = 4.324, Rp = 3.278, Re = 2.353, and GofF = 3.3766, and the lattice parameter was
estimated to be a = 0.729939(4) nm. The atomic sites and positions are shown in Table 5.2.
The mass fractions of Mg3Y, Mg, and  - Y2O3 were estimated to be 78.35%, 13.94%, and7.71%, respectively.

99

Chapter 5 High-pressure synthesis of a novel compound in the Mg – Y binary system

Fig. 5.3

Rietveld refinement of Mg – 25 at% Y prepared at 6 GPa and 700 °C for 2 h.

Fig. 5.4

Crystal structure of refined Mg3Y.

Table 5.2 The atomic sites and positions of Mg3Y.
Atom

g

x

y

z

B

Mg1

1

0.5

0.5

0.5

1.6979

Mg2

1

0.25

0.25

0.25

2.12737Y

0.690726

0009.77E-02100

Chapter 5 High-pressure synthesis of a novel compound in the Mg – Y binary system

5.5 Effect of pressure on the presence of the Mg3Y phase
1. Effect of pressure on the atomic radius ratio of the constituent elements
In previous research studies and the previous chapters of this work, novel compounds
were successfully synthesized by high-pressure synthesis due to the adjustment in theatomic radius ratio of the constituent elements such as the high-pressure phase of Mg54Cu17
with a Mg51Zn20-type structure [13], MgCu with a CsCl-type structure [13], and Mg44Co7
with a Mg44Rh7-type structure [14].
Under ambient pressure, the atomic radii of Mg and Y are 0.160 and 0.181 nm,
respectively. The atomic radius ratio of Mg and Y (rMg/rY) was calculated to be 0.884, andthe atomic radius ratios of Mg and the other RE elements (rMg/rRE), which crystallize as
Mg3RE with BiF3-type structures under ambient pressure were also calculated, as shown in
Fig. 5.5. At 6 GPa pressure, the rMg/rY value was extrapolated to 0.882, which was slightly
lower than the rMg/rY value under ambient pressure as a result of the compressibility of Mg
is slightly more than that of Y. It can be noted that the rMg/rY value under ambient pressure
is in the range of the rMg/rRE values. The rMg/rY values from under ambient pressure to 6
GPa were found to be between the rMg/rRE values, where RE represents Nd and Sm.
In previous research studies, the atomic radius ratio factor has been proposed to
explain the reason why novel compounds can be synthesized[10,13,14]. From the rMg/rY
results, it seems that the application of pressure to adjust the atomic radius ratio of theconstituent elements is not the main reason why Mg3Y is obtained. This indicates that otherfactors should be considered when novel compounds are being synthesized.

101

Chapter 5 High-pressure synthesis of a novel compound in the Mg – Y binary system

Fig. 5.5

Values of the atomic radius ratios of Mg and the each rare earth elements
which exhibit Mg3RE phases under ambient pressure, except for Mg and Y(blue circles) and those of Mg and Y under ambient pressure (circle mark)and 6 GPa (red triangle) [15]. The type of crystal structures of each of therare earth elements are shown under the plot.

2. Effect of pressure on the crystal structures of the constituent elements
Under ambient pressure, Mg and Y exhibit HCP crystal structures. As described in theprevious chapters, there are no changes to the Mg crystal structure under the experimental
conditions detailed in this study. In terms of Y, it exhibits a sequence of changes in its crystal
structure in the order of HCP  Sm-type HCP  DHCP  FCC upon an increase in thepressure, as shown in Fig. 5.6. The changes in the crystal structure according to thissequence are related to the increase in the number of electrons in the d orbital and thedecrease in the atomic number of the rare earth element [16].

102Chapter 5 High-pressure synthesis of a novel compound in the Mg – Y binary system

Fig. 5.6

The pressure-temperature phase diagram of yttrium from Ref. [17]The ground-state electron configuration of Y is [Kr] 4d1 5s2 and the excited-stateelectron configuration of Y has been predicted to change to [Kr] 4d2 5s1 upon a promotionenergy of 130 kJ/mol [18]. The increase in the number of electrons in the d orbital of RE
elements from d1 to d2 was indicated that instead of the HCP structure, the Sm-type HCP
and DHCP structures, respectively, were stabilized, but upon an increase from d2 to be d3
instead of the DHCP structure, FCC structures were stabilized instead [16]. The changes in
the crystal structure of Y from HCP to Sm-type HCP and from Sm-type HCP to DHCP werereported to occur under 10 – 15 GPa and at 25 – 33.7 GPa pressure, respectively, at room
temperature [19,20]. Moreover, the electron configuration of Y at -273 °C was determinedfrom an analytical calculation using the extreme pressure polarizable continuum model,
which showed that it changed from the ground-state (4d1 5s2) to the first excited-state (4d2
5s1) and to the second excited-state (4d3 5s0) under pressure of approximately 20 and 50
GPa, respectively [21], therefore, the change in the crystal structure of Y in the order of
HCP  Sm-type HCP  DHCP at -273 °C should occur at a pressure of 20 – 50 GPa. If
the pressures under which the HCP to Sm-type HCP and Sm-type HCP to DHCP structuralphase transitions of Y occur decrease with an increase in the temperature, it is possible thatthe crystal structure of Y changed from HCP to either Sm-type HCP or DHCP structures
under the experimental conditions (700 °C, 2 – 6 GPa) used in this study.
Furthermore, considering the electronegativity of Y, it is close to those of heavy-rareearth (hRE) metals and the same as that of Dy. The differences in the electronegativities of103

Chapter 5 High-pressure synthesis of a novel compound in the Mg – Y binary system

the Mg3RE compounds (∆ χMg3RE ), calculated using Eq. 3.1, are given in Table 5.3. As theelectronegativities according to the Martynov–Batsanov scale of rare earth elements fromCe onwards are not available and the electronegativities of those elements have beenreported on the Pauling scale [22], the differences in the electronegativities of the Mg3Y
and Mg3La were calculated using both scales and those of the Mg3RE compounds (RE =Ce, Pr, Nd, Sm, Gd, Tb, and Dy) were calculated using the Pauling scale. Taking the Mg
and RE to be elements B and A, respectively, according to the ∆ χMg3RE value on thePauling scale, it is close to the ∆ χMg3Y value.
The trend in the change in the electronegativity of Y with an increase in the pressure
from ambient pressure to 6 GPa is smaller than that of Mg. Therefore, the ∆ χMg3Y value
under high pressure is lower than the value under ambient pressure. The ∆ χMg3Y value iscloser to the ∆ χMg3RE value at high pressure than under ambient pressure. The
experimental results determined that Mg3Y was obtained as almost a single phase during
its synthesis under a pressure of 4 GPa. Based on the calculations of the electronegativitiesof each element as a function of pressure [21], the electronegativities of Mg upon increasing
the pressure to 4 and 6 GPa were estimated to increase by approximately 0.124 and 0.16,
respectively, according to the Pauling scale. Also, the electronegativities of Y were
estimated to increase by approximately 0.099 and 0.13, respectively, according to thePauling scale. The calculated ∆ χMg3Y value decreased from -0.045 to -0.058 and -0.059 at
4 and 6 GPa, respectively, values that are between those of ∆ χMg3Sm and ∆ χMg3Gd .

104

Chapter 5 High-pressure synthesis of a novel compound in the Mg – Y binary system

Table 5.3 The differences in the electronegativities of Mg3RE compounds under ambientpressure∆ χMg3RE∆ χMg3RE(Martynov–Batsanov scale)(Pauling scale)

Mg3La

0.02

-0.105

Mg3Ce--0.095

Mg3Pr--0.090Mg3Nd--0.085

Mg3Sm--0.070Mg3Gd--0.055

Mg3Tb--0.105

Mg3Dy--0.045

Mg3Y

0.05-0.045

Mg3RE

As discussed above, according to the effect that pressure has on the aforementioned factors,
it can be considered that Mg3Y was successfully synthesized via high-pressure synthesis
because the crystal structure of Y transformed to either a Sm-type HCP structure, the same asthat of Sm, or a DHCP, the same as that of Nd. The rMg/rY values under ambient pressure were
up to 6 GPa were determined to be between the rMg/rNd and rMg/rSm values. Assuming that the
crystal structures of the RE elements correlate with the rMg/rRE values in terms of Mg3RE
compound formation, the lower limit of the rMg/rRE values for RE elements with an HCP
structure are values that are in the range of 0.884 – 0.89, which lie between the rMg/rY andrMg/rGd values. The upper limit of the rMg/rRE values for RE elements with a DHCP structure isthe values that are in the range of 0.88 – 0.89, which lie between the rMg/rNd and rMg/rSm values.
Furthermore, the decrease in the value of ∆ χMg3Y approaches that of ∆ χMg3Sm , thusincreasing the possibility of obtaining a Mg3Y compound.

105

Chapter 5 High-pressure synthesis of a novel compound in the Mg – Y binary system

5.6 Thermal stability measurement
Thermal stability measurements were carried out by DSC analysis under a flow of Ar in thetemperature range of between room temperature to 450 °C at a rate of 10 °C/min. The DSC
analysis of Mg3Y prepared at 700 °C and 6 GPa for 2 h and the XRD patterns of Mg3Y before
and after the DSC analysis are shown in Figs. 5.7 and 5.8, respectively. From the DSC curve,
an endothermic peak was observed. Judging from the DSC and the XRD analysis, Mg3Y
decomposed into Mg24Y5 and Mg2Y via an endothermic reaction that occurred at 315 °C. Thechange in enthalpy of the decomposition reaction was estimated to be 0.974 kJ/mol, where thedecomposition reaction of Mg3Y can be expressed using Eq. 5.1.

14 Mg3 Y →315 °C

9 Mg2 Y + Mg24 Y5

Eq. 5.1

High-pressure phase compounds such as MgCu, MgNi, Mg6Ni, and Mg44Co7 mainly
decompose via exothermic reaction [14,23,24] because they are metastable state compounds.
However, Mg3Y, which was prepared using a high-pressure synthesis method in this study,
decomposed via an endothermic reaction. Considering the enthalpy of formations ofcompounds in the Mg – Gd and Mg – Dy systems, in which Mg3RE, Mg2RE, and Mg24RE5 arestable phases, among these, the Mg2RE and Mg24RE5 phases are the most and least stablephases, respectively, for each system [25]. Based on the calculation of the change in enthalpy
using the standard enthalpy of formation data in a report (Ref.[25]) for the following reaction:
14 Mg3 RE →9 Mg2 RE + Mg24 RE5

Eq. 5.2

where RE represents either Gd and Dy, the changes in enthalpy of both are positive values(85.479 and 64.74 kJ/mol, respectively), which indicate that the decomposition reaction of
Mg3RE into Mg2RE and Mg24RE5 (RE = Gd and Dy) is also an endothermic reaction the sameas that of the decomposition reaction of Mg3Y. From this result, it was determined that Mg3Y
is more stable than Mg24Y5.

106

Chapter 5 High-pressure synthesis of a novel compound in the Mg – Y binary system

Fig. 5.7

DSC curve of Mg3Y prepared at 700 °C and 6 GPa for 2 h.

Fig. 5.8

XRD patterns of Mg3Y prepared at 6 GPa and 700 °C for 2 h
before and after the DSC analysis.

107

Chapter 5 High-pressure synthesis of a novel compound in the Mg – Y binary system

5.7 Hydrogenation properties
The hydrogenation of Mg3Y was carried out in a SUS cell at 200 °C and 6 – 8 MPa of H2.
The XRD patterns of Mg3Y before and after hydrogenation under various conditions are shownin Fig. 5.9. The results showed the decomposition of Mg3Y into -MgH2, -MgH2 [26,27], YH3,
and FCC phases. In the XRD pattern, the FCC peak position shifted to a higher angle comparedto the peak position of YH2, which also exhibits an FCC structure. The shift in the peak
positions to higher angles indicates that the FCC phase has a lower smaller lattice parameter.
The effect of Mg substitution in YH3 stabilized the high-pressure phase FCC-YH3 underambient pressure [28], which has a smaller lattice parameter than that of YH2 [29].
Dehydrogenation was carried out by TG analysis from room temperature to 450 °C. The
results of the weight loss due to hydrogen desorption and the desorption temperature are shownin Fig. 5.10. The total amount of hydrogen content lost from hydrogenated Mg3Y at 6 MPa and
8 MPa were 0.72 and 3.52 mass% at hydrogen desorption temperatures of 367 °C and 350 °C,
respectively. The increase in the hydrogenation pressure affected the amount of hydrogencontent in the sample due to an increase in the kinetics of the hydride phases transformation
rate [30]. After dehydrogenation, Mg and YH2 peak patterns were found to be present in XRD
results, as shown in Fig. 5.9. The total amount of hydrogen content lost from hydrogenated
Mg3Y measured by TG analysis represented the hydrogen desorption of MgH2 to Mg and FCCYH3 to YH2.
High-pressure hydrogenation of Mg3Y was carried out at 700 °C and 6 GPa for 2 h. The
result after hydrogenation is shown in the XRD pattern in Fig. 5.11, in which Mg, Y, YH2, andYH3 peak patterns can be observed. Among the phases present, the YH3 phase is the only phasethat desorbs hydrogen at a temperature lower than 450 °C. Therefore, the amount of hydrogencontent lost from the high-pressure hydrogenated Mg3Y was estimated to be less than 0.1
mass% hydrogen.
As was suggested earlier, the reason for the interstitial sites of the H atoms in the Mg3La
lattice being smaller than the atomic size of H might be related to its decomposition into MgH2
and LaH3 [4], therefore the interstitial sites of the H atoms in the Mg3RE compounds (Fig. 5.12)
were calculated, as presented in Table 5.4. The calculated spaces of the interstitial sites in allof the Mg3RE compounds are smaller than 0.04 nm, smaller than the radius of a H atom. Mg3La,
synthesized from MgH2 and LaH3 as starting materials at 700 °C under a pressure of more than
2 GPa for 2 h was subjected to high-pressure hydrogenation to yield Mg3LaH9 [9], as shown in
Fig. 5.13. At high-pressure in the order of GPa, the interstitial sites of the H atom in the Mg3La
lattice increased in size due to the effect of pressure on the decrease in the atomic sizes of Mg

108

Chapter 5 High-pressure synthesis of a novel compound in the Mg – Y binary system

and La. In terms of the calculations on Mg3Y, the interstitial sites of the H atoms in thesynthesized lattice are smaller than 0.04 nm, which is the minimum space size that can occupya H atom [31]. Assuming that the aspect ratio of the Mg3Y lattice is constant at any pressure,
as well as the fact that the value of the atomic radius ratio of Mg and Y (rMg/rY) hardly changes
from that under ambient pressure, as shown in Fig. 5.5, the interstitial sites in Mg3Y also barelychange at high pressure, which might be the cause of the decomposition of Mg3Y during thehydrogenation process.

Fig. 5.9

XRD patterns of Mg3Y (a) before hydrogenation, (b) after hydrogenation at
200 °C and 6 MPa of H2, (c) after hydrogenation at 200 °C and 8 MPa of H2,(d) hydrogenated in various conditions after dehydrogenation by TG analysis.

109

Chapter 5 High-pressure synthesis of a novel compound in the Mg – Y binary system

Fig. 5.10

Fig. 5.11

TG curve of hydrogenated Mg3Y.

XRD patterns of Mg3Y before and after high-pressure hydrogenation at 700 °C
and 6 GPa for 2 h.

110

Chapter 5 High-pressure synthesis of a novel compound in the Mg – Y binary system

Fig. 5.12

O-site in a Mg3RE with a BiF3-type structure.

Table 5.4 Lattice parameters (a) and calculated maximum space sizes of O-sites in each Mg3RE
unit cell
Mg3RE

Lattice parameter(a / nm)

The calculated maximum
space size of the O-site (nm)Mg3La0.74930.0273Mg3Ce0.74440.0261

Mg3Pr

0.74150.0254

Mg3Nd

0.73910.0248Mg3Sm

0.73710.0243Mg3Gd

0.73100.0228Mg3Tb

0.72960.0224Mg3Dy

0.72670.0224

Mg3Y

0.72990.0225111

Chapter 5 High-pressure synthesis of a novel compound in the Mg – Y binary system

Fig. 5.12

XRD patterns of (a) Mg3La before hydrogenation, (b) Mg3La high-pressurehydrogenated at 700 °C and 5 GPa for 2 h, (c) MgH2 – 25 at% LaH3 preparedat 900 °C and 5 GPa for 2 h. The unmarked peaks belong to Mg3LaH9.

5.8 Summary
In this chapter, a novel compound in the Mg – Y binary system was successfully
synthesized via high-pressure synthesis. Mg3Y, a cubic BiF3-type structure belonging to thespace group Fm–3m (No. 225) with an estimated lattice parameter of a = 0.729939(4) nm wassynthesized at 700 °C under a pressure of more than 2 GPa for 2 h. Mg3Y was found to be
thermally stable up to 315 °C before decomposing into Mg24Y5 and Mg2Y phases via
endothermic reaction. At 200 °C and 8 MPa of H2, Mg3Y was observed to disproportionatedinto ambient-pressure and high-pressure phases of both MgH2 and YH3. The dehydrogenation
temperature of hydrogenated Mg3Y was determined as 350 °C, where the experimental
maximum hydrogen content was estimated to be 3.51 mass%. High-pressure hydrogenation of
Mg3Y at 6 GPa did not result in the hydride of the compound in the form of Mg3YHx.

112

Chapter 5 High-pressure synthesis of a novel compound in the Mg – Y binary system

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hydride with promising hydrogen storage properties, Int. J. Hydrogen Energy 32 (2007)
3929–3935.
https://doi.org/10.1016/j.ijhydene.2007.05.026.
[2] L.Z. Ouyang, F.X. Qin, M. Zhu, The hydrogen storage behavior of Mg3La and
Mg3LaNi0.1, Scr. Mater. 55 (2006) 1075–1078.
https://doi.org/10.1016/j.scriptamat.2006.08.052.
[3] L.Z. Ouyang, X.S. Yang, H.W. Dong, M. Zhu, Structure and hydrogen storage
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Chapter 6
Conclusions
In this thesis, the exploration of novel Mg – X compounds (X = Co, Zn, Y) was carried out
via high-pressure synthesis. Novel compounds in the Mg – Co and Mg – Y binary systems withthe chemical formulas Mg44Co7 and Mg3Y, and the extended solid solubility of Zn in Mg in theform of Mg – 10 at% Zn were obtained. The successful synthesis of novel compounds and theextension of the solid solubility of Zn via high-pressure synthesis are dictated by three factors:(1) the atomic sizes of the constituent elements, (2) the electron configurations of theconstituent elements, and (3) the electronegativities of the constituent elements. These threefactors were discussed in terms of the atomic radius ratios of the Mg and X elements (rMg/rX),
the crystal structures, and the differences in the electronegativities of the compounds (∆ χAxBy ).
Under the influence of pressure,
1. All the atomic radius ratios (rMg/rX) of the compounds synthesized under highpressure changed to values that closed to those of other compounds with the same typestructures that are stable under ambient pressure. However, the change in the atomic radiusratio of Mg and Zn may still not be sufficient to form compounds with those types ofstructures.
2. The crystal structures of the X elements that are determined by the number ofvalence electrons tended to be the same crystal structures as the other elements in theirgroups at the synthesis condition. It resulted from the changes in the electron configurationof each element.
3. The differences in the electronegativity of the compounds (∆ χAxBy ; where x < y)synthesized under high-pressure conditions changed due to the changes inelectronegativities of the constituent elements with different rates. The differences in theelectronegativity of the synthesized compounds ( ∆ χMg44Co7 and ∆ χMg3Y ) at synthesispressures became closer to those of the other compounds of each type structure than theirvalues under ambient pressure.
The changes in the atomic radii, crystal structures and electronegativities of elements
involve the changes in electron configurations that have an influence on the change in chemical
reactivity. Therefore, the changes in these three factors by the effect of pressure increased thepossibility to obtain the novel compounds. These three factors should be considered together
in the design and exploration of novel compounds.

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Chapter 6 Conclusion

Mg44Co7 was obtained at 700°C under a pressure of more than 5 GPa over 8 h. The crystal
structure of Mg44Co7 with a Mg44Rh7-type structure belongs to the space group F–43m(No.216), with an estimated lattice parameter of a = 2.0127(1) nm, decomposes into Mg and
MgCo phases at 390°C under an Ar flow via an exothermic reaction. Hydrogenation of
Mg44Co7 in the temperature range of 300 – 350 °C and 8 MPa of H2 resulted in its
decomposition into MgH2, Mg6Co2H11, and Mg2CoH5. The experimental maximum hydrogencontent of hydrogenated Mg44Co7 was estimated to be 4.2 mass%.
The solid solubility of Zn in Mg was extended from 2.4 at% Zn under ambient pressure to
10 at% Zn at 500°C and 6 GPa for 8 h, yielding Mg – 10 at% Zn. The crystal structure of Mg– 10 at% Zn belongs to the space group P63/mmc (No.194), with estimated lattice parametersof a = 0.3174(2) nm and c = 0.5148(3) nm. The hydrogenation of Mg – 10 at% Zn at 300 °C
and 5 MPa of H2 decomposed into (Mg – x at% Zn) H2 and Mg21Zn25. The experimentalhydrogen content was estimated to be 2.8 mass%.
Mg3Y with a BiF3-type crystal structure belonging to the space group Fm–3m (No. 225)
with a lattice parameter of a = 0.729939(4) nm was obtained at 700°C under a pressure of morethan 2 GPa for 2 h. Mg3Y was found to decompose into Mg24Y5 and Mg2Y at 315°C under an
Ar flow via an endothermic reaction. Mg3Y hydrogenated at 200°C and 6 – 8 MPa of H2
decomposed into the ambient-pressure and high-pressure phases MgH2 and YH3. The
experimental maximum hydrogen content of hydrogenated Mg3Y was estimated to be 3.51
mass%.
The Mg44Co7, Mg – 10 at % Zn, and Mg3Y could be defined as metastable phases in theirsystems. From the hydrogenation results of the synthesized compounds in this study, thehydrides of the parent compounds were not obtained, but the synthesized compounds exhibitedthe disproportionation reactions with hydrogen that gave beneficial results in hydrogenation
properties. Their hydrogen desorption temperatures were lower than the desorption temperatureof conventional MgH2. The disproportionation of Mg44Co7 and Mg3Y with hydrogen leaded to
the formation of Mg2CoH5 and YH3, respectively, that could act as catalysts for thedehydrogenation of MgH2. And the disproportionation of Mg – 10 at% Zn with hydrogen
leaded to the formation of (Mg – x at % Zn) H2 that exhibited the decrease in the thermal
stability of MgH2 due to the substitution of Zn in MgH2.

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Suggestions and future directions
To synthesize novel compounds via high-pressure synthesis, pressure-dependent factors interms of the atomic radius ratios of the constituent elements, the crystal structures of theconstituent elements, and the electronegativities of the compounds need to be taken into
consideration.
Since these factors determine compound formation (compound composition and structure),
not only these three factors but also including other factors such as valence electron
concentration and coordination number should be taken into account. Therefore, further
research should be focused on determining the effect that pressure has on the other factors inorder to support the knowledge for the design and exploration of novel materials.

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Chapter 6 Conclusion

List of Achievements
Published Paper(s)1. High-pressure synthesis of a new compound of the Mg–Co system
Siree Burapornpong, Toru Kimura, Atsunori Kamegawa
International Journal of Hydrogen Energy 45 (2020) 15553–15559.
doi:10.1016/j.ijhydene.2020.04.020
2. High-pressure synthesis of a novel compound of Mg–Y system
Siree Burapornpong, Shota Tsuchiya, Toru Kimura, Atsunori Kamegawa
Journal of Alloys and Compounds 850 (2021) 156754.
doi.org/10.1016/j.jallcom.2020.156754
Presentation at an International Conference
1. High-pressure synthesis of new compound in Mg–Co system
Siree Burapornpong, Toru Kimura, Atsunori Kamegawa
16th International Symposium on Metal-Hydrogen Systems
Guangzhou, China, October 30, 2018.
Presentations at Domestic Meetings
1. Investigating novel compound in Mg-rich side of Mg–Co system by high pressure synthesis
Siree Burapornpong, Toru Kimura, Atsunori Kamegawa
The Japan Institute of Metals and Materials 2018 Autumn Annual Meeting
Tohoku University, Sendai, September 20, 2018.
2. High-pressure synthesis of novel Mg–Co compound
Siree Burapornpong, Toru Kimura, Atsunori Kamegawa
The 6th Seminar on Next Generation Hydrides Research, The Japan Institute of Metals andMaterials
Hiroshima University Faculty Club, Hiroshima, November 14, 2019.

119Acknowledgment
This dissertation was completed with the support and assistance of many people.
First of all, I would like to express my sincere thanks to supervisor Prof. Atsunori
Kamegawa for the continuous support of my doctoral study and research, for his valuableguidance and knowledge throughout my doctoral program. His guidance is not only guidanceon research and writing dissertation but also working attitude and communication skills.
Besides my supervisor, I would like to thank my dissertation committee: Prof. Hideyuki
Saitoh, Prof. Chihiro Sekine and Assoc. Prof. Naoya Sawaguchi for their constructive
comments and valuable suggestions.
My sincere thanks to Dr. Toru Kimura for advising on research and access to researchfacilities.
I am also thankful to all who have been members of Hydrogen functional material
laboratory of my time here for research discussion and laboratory activities.
For over 6 years in Muroran Institute of technology, I would like to thank the academic
support of Muroran Institute of Technology and all staff of the Center for international relations
for supporting and giving much advice to me since the first time I came here to participate
exchange program in 2013. They also gave opportunities for me to participate in many
activities for learning Japanese cultures and spreading my country's cultures.
Finally, I would like to thank my family and my friends for giving me moral support
throughout the writing of this dissertation. I am very thankful for supporting and believing in
my decision all along.

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