反応焼結法による希土類単硫化物 RESxHo)の合成メタデータ

言語: English
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公開日: 2021-06-23
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キーワード (En):作成者: チャン, ヌ ヴィエン
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URL

https://doi.org/10.15118/00010386(RE = Gd,

Synthesis of rare-earth monosulfide RESx
(RE = Gd, Ho) compacts by reaction sintering
反応焼結法による希⼟類単硫化物 RESx
(RE = Gd, Ho の合成)

Dissertation

Submitted to Muroran Institute of Technology
Division of Production Systems Engineering
In Partial Fulfillment of the Requirements
For the Degree of Doctor of Engineering

By
Tran Nhu Bien
Muroran Institute of Technology,
Muroran, Hokkaido, Japan

December 2020
I

Certificate of Approval

This dissertation entitled:

Synthesis of rare-earth monosulfide RESx (RE = Gd, Ho)compacts by reaction sintering
反応焼結法による希⼟類単硫化物 RESx (RE = Gd, Ho の合成)

Written by Tran Nhu Bien
has been approved for the Division of Production Systems Engineering
Muroran Institute of Technology, Hokkaido, Japan

Date:

Professor Dr. Shinji Hirai
Professor Dr. Chihiro Sekine

Signed by the final examining committee:
Professor Dr. Isao Saeki

Professor Dr. Chihiro Sekine

Professor Dr. Toshihiro Kuzuya

The final copy of this dissertation has been examined by the signatories, and we findthat both the content and form meet acceptable presentation standards of scholarly
work in the above-mentioned discipline.

IIDedication
To my supervisor
To my family
To my wifeIIIAcknowledgments
I extend my deepest gratitude to my supervisor, Prof. Hirai Shinji. His supportand encouragement during my time as a Ph.D. student was invaluable and very
important. I would like to express my deepest thanks to Prof. Sekine, my co-supervisor,
who was always most helpful when called upon. During the medical leave of Prof.
Hirai Shinji, special advisor Dr. Kazuhei Wakiya stood in as superintendent, and I
would like to express my gratitude for his guidance and suggestions. I would also liketo thank Prof. Saeki, Prof. Kamegawa Atsunori, Prof. Toshihiro Kuzuya, and Dr.
Kawamura for their valuable comments and instructions.

I would like to express my sincere thanks to Prof. Inga Vasilyeva and Dr.
Ruslan Nikolaev from the Nikolaev Institute of Inorganic Chemistry (Russian) for
their support in terms of melting-point measurements, quick responses to all myrequests, and for reviewing my article. Prof. Vitalij K. Pecharsky and Prof. Arjun K.
Pathak are likewise acknowledged for supporting me during my internship at Ames
Lab and for their suggestions for my studies.

I am fortunate to have fantastic friends, lab mates, colleagues, and a researchteam, including Hoang Anh Tuan, Mohammed Alharbi, and Akie Kuroda; I deeply
appreciate our talks and discussions, work-related or otherwise. I would also like tothank all my friends and my laboratory staff for all the precious help during my studies
at the Muroran Institute of Technology.

IVLast but not least, my appreciation also goes out to my family, my parents fortheir endless love, care, and encouragement throughout my life and my studies in Japan,
and to my younger sister for always letting me know how proud she is of me. Lastly,
I would like to express my gratitude to my wife and my children for always being there
for me. Without their tremendous understanding and encouragement over the past few
years, it would have been impossible for me to complete my studies.

VAbstract
Rare-earth monosulfides, or RES (RE = Gd, Ho), have attracted significant
attention because of their potential application as cathodes in organic light emitting
diodes, thermal-expansion compensators, thermoelectric generators, and magnetic
recording and electronic devices. However, the synthesis of pure RES is stillchallenging because the RE metal, as a raw material, is easily oxidized and the current
synthesis methods are complex. This study examined simple synthesis methods for
obtaining high-purity RES. Stoichiometric and non-stoichiometric GdSx and HoSx
(0.68 ≤ x ≤ 1.2) were synthesized by the direct reaction sintering of -Gd2S3 and GdH2
for GdSx and -Ho2S3 and Ho metal for HoS using a pulse electric current sintering
apparatus.
The synthesized compacts were characterized by microscopic analysis using
an X-ray diffractometer, EDX analyzer, and an O/N/H analyzer. The X-ray diffraction
results showed that GdSx and HoSx sintering compacts can be obtained without an
impurity phase by optimizing the sintering temperature. The actual composition for
the GdSx and HoSx sintering compacts can be varied from GdS1.10 to GdS0.75 andHoS1.19 to HoS0.83, respectively. The study revealed that the rare-earth and sulfur
defects arising from rare-earth additions within the homogeneity range strongly affectthe lattice parameter, melting point, and electrical resistivity. The resistivity of the
stoichiometric compound shows a metallic behavior down to several Kelvin, while in
the off-stoichiometric compounds, semiconductor-like behavior appears below themagnetic ordering temperature. The lattice parameter and the melting point become
the highest when the composition is close to stoichiometric. These insights about thesynthesis process and the relation between composition and properties provide
guidelines for further general problem solving in O-free RES preparation.
VI概要

希⼟類単硫化物は NaCl 型の結晶構造をもち，光学デバイス，太陽電池，フォトダイ
オード，磁気記録デバイスなどのエレクトロニクス分野における応⽤に不可⽋な材料である。しかし，希⼟類⾦属と硫⻩の蒸気圧が⼤きく異なる点，単体の希⼟類⾦属が極めて酸化しやすい点，これまでの試料作製⽅法が⾮常に多くの反応プロセスを伴う複雑なものであることなどから，純良な希⼟類単硫化物試料の合成は困難であった。本研究では，これまで研究例のほとんどない，Gd, Ho を含む希⼟類単硫化物に着⽬し，CS2 ガス
硫化法により作製した希⼟類三⼆硫化物（-Gd2S3, -Ho2S3）と希⼟類単体（Ho）もしくは希⼟類⽔素化物（GdH2）を出発物質とし，放電プラズマ焼結（SPS）装置を⽤いた
反応焼結法により，物性測定が可能である⼤きなサイズの希⼟類単硫化物 GdS, HoS の純良な単⼀相焼結体の合成に成功した。また，出発原料の希⼟類と硫⻩の⽐率を変える
ことにより，⾮化学量論⽐組成の GdSx，HoSx の単⼀相焼結体を広い組成範囲で合成することにも成功した。さらに，得られた試料について，結晶構造解析，元素組成分析，
微量に含まれる不純物の同定を⾏い，熱物性，電⼦物性を詳細に調べた。その結果，格⼦定数，融点，電気伝導率の変化が，希⼟類と硫⻩の組成⽐（x）と密接に関係していることを確認した。

本研究では，従来の⽅法よりも単純なプロセスで不純物の少ない純良な希⼟類単硫化物の⼤型焼結体の合成が可能であることを⽰した。さらに，出発原料の⽐率を変えることで，⾮化学量論⽐組成の希⼟類単硫化物における希⼟類と硫⻩の組成⽐（x）を制御することが可能であることを⽰した。さらに，希⼟類と硫⻩の組成⽐（x）を変えることで電気伝導率などの物性を⼤きく変化させることが可能であることから，今回得られた結果は，物性値の最適化を⾏う上でも重要な知⾒であると考えられる。これらの結果
は，Gd, Ho 以外の希⼟類単硫化物にも応⽤が可能であると考えられ，今後の希⼟類単硫化物の設計の幅を広げることにつながり，次世代の⾼性能デバイスに不可⽋な材料開発の発展に寄与するものであると考えられる。

VII

Contents
Dissertation ..................... エラー! ブックマークが定義されていません。
Dedication ....................... エラー! ブックマークが定義されていません。
Acknowledgments .......... エラー! ブックマークが定義されていません。
Abstract ........................... エラー! ブックマークが定義されていません。
List of Figures ................. エラー! ブックマークが定義されていません。
Chapter 1 Introduction.. エラー! ブックマークが定義されていません。
1.1

Rare-earth mono-sulfidesエラー! ブックマークが定義されてい

ません。
1.2

Review of synthesis of rare earth sulfidesエラー! ブックマークが定義されていません。
1.2.1 Direct reaction of the elementsエラー! ブックマークが定義されていません。
1.2.2 EuS .................エラー! ブックマークが定義されていません。
1.2.3 Reaction of the RE metals on RE2S3 sulfides.エラー! ブックマークが定義されていません。
1.2.4 Synthesis of RES (RE = Gd, Ho)エラー! ブックマークが定義されていません。
1.3

Purposes ............ エラー! ブックマークが定義されていません。

Chapter 2 Experimental エラー! ブックマークが定義されていません。
2.1

Synthesis methodエラー! ブックマークが定義されていません。

2.1.1 CS2 gas sulfurizationエラー! ブックマークが定義されていません。
2.1.2 The pulsed electric current sintering apparatus (SPS)エ ラ ー ! ブックマークが定義されていません。
VIII

2.1.3 Synthesis method of RES (RE = Gd and Ho)エラー! ブックマー
クが定義されていません。
2.2

Characterization by microscopic analysisエラー! ブックマークが定義されていません。
2.3

Apparatus for investigation of melting point measurementsエ ラー! ブックマークが定義されていません。
2.4

Electrical resistivity measurementエラー! ブックマークが定義されていません。
Chapter 3 Results and discussionエラー! ブックマークが定義されてい
ません。
3.1

Synthesis of RES (RE = Gd and Ho) compactsエラー! ブックマークが定義されていません。
3.1.1 GdS .................エラー! ブックマークが定義されていません。
a.

Synthesis of GdSx compactsエラー! ブックマークが定義されて

いません。
b.

Density and lattice constant behaviorエラー! ブックマークが定義されていません。
c.

Microstructure observationエラー! ブックマークが定義されていません。
3.1.2 HoS................. エラー! ブックマークが定義されていません。
a.

Synthesis of HoSx compactsエラー! ブックマークが定義されて

いません。
b.

Lattice constant and densityエラー! ブックマークが定義されて

いません。
c.

Microstructures and EDX analysisエラー! ブックマークが定義されていません。
IX

3.2

Electrical propertiesエラー! ブックマークが定義されていません。
3.2.1 Electrical properties of GdSエラー! ブックマークが定義されていません。
3.2.2 Electrical properties of HoSエラー! ブックマークが定義されていません。
3.3

Melting point measurementエラー! ブックマークが定義されていません。
3.3.1 Melting point of GdSエラー! ブックマークが定義されていません。
3.3.2 Melting point of HoSエラー! ブックマークが定義されていません。
Chapter 4 Conclusions ... エラー! ブックマークが定義されていません。
References ....................... エラー! ブックマークが定義されていません。

XList of Figures

Figure 1.1 Crystal structure of RES (RE: Rare-earth) ..................................................3
Figure 2.1 Schematics of the experiment apparatus used for sulfurization reaction ..14
Figure 2.2 The pulsed electric current sintering apparatus .........................................16
Figure 2.3 Process of synthesis of rare earth monosulfides ........................................17
Figure 2.4 Potential diagram of Gd-O2-S2 system at 1773, 1973 and 2173 K ...........19
Figure 2.5 Scheme of a thermo-microscopic setup: (1) sample, (2) molybdenum
crucible, (3) W/W–Re thermocouple, (4) tungsten heater, (5) sample, (6) IR
photodiode, (7) optical quartz window, (8) microscope, (9, 10) vacuum and inert gas
valves, and (11) computer. ..........................................................................................26
Figure 2.6 Electrical resistivity measurement system by laboratory-built .................28
Figure 2.7 Electrical resistivity measurement by the four probe method ...................29
Figure 3.1. The image of the sintering die (a) and GdS compact samples (b) ...........30
Figure 3.2. XRD results of the compacts sintered at 1373, 1573, and 1773 K...........31
Figure 3.3. XRD results of compacts when the preparation ratios of GdH2 are adjusted
at a sintering temperature of 1773 K. .........................................................................32
Figure 3.4. Effect of the actual composition of GdSx on lattice constant of sinteredcompacts. ....................................................................................................................36
Figure 3.5.Optical microstructures of cross-sections of sintered compacts formed at (a)1573 and (b) 1773 K when the actual composition was GdS1.21 ................................40

XI

Figure 3.6.Optical microstructures of cross-sections of the sintered compact formed at
1773 K when the actual compositions were (a) GdS1.01, (b) GdS0.96, (c) GdS0.92, and(d) GdS0.82 ...................................................................................................................42
Figure 3.7. Optical microstructures and SEM microstructures of the cross-section of
the sintered compact formed at 1773 K when the actual compositions were (a, b)
GdS0.92, (c, d) GdS0.82, (e, f) GdS0.77 ...........................................................................44
Figure 3.8 Images of the graphite punch and die after sintering at 1973 K with theheating rates of (a) 5, (b) 10, and (c) 25 K/min ..........................................................49
Figure 3.9 Images of the sintered compacts at sintering temperatures of (a) 1773, (b)1873 and (c) 1973 K when the preparation ratio of the raw materials is HoS1.0 ........51
Figure 3.10 XRD patterns of the sintered compact surface in Figure 4.2 ..................52
Figure 3.11 Powder XRD patterns of the HoSx compacts sintered at 1973 K withvarious preparation ratios (Table 4.1) .........................................................................54
Figure 3.12 Surface change of the preparation ratio of the HoS1.10 compact sintered at1973 K (a) before and (b) after air exposure ..............................................................56
Figure 3.13 XRD patterns of the sintered compact surface in Figure 5 (a) before and(b) after air exposure for 14 days ................................................................................57
Figure 3.14 Relationship between the actual ratio and the lattice constant of HoSx ..61
Figure 3.15. Optical microstructures of the cross-sections of the sintered compact
formed at 1973 K, where the actual ratios are (a) HoS1.19, (b) HoS1.11, (c) HoS1.01, (d)HoS0.99, (e) HoS0.94, and (f) HoS0.85 ............................................................................63
Figure 3.16. Dependence of actual composition on electrical conductivity (σ) at room
temperature of sintered compacts formed at a sintering temperature of 1773 K ........67
Figure 3.17. Temperature dependence of the electrical resistivity of the sinteredcompacts shown in Figure 3.1 ....................................................................................68

XII

Figure 3.18 Relationships between the electrical conductivity (σ) measured at room
temperature and the actual composition of the HoSx compacts sintered at 1973 K ...70
Figure 3.19 Temperature dependence of the electrical resistivity (ρ) versus the actual
composition of the HoSx compacts sintered at 1973 K ..............................................71
Figure 3.20. Heating curves (a) and liquidus line in the homogeneous range (b) for thecompacts sintered at 1773 K .......................................................................................73
Figure 3.20 Melting points versus actual compositions of the HoSx compacts sinteredat 1973 K .....................................................................................................................75

XIII

List of Tables
Table 2.1. Equilibriums in the Gd-S-O system at 1773 K ..........................................21
Table 3.1. Composition of GdSx determined from the preparation ratios and actual
composition of GdSx determined from chemical analysis ..........................................34
Table 3.2. Effect of Gd content of GdSx on the apparent density and lattice constant ofsintered compacts ........................................................................................................38
Table 3.3. Results of EDX point analysis at the positions shown in Figure 3.6, Figure
3.7, and Figure 3.8 ......................................................................................................48
Table 3.4 Preparation ratios of the raw materials and the actual ratios determined fromthe chemical analysis for HoSx sintered at 1973 K .....................................................59
Table 3.5. Apparent density of the sintered compacts formed at 1973 K...................61
Table 3.6. Apparent density of the sintered compacts formed at 1973 K...................65

XIV

Chapter 1 Introduction
1.1 Rare-earth monosulfides
The rare-earth monosulfides (RES) form the cubic NaCl-type structure asshown Figure 1 [1]. They have attracted much attention because of their potential
use as a cathode in organic light emitting diodes [2], [3], thermal-expansioncompensators [4], thermoelectric generators, magnetic recording and electronic
devices [5], and magnetic refrigerants [6]. Magnetic measurements of RES showedthat most rare-earth (RE) atoms are trivalent, meaning that an unbonded electron,
Ln3(e)S2, is present. The extra electron is delocalized in the 5d conduction band,
resulting in the metallic-type conduction. In contrast, EuS, SmS, and YbS contain
divalent RE atoms, thus, there are no extra electrons, and these compounds behave
as semiconductors or semimetals [1]. The following section provides a brief review
of RES.

1.1.1 CeS
CeS has a high melting point, low vapor pressure, low chemical reactivity
with molten metals, and other characteristics which make it suitable for refractorymaterials. The change in free energy during the formation of CeS has a larger
negative value than that for Ce2S3. This may account for the chemical inertness of
CeS toward many molten metals. Additionally, CeS possesses outstanding
conducting properties. The electrical resistivity is as low as 0.72  10-4 cm at 293
1

K and 1.12  10-4 cm at 1273 K. In contrast, the thermal conductivity is as low
as 39 W/(mK)-1, which is comparable to alumina. Because of these conducting
properties, CeS is a potential candidate to be applied in induction heating.

1.1.2 SmS
SmS has received extensive attention because it shows a pressure-induced
transition from the semiconducting to the metallic state [7], [8]. The
semiconducting–metallic transition is driven by the variable oxidation state of Sm.
Samarium has two possible ground state configurations in SmS, including thenonmagnetic Sm2+ with the 4f6(7F0) configuration and the magnetic Sm3+ with the
4f5(6H5/2) configuration [9]. The Sm valence state changes from a predominantly 2+
state (black, semiconducting state) to a mainly 3+ state (golden, metallic state).
Consequently, the color of SmS is changed by the transition, earning it the name of
the black–golden transition. The importance and uniqueness of this material isbased on its switching characteristics driven by pressure. For other switchable
materials, such as VO2, the switching requires proper heating/cooling cycles [5].
Furthermore, other studies of SmS showed the applicability of this material in highquality sensing and memory devices, as well as very presumably in opticalswitching devices, due to the laser-induced transition of SmS [10]–[12].

21.1.3 EuS
EuS shows a ferromagnetic transition at 16.2 K. EuS has attracted interest as
model crystals for the investigation of ferromagnetism in semiconductors [13]. The
dominant exchange interaction is among the nearest neighbor Eu2+ ions [14].
Europium shows a large magnetic entropy change at approximately theferromagnetic transition temperature. Therefore, EuS has potential application inmagnetic refrigerant materials [6], [15].

Figure 1.1 Crystal structure of RES (RE: Rare-earth)31.1.3 GdS and HoS
GdS is an antiferromagnetic with an approximate Néel temperature of 60 K.
Any increase in the sulfur content up to Gd3S4 or Gd2S3 may lead to a structuraltransition to the Th3P4-type and the appearance of the ferromagnetic order. In
contrast, excess Gd generates an antiferromagnetic–ferromagnetic transition in
adjunction with a semiconductor–metal transition [16]–[18]. Therefore, pure GdS(and that with added Gd) has actual and potential applications in the electronics
fields, such as optical devices, photovoltaics, photodiodes, and magnetic recording
devices [18]. GdS easily forms defects that affect the physical and chemical
properties. Vasilev and Beckenbaug reported that the electrical conductivity isstrongly dependent on sulfur vacancies [19], [20].

HoS also crystallizes with the cubic NaCl type-structure. Neutron
diffraction measurements showed that HoS exhibits a Type-II antiferromagnetic
order at the Néel temperature of TN = 21 K [21]–[23]. HoS has been widely studied
because of its intriguing physical properties and potential applications in electronic
and magnetic recording devices [15], [24]–[27]. Furthermore, HoS possesses alarge specific heat, close to the liquid H temperature, hence, it is expected to be
applied to cryopanels close to this temperature.

As mentioned above, GdS and HoS possess intriguing properties. However,the practical application of the RES (GdS and HoS in particular) has not yet been
consistent with its inherent ability. This may be because they are difficult to prepare4as they require clean stoichiometric crystals. Therefore, caution is recommended
when interpreting results that could be sensitive to the sample quality. However, noprevious literature exists on the effects of impurities, such as O, on the chemicaland physical properties of RES.

5

1.2 Review of the synthesis of rare-earth sulfides
This section briefly reviews the various methods used to synthesize RES.

1.2.1 Direct reaction of elements
RES can be obtained from a direct reaction between rare-earth elements andsulfur vapor. In this method, 0.5–1.0 mm-thick slices of RE metals are reacted insulfur vapor where the temperature is progressively raised to 1050 °C andmaintained for two or three days. The product is crushed, pressed in an agate mortar,and re-sintered for two or three days. To obtain solid samples, the sample are sealed
under vacuum in a tantalum crucible, and the temperature is maintained at slightly
higher than the melting point, and thereafter cooled down to room temperature
within 30–60 min [28].

Alanko et al. reported another method in which RE elements andmechanochemical processes were used [29], where the driving force for thechemical reactions was partially supplied by energetic milling media impacts. In atypical synthesis, 5 g of the rare-earth ingots and sulfur powder were milled in an
argon atmosphere glovebox with PO2 < 30 ppm. Two milling media sizes (5– and
10–mm diameter) were used to help randomize the media trajectories and increase
the contact area with the vessel interior during milling. The milling was conducted
at 500 rpm for between 50 min and 12 h. However, a small amount of RE2S wasdetected as an impurity.

61.2.2 EuS
EuS is a special case because it can be directly obtained by the action of an
H2S stream on Eu2O3 contained in a graphite crucible at 1100–1200 oC [30]. The
action of an H2S stream on the oxalate [31] or on the sulfate [32] was also used.

In contrast, our research group synthesized EuS by reacting CS2 gas andEu2O3 powders [33] and obtained single-phase EuS powders at 800 C for 8 h.

1.2.3 Reaction of the RE metals on RE2S3 sulfides
CeS
Eastman et al. used this method first used synthesize CeS powders [34]. A
mixture of Ce2S3 and CeH3 was heated to approximately 400 °C to dissociate thehydride and form powdered Ce metal, followed by heating the sample to 2000 °C
in a vacuum.

In contrast, Hirai et al. found that CeS powders can be synthesized by reacting
Ce2S3 powder with Ce metal using a spark plasma sintering apparatus at 1273 K for
10.8 ks [15]. In this method, Ce2S3 was obtained from the CS2-gas sulfurization ofceria (CeO2) powder [19]. The synthetic CeS powders contained a small amount ofCe, Ce2O2S, and β-Ce2S3 as impurities [15]. However, for the CeS sintered compact
prepared by spark plasma sintering, the O content gradually decreased as the7sintering temperature increased due to evaporation of the volatile CeO [15]. Singlephase CeS compact was formed by sintering at 2173 K [15]. To evaluate theactivation energy for the densification of CeS, a CeS powder was prepared bymilling an initial sintered compact and utilizing it as an ingredient for hot-press
sintering experiments [15]. The densification data during hot-press sintering wasanalyzed using a kinetic equation and the analysis showed that boundary diffusion
was the rate-limiting process, as similarly reported for the sintering of Ce2S3, SiC,
and Al2O3 [15]. This result suggests that the boundary diffusion model can explain
the densification data with an apparent activation energy of 479 kJꞏmol-1 [15].

The direct action of RE metals on RE2S3 were also used for all RES powders(except for EuS, TmS, and YbS) [35]. The advantage of this method is the formationof homogeneity ranges with low S contents from the RES powders. In this method,
the mixtures of metal and RE2S3 were heated to approximately 1350 C in a Ti
boride vessel. The use of graphite vessels must be avoided to prevent the formationof RE carbides. For the stoichiometric composition, the reaction proceeded for only
1 h. In contrast, for the various non-compositions, raw materials were thermallyheated for several hours to obtain homogeneous phases [35].

81.2.4 Synthesis of RES (RE = Gd, Ho)
Several applicable reports, among the RE sulfides literature, exist
concerning the GdS and HoS synthesis methods.

The synthesis of pure GdS remains a challenge because Gd, as a raw
material, is easily oxidized. Therefore, sharp distinctions between thethermodynamic properties of Gd and S (melting and boiling points) and the low
kinetics of their interaction generate a spatial chemical inhomogeneity.
Additionally, the considerable Gd affinity to O leads to undesired oxidized byproducts that are difficult to detect if they are miniscule. Therefore, in mostsynthesis methods, Gd2S3 is used as the starting material to reduce the effect ofoxidation. Because both α-Gd2S3 and GdH2 are stable in air at room temperature[36], [37], α-Gd2S3 can be prepared by sulfurization of Gd2O3 powder using CS2 or
H2S gas [38], [39]. For a considerable period of time, GdS has been synthesized byreacting Al with equimolar Gd2S3 and Gd2O2S [40] according to the following
reaction:

3Gd2S3 + 3Gd2O2S + 4Al ＝ 12GdS + 2Al2O3s(1.1)

To remove Al2O3, the product is treated by heating above 1700 ℃ for 2 h
under vacuum because Al2O3 can be eliminated by sublimation as a volatile Al
suboxide under vacuum at this temperature.

9

GdS has also been synthesized by using the carbothermic reduction of
Gd2O3 [41]. After pelletizing a mixture of Gd2S3, soot, and Gd2O3 (according to themole ratio as required by Eq.(1.2)), the mixture was vacuum heat treated at 1600 ℃
for 23 hours, followed by grinding, repelletization, and a final heat treatment at
1700 ℃ for 2 h. The analytical composition of the obtained GdS was 82.81% Gd
and 16.59% S, and it contained 6% free carbon as an impurity.

Gd2O3 + 2Gd2S3 + 3C ＝ 6GdS + 3CO(1.2)

GdS also can be prepared by a two-stage process from metal filings andpowdered S [42]. First, the reaction of Gd with excess S was conducted in a sealed
evacuated fused silica tube maintained at 500 °C for several days. Next, after adding
excess Gd, GdS exhibiting a gold color was obtained by heat treating it in a Knudsen
cell at 900 °C for 6 h and then at 1950 °C for 24 h to eliminate the excess Gd.

Furthermore, single crystals have been grown from molten GdS [20], [43].
After pre-reacting a mixture of Gd chips and S in a sealed silica tube at 1600 °C
and forming a pellet, the pellet was heated above the melting point in a Ta tube or
W crucible. Then, after sufficient homogenization of the molten metal, a single
crystal of GdS was grown by annealing for several hours at a temperature 2050 °C
lower than the melting point.

10Moreover, GdS nanoparticles were produced in solution at ambient
temperature using the ionic starting materials of GdCl3 and Na2S in the presence of
dextrose [44].

HoS was synthesized from the direct reaction of elements [22], [45].
However, high purity was difficult to achieve when the Ho and S elements wereused. Furthermore, it was reported that the HoS composition was difficult control
[23].

11

1.3 Purposes
RES have been intensively studied since late last century. However, thesynthesis of RES is still challenging owing to the complexity of the current
synthesis methods. In particular, only some reports on the GdS and HoS synthesis
methods are available. Furthermore, most RES prepared by the previously
mentioned methods contain impurities and it has not been well clarified how thesample quality impacts their properties, though RES easily forms defects. Therefore,
it is important to systematically study how defects affect the chemical and physicalproperties of RES.
In this study, a simple RES (RE = Gd or Ho) synthesis method wasdeveloped by combining the CS2 sulfurization of RE oxides and reaction sinteringwith a pulse electric current sintering apparatus. The processing conditions,
including the heating rate, temperature, and duration of sintering, were examined to
produce single-phase and chemically homogeneous compacts.
Additionally, the effect of defects by the RE metal addition on the lattice
constant, melting point, and electrical resistivity were studied. Moreover, thedetailed features of the microstructure and experimental density results wereconsidered to estimate the purity, porosity, and grain boundary state to understandthe relationship between the microstructure and the macroscopic physical propertiesof compacts. The capabilities of the sintering process provided with highly dense
compacts as a peculiar interface were also considered.

12Chapter 2 Experimental
2.1 Synthesis method
GdS and HoS were synthesized by the CS2 gas sulfurization of Ho and Gd
oxides and reaction sintering with a pulsed electric current sintering apparatus(SPS).

13

2.1.1 CS2 gas sulfurization
Figure 2.1 depicts a schematic diagram of the sulfurization apparatus. RE
sesquisulfides (α-Gd2S3 and δ-Ho2S3) were synthesized according to reaction (2.1)[46]:

2Ln2O3 + 3CS2 → 2Ln2S3 + 3CO2

(2.1)

Figure 2.1 Schematics of the experiment apparatus used for sulfurization reaction
Gd2O3 powder (99.99%, Shin-Etsu Chemical Co., Ltd., Tokyo, Japan) witha 2.2354 m mean particle size was placed in a quartz boat and inserted into a silica
glass tube. After flushing with Ar gas at room temperature, the boat was heated to1273 K before the Ar gas was replaced by CS2 gas. The CS2 gas was introduced
into the reaction tube by passing the Ar carrier gas through a bubbler that contained
reagent grade liquid CS2. The flow rate of Ar gas was fixed at 80 mL/min. Afterbeing held at this temperature for 2 h, the boat was gradually cooled to room
temperature under an Ar atmosphere.

14

For the synthesis of δ-Ho2S3, Ho2O3 powder with an average 1.6 m particle
size (99.99%, Shin-Etsu Chemical Co., Ltd., Fukui, Japan) was used as a starting
material. The sulfurization was performed under the conditions presented in Ref.
[47].

2.1.2 Pulsed electric current sintering apparatus
Pulsed electric current sintering is also known as spark-plasma sintering(SPS). Figure 2.2 presents a photograph of the equipment used in this study. The
SPS enables the high-speed synthesis of materials and components from metals,
ceramics, polymers, and composites. The SPS is often used for the synthesis ofmaterials that are difficult to deform by conventional pressure-less sintering or hot
isostatic pressing [48]–[50]. The sintering conditions are described in Section 2.1.3.

15

Figure 2.2 The pulsed electric current sintering apparatus

162.1.3 RES (RE = Gd, Ho) synthesis method
Figure 2.3 presents a flowchart of the preparation process of RES (RE =Gd or Ho). The synthesis of RES (RE = Gd or Ho) is based on the direct reaction
between rare-earth sesquisulfide (-Gd2S3 and δ-Ho2S3) and gadolinium hydride(GdH2) for -Gd2S3 and Ho metal for δ-Ho2S3. In this study, the rare-earth
sesquisulfide (-Gd2S3 and δ-Ho2S3) was prepared by the sulfurization of a RE
oxide as described in Section 2.1.1.

Figure 2.3 Process of synthesis of rare earth monosulfides

17

GdS
The -Gd2S3 and GdH2 (Kojundo Chemical Laboratory Co., Ltd) powders
were mixed at the nominal compositions shown in Table 3.1 in air using an agate
mortar. In this study, the preparation ratios were described as the mass percent of
GdH2 in the mixture (chemical formula calculated from the preparation ratios).
Next, 3 g of the mixture was mounted in a graphite die with a 15 mm inner diameterand introduced into a SPS (SPS-511L, Sumitomo Coal Mining Co., Tokyo, Japan).
The apparatus chamber was then evacuated to 7.0 × 10−3 Pa. The sintering was
performed at a temperature ranging from 1373 to 1773 K for 1 h under a uniaxialpressure of 20 MPa. This sintering temperature was determined based on theisothermal potential diagrams discussed below. Both of the heating and coolingrates were fixed at 25 K/min.

Figure 2.4 shows the isothermal potential diagrams of the GdSO system
at 1773, 1973, and 2173 K for predicting the single-phase GdS formation state [51].
The diagrams were drawn based on data from the MALT software (Materialsoriented Little Thermodynamic Database, Kagaku Gijutsu-Sha Group, Tokyo,
Japan) [52]. The diagram was taken as a base for the accurate interpretation of thephase state of the prepared compacts, especially when O-consisting impurities were
present. Based on the GdSO diagram, the equilibrium states at specific points
and lines at 1773 K are shown by the red circles and lines in Figure 2.4 and arelisted in Table 2.1.

18

Figure 2.4 Potential diagram of Gd-O2-S2 system at 1773, 1973 and 2173 K

19

Lines 3 and 4 indicate that GdS can be formed from Gd2S3 by S evaporation
or from gaseous Gd and S2 species when their partial pressures are extremely low.
GdS can also be formed from Gd2S3 without passing through a route in which
Gd2O2S is formed if the O content of Gd2S3 as a raw material is sufficiently small
and the S potential (log PS2) is sufficiently low. Moreover, the log PS2 gradually
increases with increasing temperature. We also predicted that GdS can be formed
from Gd2O2S even at a relatively high log PS2 and O potential (log PO2) at higher
temperatures. However, the synthesis at high temperature increases the evaporationof Gd and reduces the GdS formation area; thus, the temperatures of 1373, 1573,
and 1773 K were used to prepare GdSx.

20

Table 2.1. Equilibriums in the Gd-S-O system at 1773 K
Line and point on diagram

Coexisting phases

Reactions of the phase interaction

Line 1

Gd, Gd2O3

4Gd + 3O2 ↔ 2Gd2O3

Line 2Gd, Gd2O2S

4Gd + 2O2 + S2 ↔ 2Gd2O2SLine 3Gd, GdS

2Gd + S2 ↔ 2GdSLine 4

GdS, Gd2S3

2Gd2S3 ↔ 4GdS + S2

Line 5

GdS, Gd2O2S

Gd2O2S ↔ GdS + (Gd + O2)

Line 6

Gd2S3, Gd2O2S

3Gd2O2S ↔ Gd2S3 + (4Gd + 3O2)

Point 7Gd, Gd2O3, Gd2O2S

3Gd2O2S ↔ (2Gd + 3/2S2) + 2Gd2O3

Point 8Gd, GdS, Gd2O2S

Gd2O2S ↔ GdS + Gd + O2

Point 9

GdS, Gd2S3, Gd2O2S

3Gd2O2S + 2S2 ↔ 4GdS + Gd2S3 + 3O2

21

HoS

HoSx sintered compacts were prepared by a reaction similar to that of GdS[53]. However, for the synthesis of HoS, Ho metal powder, instead of hydride, wasinvolved in the reaction with δ-Ho2S3. Thus, the reaction of HoSx (0.68 ≤ x ≤ 1.2)
can be described by the following reaction (2.2):

xHo2S3 + (32x) Ho = 3HoSx(2.2)

The Ho powder was prepared by grinding a Ho ingot (Kojundo Chemical
Laboratory Co., Ltd.) using a diamond file. Note that the preparation of Ho powder
and the mixing of δ-Ho2S3 and Ho powders were performed in air. A series of HoSx
samples with different initial Ho2S3/Ho ratios (Table 3.4) were prepared using 6 gof each mixture in a graphite die with 10 and 15 mm inner diameters, which wasintroduced into the SPS (SPS-511L, Sumitomo Coal Mining Co., Tokyo, Japan).
Sintering was performed at the temperatures of 1773, 1873, and 1973 K for 1 h
under a uniaxial pressure of 20 MPa with 7.0 × 10−3 Pa of evacuation. 5, 10, and 25
K/min heating rates were tested to find the best value for producing samples withhigh compactness. The sintering temperatures were chosen because of the reasonsdescribed below.

Because of the similarity with the Gd–S–O system, that was well studied inRef. [53], it is plausible that Ho2O2S can appear as an impurity phase at hightemperatures according to reaction (2.3). This is because O exists as the residual O

22

of Ho2S3 and a thin-surface oxidized Ho2O3 layer, which is universally present on
the Ho particles. Furthermore, O is also present inside the sintering apparatus.

Ho2S3 + 2Ho2O3 = 3Ho2O2S(2.3)

Both O sources clearly show that the amount of Ho2O2S phase depends on thethermodynamic parameters of the sintering process, on the amount of added Ho,and the thickness of the Ho oxidized layer. Thus, sintering temperatures ofapproximately 2000 K were desirable for this study because of the detectable
vaporization of Ho2O3, as in the case of other RE oxides that began with these
conditions [42], [54], [55]. According to references [56] and [57], the experimental
values of the melting points of Ho, δ-Ho2S3, and γ-Ho2S3 were determined as 1734,
1952, and 1920 K, respectively. That is, at a temperature near 2000 K, the Ho metal
and S will be vaporized, which could change the initial S/Ho ratio. These weightlosses should be determined chemically. Thus, the temperatures of 1773, 1873, and1973 K and heating rates of 5, 10, and 25 K/min were tested to find out the adequate
compromise conditions of sintering.

23

2.2 Characterization by microscopic analysis
The synthesized products were identified by powder X-ray diffraction(XRD) with Cu-K radiation using Rint-Ultima+ (Rigaku Corp., Tokyo, Japan).
The XRD was operated at a 40 kV tube voltage, a 20 mA tube current, and a 2°/min
scan speed. The lattice parameters were calculated to a 0.002 Å accuracy from thepeaks of (111), (220), and (311) using Si as a standard. The phase analysis was
performed by considering lines with an intensity  3 cps.

The apparent density of the sintered compacts was measured by theArchimedes method ( 0.01 g/cm3) using an analytical balance (AUX120,
SHIMADZU Co., Japan). The actual composition of the samples was determinedwith the following tools:



ICP-AES (ICPE-9820, Shimadzu, Kyoto, Japan) was used to determine
the Gd and Ho content ( 0.02 at.%).



The O contents of the initial powders and sintered samples were
determined ( 0.01 at.%) with an O/N/H analyzer (EMGA-930, Horiba
Ltd., Kyoto, Japan).



The Gd (or Ho), S, and O contents were controlled through X-ray
energy dispersive spectroscopy with scanning electron microscopy(EDX–SEM; JSM-6610LA, JEOL Ltd., Tokyo, Japan). Theintermetallic compounds, Sm2Fe17 and SmFe2, made in the laboratory24

were used as the calibration standard for EDX. The accuracy of thequantification procedure was within 5%.


An optical microscope (Nikon-E400 Pol) was used to obtainmicrostructure images of the cross sections of the polished sinteredcompacts.

2.3 Apparatus for the investigation of melting point
measurements
Figure 2.5 shows the thermal apparatus for measuring the melting points.
These measurements were performed at the Nikolaev Institute of Inorganic
Chemistry, Russia.

25

Figure 2.5 Scheme of a thermo-microscopic setup: (1) sample, (2) molybdenum
crucible, (3) W/W–Re thermocouple, (4) tungsten heater, (5) sample, (6) IR
photodiode, (7) optical quartz window, (8) microscope, (9, 10) vacuum and inert
gas valves, and (11) computer.

26

The thermal stability up to the melting point was studied by using a hightemperature thermal analysis of the rapid heating and a special experimental
procedure providing melting without composition variation [58]–[60]. Obtaining
reliable values for the composition/melting point relationship of the nonstoichiometric GdSx and HoSx compacts is challenging because of their incongruent
sublimation before melting. Therefore, a high-temperature thermo-microscopic
apparatus operating at a wide range of heating rates and pressures of inert gases wasused [21–23]. The working principle of the thermo-microscopic apparatus is basedon the measurement of variations in the radiation coefficient of heating solids
located in an open Mo crucible (2) by a suitable IR photodiode (mounted on a
microscope (6)). To measure GdSx and HoSx, a 5–7 mg sample (1) was heated at a
rate of 3000 K/min and at a He pressure of 7.0 atm (9). Under these conditions, asolid-vapor equilibrium was realized inside the crucible without the vapor emerging
and condensing on the peephole window of the apparatus. The phase transition isrecorded in the dU/dtT coordinates, where dU/dt is the time derivative of thesample thermal radiation. The apparatus was calibrated with reference materials
such as Pt (2045 K), sapphire (2333 K), and Y2O3 (2573 K), which allowed for themeasurement of the temperature of the peaks with an accuracy of 2%. Three to fiveparallel measurements were conducted for each compact tested.

27

2.4 Electrical resistivity measurement
The electrical resistivity was measured by a standard DC four probe-methodin the temperature range of 4 to 300 K using a laboratory-built physical properties
measurement system for GdS and HoS with a GM refrigerator, as shown in Figure
2.6.

Figure 2.6 Electrical resistivity measurement system by laboratory-built

28

Figure 2.7 shows a schematic diagram of the four-probe method. The
electrical resistivity, ρ, is given in equation (2.4), where VD is the voltage drop
between the two voltage probes, I is the electric current, D is a distance between thetwo probes, and A is cross-section of the sample. The current was applied by acurrent/voltage source (Model 7651, Yokogawa Corp.), and the voltage wasmeasured with a nanovoltmeter (Model 2182A, Keithley Instruments).

௏ ஺ವ = ஽ூ

(2.4)

Figure 2.7 Electrical resistivity measurement by the four-probe method

29

Chapter 3 Results and discussion
3.1 Synthesis of RES (RE = Gd, Ho) compacts
3.1.1 GdS
a. Synthesis of GdSx compacts

Figure 3.1. The image of the sintering die (a) and GdS compact samples (b)Figure 3.1 shows images of (a) a sintering mold consisting of a graphite dieand punch after pulsed electric current sintering and (b) the obtained sintering
compacts. A gold-colored layer, resulting from the vaporization of Gd metal, wasobserved near the gap between the graphite die and the punch; however, this turned
white when left in air (Figure 3.1(a)). This may have been a result of the evaporationand deposition of Gd owing to the decomposition of GdH2 at high temperature [36].
In contrast, color variations (from dark copper to bright gold) of the GdS sinteredcompacts could be obtained by increasing the charge ratio of GdH2, that is, 16.16(GdS1.2), 27.82 (GdS1.0), and 38.15 % (GdS0.83) (Figure 3.1(b)).

30

Figure 3.2. XRD results of the compacts sintered at 1373, 1573, and 1773 K

31

Figure 3.3. XRD results of compacts when the preparation ratios of GdH2 areadjusted at a sintering temperature of 1773 K.

32

Figure 3.2 shows the XRD results for the compacts sintered at 1373, 1573,
and 1773 K when the preparation ratio of GdH2 was fixed at 27.82 mass% (GdS1.0).
Figure 3.3 shows the XRD results for the compacts when the preparation ratios of
GdH2 were adjusted at a sintering temperature of 1773 K. When the sintering
temperature was 1373 K, Gd2O2S peaks were detected, as were the peaks of γ-Gd2S3transformed from α-Gd2S3. As the sintering temperature increased to 1573 K, thepeaks of the -Gd2S3 phase disappeared and were completely replaced by GdS
peaks. However, small amounts of Gd2O2S phases remained; these may have been
formed according to Eq. (3.1)

2Gd2O3(s) + Gd2S3(s) ＝ 3Gd2O2S(s)(3.1)

When the sintering temperature was increased to 1773 K, single-phase GdS
was obtained. The formation of single-phase GdS was confirmed to not depend on
the preparation ratio of GdH2, except when this preparation ratio was extremely
large or small. When the preparation ratio of GdH2 was small, γ-Gd2S3 remained.
Because the formation did not depend on the preparation ratio of GdH2, thevolatilization of extra Gd not contributing to the reaction was considerable.
However, it was observed that the peak intensity increased and the peak became
sharper when the preparation ratio of GdH2 was increased. Moreover, it was
confirmed that the crystal orientation and crystallinity of GdS increased. It was
concluded from the XRD results that single-phase GdS was formed when thesintering temperature was fixed at 1773 K and when the preparation compositions
were between 21.25 mass% (GdS1.1) and 43.54 mass% (GdS0.75).

33

Table 3.1. Composition of GdSx determined from the preparation ratios and actual composition of GdSx determined from chemical analysis

Molar ratio Gd2S3/GdH2 GdH2 (wt.%)

Preparation composition (at.%)
Gd

S

GdSxi

Actual composition (at.%)
Gd

S

O

GdSxii

1:0.5

16.16

45.45

54.55

GdS1.20

45.21

54.51

0.28

GdS1.21

1:0.7

21.25

47.37

52.63

GdS1.10

47.19

52.42

0.39

GdS1.11

1:1

27.82

50.00

50.00

GdS1.00

49.45

49.82

0.73

GdS1.01

1:1.2

31.62

51.61

48.39

GdS0.94

50.65

48.51

0.84

GdS0.96

1:1.6

38.15

54.55

45.45

GdS0.83

51.64

47.39

0.97

GdS0.92

1:2

43.54

57.14

42.86

GdS0.75

54.12

44.67

1.21

GdS0.82

1:2.4

48.06

59.46

40.54

GdS0.68

54.93

42.46

2.61

GdS0.77i

Chemical formula calculated from the preparation compositionii

Chemical formula calculated from the actual composition34

Table 3.1 shows the composition of GdSx determined from (i) the
preparation ratios of GdH2 and (ii) the actual composition of GdSx determined bychemical analysis. The sintering temperature of the compacts was 1773 K, which isconsistent with the compacts identified in Figure 3.3. The x values in the GdSx
chemical formulas determined from the actual compositions were larger than thosefrom the preparation ratios. The difference in these x values increased as the GdH2
preparation composition was increased. These increases may have been due to thevolatilization of Gd leading to a decrease in the concentration of GdS formed by
the reaction shown in Eq. (3.2). The volatilization of Gd was evident in Figure 2(a).
Furthermore, the O impurity content increased with an increasing GdH2 preparation
ratio. The O contents were also found to be less than the O content of -Gd2S3 as araw material, except for that at the 48.06 mass% (GdS0.68) GdH2 preparation ratio.
These decreases in O content are thought to be due to the formation of volatile GdO
by the reaction of Gd2O2S and Gd, as shown in Eq. (3.3). When the preparation
ratio of GdH2 was 48.06 mass% (GdS0.68), the maximum O content reached 2.61%
owing to the generation of Gd2O3, as shown in Figure 3.3. Bakovets et al. indicatedthat S diffusion (Sdif) leads to a reaction with Gd2O2S, yielding Gd2S3 and O
diffusion (Odif) [61]. This is related to the prediction that GdS can be formed from
Gd2O2S at high temperatures even with a relatively high log PS2 and log PO2, asshown in Figure 2.4.
Gd2S3(s) + Gd(l) ＝ 3GdS(s)Gd2O2S(s) + Gd(s) ＝GdS(s) + 2GdO(s,v)

b. Density and lattice constant behavior
35

(3.2)
(3.3)

Table 3.2 shows the effect of the Gd content of GdSx on the apparent density
and lattice constant of the sintered compacts formed at 1773 K, which is consistent
with the compacts identified in Figure 3.3 and Table 3.1. The apparent and relative
densities were measured only for the single-phase sintered compacts. The apparent
densities approximated the theoretically calculated values. The relative density
increased as the Gd content in the preparation composition increased owing to thegeneration of a liquid Gd phase, which acted as a sintering additive.

Figure 3.4. Effect of the actual composition of GdSx on lattice constant of sinteredcompacts.
Figure 3.4 shows the relationship between the lattice constant and the actual
composition. The lattice constants reported by other authors are also provided [19],[20], [24], [42], [62]. Based on the various conditions of our experiments, the lattice36constant exhibited deviations for the same composition. However, changes in thelattice constant in response to the Gd content of GdSx were in good agreement withconventional results [62]. A linear decrease in the lattice constant was confirmed in
the actual composition range for GdS0.71 to GdS1.01 as the Gd content increased. A
similar tendency was confirmed for the Nd, Pr, Ce, and La monosulfides, and it has
been reported that the composition range that maximizes the lattice constantbecomes narrower for light RE elements. However, in the actual composition range
from GdS1.01 to GdS1.21, the lattice constant gradually decreased with a decrease in
the Gd content, which also agreed with various results [62].

37Table 3.2. Effect of Gd content of GdSx on the apparent density and lattice constant of sintered compacts
Lattice constantDensity dexp. /dcal.

Relative density(Å)(g/cm3)(%)

Chemical formula

GdS1.215.5511

6.43/ --

Gd0.99S1.05-

GdS1.115.5615

6.56/6.88

0.953

GdS0.97

0.9986 [62]GdS1.01

5.5735

6.99/7.26

0.962

GdS0.94-5.5505 [24], [20]GdS0.96

5.5684

7.16/7.23

0.990

GdS0.90-

5.5629 [42], 5.5501 [19]GdS0.92

5.5581

7.20/7.22

0.997

GdS0.89-

5.5516 [20]GdS0.825.5485

7.25/7.10

1.021

GdS0.87

0.9984 [62]GdS0.77

5.54687.28/ --

GdS0.79-

Actual composition

38

References
Relative density Lattice constant (Å)
5.5606 [24]5.5663 [42], 5.5635 [20]5.5619 [42]c. Microstructural observations
The determination and distribution of the impure phases was conducted byusing the EDX technique on the polished compacts using an optical microscope.
Point EDX analysis was applied for the Gd, S, and O elements in different selectedareas, and the chemically homogeneous areas that were found were interpreted interms of phases. Some areas were considerably smaller than the excitation volume
and could not be fully quantified by EDX, and the Gd/S and Gd/O ratios near theedge of the GdS grains could not be measured in terms of their true values because
they presented an enrichment in Gd or S. The numbers in Figure 3.5, Figure 3.6,
and Figure 3.7 indicate the positions of the EDX analysis points, and thecorresponding composition analysis results are shown in Table 3.3.

39

Figure 3.5.Optical microstructures of cross-sections of sintered compacts formed at (a) 1573 and (b) 1773 K
when the actual composition was GdS1.21

40

Figure 3.5 shows the optical cross-sectional microstructures of the sinteredcompacts formed at (a) 1573 and (b) 1773 K for an actual composition of GdS1.21. The
XRD results of the sintered compact with a preparation composition of GdS1.0 (actual
composition of GdS1.01) shown in Figure 3.2 demonstrates that unreacted Gd2S3
remained at a sintering temperature of 1373 K. As shown in Figure 3.5, Gd2O2S was
not confirmed at a sintering temperature of 1773 K. In contrast, when the sintering
temperature was 1573 K, fine Gd2O2S and elliptically grown Gd2O2S were confirmed.
Further, it was confirmed that Gd2S3, which was not detected by XRD in the
preparation composition of GdS1.0 (actual composition of GdS1.01), remained at bothsintering temperatures when the actual composition was 1.21. The compositions of
Gd2S3 as precipitates were determined as GdS1.55 for No. 1 and GdS1.47 for No. 2 which
is located near the edge of the GdS grains. The results were consistent with theunderstanding that the stable Gd2S3 precipitates are found out of the homogeneous
range of the GdS phase.

41

Figure 3.6.Optical microstructures of cross-sections of the sintered compact formed at 1773 K
when the actual compositions were (a) GdS1.01, (b) GdS0.96, (c) GdS0.92, and (d) GdS0.82
42

Figure 3.6 shows the optical cross-sectional microstructures of the sinteredcompact formed at 1773 K when the actual compositions were (a) GdS1.01, (b)GdS0.96, (c) GdS0.92, and (d) GdS0.82. The XRD results in Figure 3.3 indicate thatsingle-phase GdS was formed with these compositions. After polishing, a metallic
colored surface exhibiting metallic bonding was obtained. Similar to the XRD result,the -Gd2S3 phase was not found because it was completely replaced by the GdS
phase in all samples. The composition was determined as GdS0.94 for No. 3 and
GdS0.92 for No. 4 (Table 3.3). However, precipitates were formed along the grainboundaries and spherical precipitates were formed within the grains in the cases of
GdS0.92 in Figure 3.6(c) and GdS0.82 in Figure 3.6(d). The mean grain sizes were302 µm and 370 µm for GdS0.92 and GdS0.82, respectively, and the mean grain size
increased with an increasing Gd content.

43

Figure 3.7. Optical microstructures and SEM microstructures of the cross-sectionof the sintered compact formed at 1773 K when the actual compositions were (a, b)
GdS0.92, (c, d) GdS0.82, (e, f) GdS0.77

44

Figure 3.7 shows the optical and SEM cross-sectional microstructures of thesintered compacts formed at 1773 K when the actual compositions were (a, b)
GdS0.92, (c, d) GdS0.82, and (e, f) GdS0.77. The XRD results in Figure 3.3 confirm
that single-phase GdS was obtained (except at GdS0.77, for which the Gd content
was extremely high).

Additionally, based on point No. 5 of the GdS0.92 EDX analysis, thespherical precipitates observed within the grains (Figure 3.6(c)) were found to be
Gd with a large amount of dissolved S. They presented as a spherical shape because
they were grown from a liquid phase. Furthermore, O enrichment was detected atanalysis point No. 6 at the center of the spherical precipitates. The same
compositions were confirmed at points No. 7 of GdS0.82 and No. 10 of GdS0.77. For
each of these points, a composition region existed near the center of the Gd
precipitate in which a large amount of S was dissolved. Bases on this composition,
the precipitates may be a mixture of GdO and Gd in which a large amount of S is
dissolved. There is a possibility that gaseous GdO, generated according to Eq. (3.3),
condensed after remaining in the sintered compact, with liquid Gd gathering aroundthe condensed form.

The fine particles were likely covered by a GdO layer to a certain degree if
one of them began from the fixed O amount, which approximately ranged from 4%
to 14%. The layer was possibly linked coherently with the metallic surface [63].
The GdO/Gd precipitates were unable to rapidly interact with Gd2S3 grains; thus,
creating gradient zones that were enriched with different contents of S dissolved
around them.
45

EDX analysis was performed for points No. 8 to No. 11 of GdS0.77. Whenthe actual composition was GdS0.77, the maximum O content reached 2.61% owing
to the generation of Gd2O3, as shown in Figure 3.3. Gd precipitates, in which a large
amount of S was dissolved, together with precipitates around Gd2O3 which wereenriched with O, were observed with a surrounding matrix of GdS. Being
surrounded by the matrix, the particles oxidized only to GdO, whereas the metallic
particles located in the free intergranular spaces of the matrix could be oxidized to
Gd2O3 particles, which were found in the extended area of No. 11 for the GdO1.51
composition. This can best be explained by an O grain-boundary diffusion
mechanism during the sintering process.

Therefore, it is clear that when GdH2 was added, the unreacted excess
remained in the sintered compact as Gd2O3, Gd in which S is dissolved, and Oenriched Gd in which S is dissolved. When the amount of GdH2 added was
gradually decreased, Gd2O3 disappeared, and Gd in which S was dissolved and Oenriched Gd in which S was dissolved were present as precipitates in the grains andat the grain boundaries. Furthermore, as the amount of added GdH2 was decreased
to GdS0.96, the precipitates in the GdS matrix became smaller. When the amount of
GdH2 added was less than the stoichiometric composition, the presence of
unreacted Gd2S3 was confirmed.

46

Therefore, as a single-phase, according to XRD, GdS0.92 and GdS0.82
compacts became a heterophase through the presence of GdO/Gd and Gd2O3
particles, which could decrease the free carrier concentration of the compacts.
Sesquioxide was detected, even with XRD, in GdS0.77 in which the Gd content was
extremely high and the maximum O content reached 2.61%. Therefore, it isinteresting to consider the physical property changes of the compacts that have thismicroscopic phase inhomogeneity.

47

Table 3.3. Results of EDX point analysis at the positions shown in Figure 3.5, Figure 3.6, and Figure 3.7
EDX analysis point

Composition at EDX analysis point (at.%)Phases determined

S

Gd

O

S/Gd

No. 1

58.5

37.5

4.0

1.55

-Gd2S3

No. 2

57.6

39.0

3.5

1.47

-Gd2S3

No. 3

46.3

49.5

4.2

0.94

GdS0.94

No. 4

45.8

49.8

4.4

0.92

GdS0.92

No. 5

30.9

64.5

4.6

0.48

GdS0.48

No. 6

27.7

58.4

13.9

-

(GdS + GdO)

No. 7

25.9

53.5

20.6

-

(GdS + GdO)

No. 8

46.8

50.8

2.4

1.24

GdS0.92

No. 9

35.9

60.7

3.4

0.59

GdS0.59

No. 10

29.0

56.7

14.2

-

(GdS + GdO)

No. 11

1.95

38.98

59.07

1.51 (O/Gd)

GdO1.5148

3.1.2 HoS
a. Synthesis of HoSx compacts

Figure 3.8 Images of the graphite punch and die after sintering at 1973 K with theheating rates of (a) 5, (b) 10, and (c) 25 K/min

49

Figure 3.8 shows a graphite punch and die containing the compact with an
initial S/Ho ratio of 1, after sintering at 1973 K with the heating rates of 5, 10, and25 K/min. The sintered compacts had melted and leaked out of the die at the 10 and
25 K/min heating rates. However, only a gas phase had condensed on the die lid atthe 5 K/min heating rate. The golden color and the metallic luster of the condensate
allowed for their identification as the HoO was being deposited [42], [55], [64]. It
is presumed that in fast heating (10 and 25 K/min), the liquid Ho2S3 reacts with theresidual O before HoS is formed, resulting in the formation of Ho2O2S, which hasa relatively low melting point and leaked out. Slow heating (5 K/min) was favorable
to both processes, namely, the Ho2S3 and Ho interacted with crystallization fromthe solid HoS melt and the vapor of the HoO in a solid state. Because this heating
rate was best suited to the synthesis of pure (O-free) HoS, it was further used in theexperiment.

50

Figure 3.9 Images of the sintered compacts at sintering temperatures of (a) 1773, (b) 1873 and (c) 1973 K
when the preparation ratio of the raw materials is HoS1.0

51

Figure 3.10 XRD patterns of the sintered compact surface in Figure 3.952Figure 3.9 shows the surface of the polished compacts when the preparation
ratio of the Ho metal was fixed at 27.82% of the mass (HoS1.00) and sintered at 1773
K (a), 1873 K (b), and 1973 K (c). Gray-colored impurity phases were found in thecompact sintered at 1773 K. The impurity phase area decreased with a higher
sintering temperature. The gray color of this impurity indicated that it was the
Ho2O2S phase. This visual observation was confirmed by the XRD patterns inFigure 3.10. The intensity of the Ho2O2S peaks was detectable for the compacts
sintered at the two lower temperatures. These peaks disappeared at the 1973 K
sintering temperature. The Ho2O2S disappearance was caused by the reaction
between S and Ho2O2S, which contributed to the Ho2S3 production through areaction similar to that in the GdS synthesis [53]. The occurrence of S in suchconditions was dictated by the dissociative mechanism of the solid HoS phaseaccording to Ref. [20]. In a previous study, we established a Gd–S–O diagram close
to 2000 K [53], which was applied as a basis to demonstrate that the HoS phase in
our experiment might also be formed from Ho2O2S and S because the sintering
temperature at 1973 K was appropriate for ensuring the required values of log PS2
and log PO2.

53

Figure 3.11 Powder XRD patterns of the HoSx compacts sintered at 1973 K withvarious preparation ratios (Table 3.4)

54

Figure 3.11 shows the powder XRD patterns of the HoSx compacts sintered at 1973
K with various preparation ratios of raw materials (Table 3.4). A small amount of
Ho2O2S was detected when the preparation ratio Ho2S3/Ho was equal to 1: 2.0(HoS0.75) or 1: 2.4 (HoS0.68). By contrast, no impurity phase was detected in theother samples, indicating that the obtained sintered compacts were a single phase.
The Ho2O2S phase formation was obtained with the Ho metal excess. The Ho2O2S
existence depended on the amount of Ho metal covered by the oxide film Ho2O3,
even if the Ho2O3 phase remained as in our previous study [53].
Figure 3.12 depicts the color change of the compact surface (a) before and(b) after air exposure for 14 d, with the preparation ratio of the Ho metal fixed at
21.25 mass% (HoS1.10) and the sintering temperature fixed at 1973 K. The sinteredcompact surface before air exposure exhibited a gold color. It turned to bright pink
after air exposure for 14 d, which was a close match to the color of Ho2O3 oxide.

55

Figure 3.12 Surface change of the preparation ratio of the HoS1.10 compact sintered at 1973 K (a) before and (b) after air exposure

56

Figure 3.13 XRD patterns of the sintered compact surface in Figure 5 (a) before and (b) after air exposure for 14 days

57

The compact surface transformation was also analyzed by XRD and Figure
3.13 illustrates the obtained results. Ho2O2S and Ho2O3 were detected on thesintered compact surface after air exposure for 14 d, clearly demonstrating that theformation of these oxides causes a color change. In air, the initial HoO condensate
was transformed into Ho2O3 in a process that was very slow to proceed [54].
However, oxidation did not proceed inside the sintered compacts. The surface
demonstrated a yellow color after light polishing, which was typical of the compact.
Therefore, after fabrication, the sintered compacts were immediately placed in a
vacuum desiccator and polished again before the microscopic and electricalmeasurements.

58

Table 3.4 Preparation ratios of the raw materials and the actual ratios determined from the chemical analysis for HoSx sintered at 1973 K
Preparation ratio (at.)

Molar ratio Ho2S3/Ho

Ho (wt.%)

1:0.5

16.16

45.45

54.55

1:0.7

21.25

47.37

1:1.0

27.82

1:1.2

Ho

HoSxi

S

HoS1.20

45.43

54.1

0.46

HoS1.19

52.63

HoS1.10

47.07

52.3

0.59

HoS1.11

50.00

50.00

HoS1.00

48.47

51.1

0.42

HoS1.05

31.62

51.61

48.39

HoS0.94

49.42

50.1

0.43

HoS1.01

1:1.6

38.15

54.55

45.45

HoS0.83

50.00

49.7

0.31

HoS0.99

1:2.0

43.54

57.14

42.86

HoS0.75

50.52

47.8

1.64

HoS0.94

1:2.4

48.06

59.46

40.54

HoS0.68

52.55

45.1

2.31

HoS0.85

Chemical formula calculated from the preparation ratioii

Chemical formula calculated from the actual ratio

59

O

HoSxii

Ho

I

S

Actual ratio (at.)Table 3.4 shows the quantitative variation of the initial S/Ho ratio causedby the losses through evaporation when the actual Ho content in the sinteredcompacts was determined by ICP-AES, while O was found by a specific gas
analysis. The difference between the preparation and the actual values was obvious,
especially for the samples with large Ho metal additions. The increase of Ho in theinitial mixtures was inconsistent with the actual amount of Ho in the sinteredcompact. This result indicates that during sintering, Ho evaporated, and other gasphase processes occurred. On the contrary, most of the O content increased
according to the Ho addition. However, with the preparation ratio of HoS1.00 to
HoS0.83, the O content decreased conversely in the sintered compacts. Meanwhile,
the actual ratio for the analyzed HoS1.05 to HoS0.99 was close to the stoichiometriccomposition. This can be explained by HoO evaporation during the sintering
process. Figure 3.8(a) clearly depicts this result. As specified in the preparation of
the CeS and GdS sintered compacts [53], [55], Ce2O2S and Gd2O2S can react withmetallic additions and release volatile CeO and GdO species. In this study, the same
reaction, according to Eq. (3.4), was quite realistic and resulted in a lowering of the
O content of the sintered compact according to the data in Table 3.4. When Ho
evaporates, similar to HoO, additional Ho metal should be added to the preparation
ratio to produce stoichiometric HoS. However, if there is excess Ho, the Ho2O3
amount will be increased according to reaction (3.5).

Ho2O2S + Ho = HoS + 2HoO

(3.4)

3Ho2O2S(s) + Ho(s) = 3HoS + 2Ho2O3(3.5)

b. Lattice constant and density
60

Figure 3.14 Relationship between the actual ratio and the lattice constant of HoSx
Figure 3.14 shows the relationship between the actual ratio and the lattice
constants of HoSx sintered at 1973 K with values known from the literature [21],[23], [65], including the JCPDS 01-089-289 data. The lattice constant behavior was
in a good agreement with those found in the literature. A linear increase in the lattice
constant was confirmed with an increase of S in the actual ratio range from HoS0.85
to HoS1.01. A similar tendency was confirmed for the monosulfides of Gd, Nd, Pr,
Ce, and La. Furthermore, the maximum composition range of the stoichiometriccomposition became narrower with light RE. Additionally, the lattice parameters in
the range from HoS1.05 to HoS1.19 gradually decreased with the increase of S, which
was also consistent with the results of Ref. [65].

Table 3.5. Apparent density of the sintered compacts formed at 1973 K
ActualDensity dexp.

Density dcal.

61

Relative density

ratio(g/cm3)(g/cm3)(%)

HoS1.19

6.82

6.99

0.98

HoS1.11

7.07

7.41

0.95

HoS1.05

7.29

7.77

0.94

HoS1.01

7.06

8.05

0.91

HoS0.99

7.44

8.05

0.92

HoS0.94

7.81

8.00

0.98

HoS0.85

7.93

7.91

1.00

Table 3.5 lists the apparent density of the sintered compacts formed at 1973 K.
The nature and extension of the solid solutions for HoS were established by acorrelation of the experimental density with a theoretically calculated one after
transforming the actual S/Ho ratios in the real chemical formula based on the wellknown model with vacancies in the metal and sulfur sublattices (Table 3.5) [21],[22], [24]. Most of the specimens were determined as a single phase by XRD,
except for HoS0.94 and HoS0.85, of which a small amount of oxysulfide detected asa Ho2O2S impurity (8.07 g/cm3) may slightly increase the experimental density.
Therefore, the relative density was 98% and 100% for HoS0.94 and HoS0.85,
respectively, while 91–96% of the values were observed for the single-phasecompacts.

62

c. Microstructures and EDX analysis

Figure 3.15. Optical microstructures of the cross-sections of the sintered compact
formed at 1973 K, where the actual ratios are (a) HoS1.19, (b) HoS1.11, (c) HoS1.01,(d) HoS0.99, (e) HoS0.94, and (f) HoS0.85

63

Figure 3.15 shows the optical cross-sectional microstructures of the HoSx
sintered compact formed at 1973 K, where the actual ratios were (a) HoS1.19, (b)HoS1.11, (c) HoS1.01, (d) HoS0.99, (e) HoS0.94, and (f) HoS0.85. Figure 3.15(a) showsthat the surface of the sintered HoS1.19 compact was homogenous without any
precipitation even on the microscopic level. For HoS1.11, HoS1.01, and HoS0.99, in
Figure 3.15(b–d), some nano-sized precipitates were observed at the grainboundaries, which were not present on the XRD patterns in Figure 3.11. A welldefined two-phase microstructure was observed for HoS0.85, where large
precipitates were located between matrix grains. These Ho2O2S impurities, whose
formation became possible because of the greatest amount of initial Ho
incorporated into the sample.

Having an equiaxed grain structure, the average grain size for the HoS1.19,
HoS1.11, and HoS1.01 compacts were determined as 184, 147, and 133 µm,
respectively. The grain size decreased when the preparation ratio of the Ho metal
increased. HoS0.99 comprised a mixture of coarse grains (162 µm) and fine grains.
At the same time, the microstructures demonstrated dense grain packing without an
extended interface. This is an advantage of liquid-phase sintering at 1973 K, where
the mutual dissolution of Ho and δ-Ho2S3 provides crystallization and intergrowthof the HoS grains. Numerous line defects were observed on the crystal faces, where
no precipitates were found.

64

Table 3.6. Apparent density of the sintered compacts formed at 1973 K
Area

Chemical compositions (at.)Phases determined

Ho

S

O

S/Ho

A1

45.43

51.22

3.34

1.13

HoS1.13

A2

46.19

50.26

3.55

1.09

HoS1.09

A3

47.94

49.98

2.07

1.04

HoS1.04

A4

48.81

48.78

2.40

1.00

HoS

A5

48.55

49.16

2.28

1.01

HoS1.01

A6

49.26

48.67

2.07

1.01

HoS1.01

A7

48.97

49.06

1.97

1.00

HoS

A8

50.48

47.80

1.71

0.95

HoS0.95

A9

52.55

43.41

4.04

0.83

HoS0.83

A10

45.50

21.57

32.92

-

Ho2O2STo determine the correlation between the color and the actual compositionof the HoSx compacts, the SEM/EDX analysis of the polished samples was
performed using an optical microscope. In the areas marked as A1-A10, all threecomponents were determined for the presentation of the actual chemical formula ofthe compacts. The impurity phase inclusion was also reliably identified andpresented in Table 3.6. The atomic compositions of the matrix, determined by EDX,
were consistent with those determined by the ICPAES analysis (Table 3.4). The
analysis also presented precipitates which were observed in the actual ratio of
HoS0.85 to Ho2O2S. This result agreed with the XRD measurements in Figure 3.11.

65

3.2 Electrical properties
3.2.1 Electrical properties of GdS

Figure 3.17 shows the dependence of the preparation composition (actual
composition) on the electrical resistivity (ρ) at room temperature of the sinteredcompacts formed at a 1773 K. The behavior of  is similar to the previously
reported behavior of GdSx with 0.68 ≤ x ≤ 1 [19]. It has also been reported that theelectrical conductivity depends strongly on the x value owing to free carrier
localization and additional electron scattering at lattice defects [19].

66Figure 3.16. Dependence of actual composition on electrical conductivity (σ) atroom temperature of sintered compacts formed at a sintering temperature of 1773
K

67

Figure 3.17. Temperature dependence of the electrical resistivity of the sinteredcompacts shown in Figure 3.1

68Figure 3.18 shows the temperature dependence of electrical resistivity of thesintered compacts shown in Figure 3.1(b), and those reported in the literature [66].
It can be seen that the Néel temperature of GdS is approximately 56.6 K [67]. Thus,
when the actual compositions were GdS1.11 and GdS1.21, the electrical resistivity
showed a semiconductor behavior at temperatures of 56.6 K or less. In contrast, theelectrical resistivity showed metallic behavior at temperatures above 56.6 K. Whenthe actual composition was in the GdS0.77 to GdS1.01 range, the electrical resistance
had a positive temperature coefficient and thus showed metallic behavior.

Herein, an abnormal behavior of the resistivity temperature dependencies
for GdSx with an excess of Gd was detected, which was the resistivity of GdS0.77,
GdS0.82, and GdS0.92 that were located closer to the GdS1.11 compact than to GdS0.96.
The correlation between the abnormal and O-induced defects was caused by thevalues of 2.61, 1.21, and 0.97 at.% for GdS0.77 GdS0.82, and GdS0.92 compacts,
respectively, and imperfections in the microscopic phase with the low-dimensional
GdO/Gd and Gd2O3 particles. Moreover, the GdS1.01 and GdS0.96 compacts were
free from such defect types, demonstrating the lowest resistivity. This suggests thatmicroscopic O-induced imperfections complicate the correct interpretation of theelectric properties of the GdSx phase purity only through XRD; therefore,
microscopy is an indispensable analysis technique to study the actual RES
characteristics.

3.2.2 Electrical properties of HoS
69

Figure 3.18 Relationships between the electrical conductivity (σ) measured atroom temperature and the actual composition of the HoSx compacts sintered at1973 K70

Figure 3.18 shows the relationships between the S content (x) of the actual
ratios and the electrical conductivity (σ) of the HoSx sintered at 1973 K andmeasured at room temperature. The conductivity exhibited a maximum when x was
close to the stoichiometric sample, namely HoS0.99. A similar tendency was also
reported in the isostructural compounds of PrSx, NdSx, and LaSx in Figure 3.18 [19].
Our maximum value for HoS0.99 was lower than that for PrS and ErS, but higher
than that obtained previously for HoS [21].

Figure 3.19 Temperature dependence of the electrical resistivity (ρ) versus theactual composition of the HoSx compacts sintered at 1973 K

71

Figure 3.19 presents the temperature dependence of the electrical resistivity(ρ) of HoSx as the actual ratio of compacts sintered at 1973 K. All HoSx showed
metallic behaviors down to 50 K. However, below 20 K, a semiconducting-likeupturn was observed, except for HoS0.99. A similar behavior was also observed in
the nonstoichiometric GdSx [53], which was explained by some latticeimperfections arising from the partial oxidization of the grains and the S defects.
For HoS0.99, whose chemical composition was the closest to stoichiometric, theupturn of ρ was not observed. Meanwhile, ρ decreased with the decreasing
temperature below the Néel temperature of 21 K. Therefore, HoS0.99 and HoS1.19
presented the lowest and highest ρ, respectively. The difference is easily explainedby focusing attention on their microstructures.

72

3.3 Melting point measurement
3.3.1 Melting point of GdS

Figure 3.20. Heating curves (a) and liquidus line in the homogeneous range (b)for the compacts sintered at 1773 K
73

Figure 3.20 shows the high-temperature thermal analysis results fordetermining the melting point of GdSx sintered at 1773 K. As shown, in thistemperature range, curves with either one or two peaks were observed. It is clearthat the former sintered compacts were single-phase GdSx, whereas the latter were
multiphases of GdSx and an impurity phase. In the case of GdS1.21, the peak at 2200
± 40 K coincided with the melting point of Gd2S3, which has been reported as 2160
[40] and 2180 K [68]. For GdS0.77, a peak at approximately 2650 K coincided withthe melting point of Gd2O3, which has been reported as 2595 K [68]. Figure 3.20(b)shows the relationship between the actual composition and the melting point in thecomposition range in which formation of a single phase was also confirmed in
Figure 3.20(a). The melting points of GdS0.82, GdS0.96, GdS1.01, and GdS1.11 were2593, 2623, 2633, and 2628 K, respectively.

74

3.3.2 Melting point of HoS

Figure 3.21 Melting points versus actual compositions of the HoSx compacts
sintered at 1973 K

75

Figure 3.21 depicts the melting point of HoSx together with the data of other
RES taken from Refs. [53], [69], [70]. The melting point increased as x approached
the stoichiometric composition of 1.00. Within the experimental error, 30 K, themelting points of HoS1.19, HoS1.11, and HoS1.05 were 2530, 2548, and 2583,
respectively. Being close to stoichiometry, both compacts with HoS1.01 and HoS0.99
showed the same highest melting point value of 2608 ± 30 K. This similar tendency
was found in the isostructural compounds of CeS [69], LaS [70], and DyS [70].

76Chapter 4 Conclusions
Large and dense homogeneous non-stoichiometric GdSx (HoSx) was
successfully synthesized by the reaction sintering of Gd2S3 and GdH2 (Ho2S3 andHo) using SPS.

Additionally, the effects of metal Gd (or Ho) addition within thehomogeneity range of the GdS (or HoS) phase on the lattice constants,
microstructure, melting point, and electrical resistivity were studied. The lattice
constant and the melting temperature gradually increased when the composition
was close to the stoichiometric composition. The electrical conductivity also
increased when the actual composition was close to stoichiometric. In contrast,
when the composition was far from stoichiometric, a semiconducting like-behavior
appeared at the magnetic ordering temperature.
The developed synthesis process could be applied to synthesize other RES.
Additionally, this study demonstrated two practically important innovations thathave not been reported previously, namely, a methodology that describes the
preparation of stoichiometric compacts enriched by RE metal with no or a minimum
content of impurities, and how to systematically identify the effect of defects on thechemical and physical properties in cases where they are quite rare.

77

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List of Publications
Journal Paper
1. Tran Nhu Bien, Shinji Hirai, Inga G. Vasilyeva, Ruslan Nikolaev, Chihiro
Sekine, Atsunori Kamegawa, “Study of non-stoichiometric GdSx (0.68 ≤ x ≤1.2) processed by reaction sintering,” J. Alloys Compd., vol. 831, p. 154691,
2020, doi: 10.1016/j.jallcom.2020.154691.
2.

Tran Nhu Bien, Shinji Hirai, Inga G. Vasilyeva, Ruslan Nikolaev, Chihiro
Sekine, Atsunori Kamegawa, Kazuhei Wakiya, Yukihiro Kawamura“Composition and microstructure of holmium monosulfide compacts
processed by reaction sintering,” J. Alloys Compd., Available online 10
November, 2020, doi: 10.1016/j.jallcom.2020.157872.

3. Nikolaev R.E., Sulyaeva V.S., Alekseev A.V., Sukhikh A.S., Polyakova
E.V., Pomelova T. A., Kuzuya T., Hirai S., Tran Nhu B., Growth
Mechanism of Helical γ-Dy2S3 Single Crystals, CrystEngComm, 2021, doi:
10.1039/D0CE01750G.
International Proceedings:
1. Tran Nhu Bien, Shinji Hirai et. al, “Synthesis of GdSx (0.68 ≤ x ≤ 1.2) byReaction Sintering in Pulse Electric Current Sintering Method and Its
Properties,” The 13 Pacific Rim Conference of Ceramic Societies, October27, 2019 (Poster), Okinawa, Japan.
Proceedings of Local Presentation:
1. Tran Nhu Bien, Shinji Hirai et. al, “Synthesis of Light Rare Earth
Sesquisulphides Utilizing Phase Transformation Induced by the Addition of
Titanium”,

Muroran-IT

Rare

Earth

Workshop,

October

2017,

Kasumigaseki-Tokyo, Japan
2.

Tran Nhu Bien, Shinji Hirai et. al, “Preparation of GdS by Reaction
Sintering in Pulse Electric Current Sintering Method”, Muroran-IT Rare
Earth Workshop, November 2018, Toyako-Hokkaido, Japan86