Journal of Nuclear and Radiochemical Sciences, Vol. 21, pp. 15-27, 2021

ArticlesPhase transformation of mixed lanthanide oxides in an aqueous solution
Md. Moniruzzaman*1, Taishi Kobayashi*2 , Takayuki Sasaki
Department of Nuclear Engineering, Kyoto University, Kyotodaigaku-katsura, Nishikyo-ku, Kyoto 615-8540,
Japan
Received March 15, 2021; Accepted May 10, 2021; Published online May 26, 2021
Binary and ternary mixed lanthanide oxides including (La,Nd)2O3, (La,Eu)2O3, (Eu,Tm)2O3, (La,Nd,Eu)2O3, and(La,Eu,Tm)2O3 were synthesized at 1000 °C for 4 h, followed by characterization using X-ray diffraction (XRD)
techniques before and after contact with an aqueous solution at neutral pH and 60 °C for 4 weeks. The XRD patterns of the mixed oxides confirmed a tendency to form oxide or hydroxide solid solutions depending on theirmixing ratios. A complete solid solution of (La,Nd)2O3 underwent transformation to (La,Nd)(OH)3, while theseparated La2O3 + Nd 2O3 were converted to La(OH)3 + Nd(OH)3. A similar trend was observed for (La,Eu)2O3,
while (Eu,Tm)2O3 resulted in separate Eu(OH)3 and Tm 2O3 after contact with an aqueous solution because of theresistance of Tm 2O3 to the hydration reaction. The formation of ternary oxide solid solutions and their transformation in an aqueous solution could be satisfactorily explained by the trends observed for binary mixed lanthanides.

1. Introduction
The thermodynamic properties of actinide and lanthanide
elements play important roles in the context of nuclear technology. In particular, lanthanide fission products are known toaccumulate in the nuclear fuel and exist as oxides that may
form a solid solution with a UO2 matrix.1,2 For safe treatment
and subsequent disposal of spent nuclear fuel, the solubility ofsuch UO2-based oxide solid solutions is a key factor to reliably
predict the migration behavior of radionuclides under disposal
conditions. Although a number of studies have investigated
the structure, stability, and solubility behavior of these multicomponent solid phases, we focused on exploring the thermodynamic properties of single-, binary-, and ternary-component
solid phases to better understand those of multi-component
solid phases. Since decay heat of the radioactive waste istransferred to groundwater aquifers through surrounding engineered and geological barrier systems in the repositories,3,4 thepresent study investigated binary and ternary mixed lanthanide oxides of the (L1,L2)2O3 and (L1,L2,L3)2O3 type after aging
at an elevated temperature to define their structures and stabilities in aqueous systems.
Numerous studies have been conducted that examined thestructure, stability, and solubility behavior of pure lanthanideoxides, as described in comprehensive reviews of literaturedata.5,6 Moreover, many reports detailed the synthetic methods for the preparation of binary mixed lanthanide oxides,
including their solid solutions, and investigated their structures for the development of functional materials.7–16 The ionicradius of lanthanides plays an important role in the formationof mixed lanthanide oxides. Heiba et al. synthesized (Er1-u
Gd u)2O3 powder samples with various mixing proportions of
Gd and Er and confirmed the formation of complete solid solutions for all mixing ratios.9 On the other hand, Hirsch et al.
showed the for mation of separated solid solutions for
Nd 2O3:Lu 2O3 synthesized at 1350–1500 °C, namely, a Lu- andNd-dominated solid solution with a cubic (Lu1-xNd x O3) andhexagonal (Nd1-xLu xO3) structure, respectively.10 In contrast to*1Corresponding author 1: Email: mohammad.moniruzzaman.23c@st.
kyoto-u.ac.jp*2Corresponding author 2: Email: kobayashi@nucleng.kyoto-u.ac.jp

the abundant knowledge on the structures of synthetic mixed
lanthanide oxides, no studies have investigated their solid
phase transformation occurring upon contacting aqueous solutions. For a comprehensive model of the radionuclide migration behavior during radioactive waste management, thethermodynamic properties of lanthanides in aqueous systems
need to be clarified.17,18
Recently, we’ve examined pure oxides of light, medium,and heavy lanthanides (La2O3, Eu 2O3, and Tm 2O3) after being
in contact with aqueous sample solutions, and revealed that
La2O3 was converted to La(OH)3 under neutral to alkaline pH
conditions at 25 °C.19 On the other hand, Eu 2O3 remained stable at 25 °C and underwent transformation to Eu(OH)3 only
after aging at 90 °C, while Tm 2O3 was stable even after aging
at 90 °C.19 This study deals with binary and ternary mixed
lanthanide oxides before and after contact with an aqueous
solution. Since the solid phase transformation tendency upon
contacting aqueous solutions differs from lighter to heavier
lanthanides, binary [(La,Nd)2O3, (La,Eu)2O3, and (Eu,Tm)2O3]
and ternary [(La,Nd,Eu)2O3 and (La,Eu,Tm)2O3] systems wereselected and synthesized at various mixing ratios. The mixed
oxides were then immersed in an aqueous solution within theneutral pH region at 60 °C for 4 weeks. The aging temperature of 60 °C was used as a moderate elevated temperature
condition in the repository due to the decay heat emission
from the radioactive waste.3 The solid phases were examined
before and after immersion by powder X-ray diffraction(XRD), and the phase transformation of mixed lanthanideoxides was analyzed with the aid of the obtained lattice parameters.
2. Experimental
2.1 Materials. Reagent grade sodium perchlorate monohydrate (NaClO4·H2O, 98%), nitric acid (HNO3, 60 %), perchloric acid (HClO4, 60%), sodium hydroxide (NaOH, 97%), andpolyvinyl alcohol (PVA, 3,500, 78% hydrolyzed) were purchased from Wako Pure Chem. HEPES (C8H18N2O4S, 99.0%,
Dojindo) was used to adjust the pH of the sample solutions to aneutral pH region. Lanthanum oxide (La 2O3, 99.99%), neodymium oxide (Nd 2 O 3 , 99.9%), europium oxide (Eu 2 O 3 ,99.9%), and thulium oxide (Tm 2O3, 99.9%) were purchased© 2021 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 05/26/2021

J. Nucl. Radiochem. Sci., Vol. 21, 2021

from Wako Pure Chem. Deionized purified water (Milli-Q,
Millipore) was used to prepare all the solutions. The synthesisof the solid phases and preparation of the sample solutions
were performed under atmospheric pressure at 25 °C. The pHcvalues of the sample solutions were measured using a pH
meter (D-72, Horiba Ltd.) with a combined glass electrode(9615-10D, Horiba Ltd.). The reference electrode was filledwith 3.6 M NaCl and 0.41 M NaClO 4 (Wako Pure Chem.)
instead of 3.3 M KCl to prevent precipitation of KClO4 at thejunction of the electrode and sample solutions. The electrode
was calibrated against standard HClO 4 and NaOH solutions(pHc 1, 2, 3, 11, 12, and 13; Wako Pure Chem.) at I = 0.1 M
using NaClO4 at 25 °C to correct the experimentally measured
pHexp to the pHc values.
2.2 Synthesis of binary and ternary mixed oxide. Threetypes of binary mixed oxides, i.e., (La,Nd)2O3, (La,Eu)2O3, and(Eu,Tm) 2 O3, and two types of ternary mixed oxides, i.e.,(La,Nd,Eu)2O3 and (La,Eu,Tm)2O3, were synthesized by usinga polymeric steric entrapment method. 20,21 An appropriate
amount of PVA was dissolved in Milli-Q water to obtain a 5
wt% PVA stock solution. Ln (Ln = La, Nd, Eu, and Tm)
nitrate stock solutions with a concentration of [Ln] = 0.3 M
were prepared by dissolving La 2 O 3 , Nd 2 O 3 , Eu 2 O 3 , or
Tm 2O3(cr) in nitric acid. For the preparation of (La,Nd)2O3,
aliquots of La and Nd stock solutions were mixed at molar
ratios of La:Nd = 1:9, 3:7, 4:6, 5:5, 7:3, and 9:1, and an equivalent molar amount of PVA stock solution was then added to themixed Ln solutions. The resulting solutions were stirred for 1
h using a magnetic stirrer and heated at 200 °C for approximately 2 h. After drying, the solids were crushed in a mortar,and further heated at 1000 °C for 4 h using a muffle furnace(FO-100, Yamato Scientific Co.). Mixed oxides (La,Eu)2O3
and (Eu,Tm)2O3 were prepared in a similar manner. As reference, oxide mixtures including La 2O3+Nd 2O3, La 2O3+Eu 2O3,
and Eu 2O3+Tm 2O3 were prepared without heating by mixing
each oxide at the same weight ratio and grinding for 10 min.
For the preparation of (La,Nd,Eu)2O3, aliquots of La, Nd,
and Eu stock solutions were mixed at molar ratios of La:Nd:Eu= 4.75:4.75:0.5, 4.5:4.5:1.0, and 3.5:3.5:3.0. An equivalent molar
amount of PVA stock solution was then added to the mixed
solutions, and the ternary mixed oxides were obtained using asimilar procedure as that used for the synthesis of binary
mixed oxides. For the preparation of (La,Eu,Tm)2O3, the molar
ratios of La, Eu, and Tm stock solutions were 0.5:4.75:4.75,
1.0:4.5:4:5, 3.0:3.5:3.5, and 4.5:4.5:1.0. For comparison, mixt ures of pure oxides such as La 2 O 3 +Nd 2 O 3 +Eu 2 O 3 andLa2O3+Eu2O3+Tm2O3 were also prepared without heating.
The synthesized mixed oxides and prepared mixtures ofpure oxides were analyzed using XRD (Miniflex, RIGAKU)
with Cu-Kα (λ = 0.154 nm) in the scattering angle range of 2θ
= 10–60° at a scan rate of 5°/min. To determine the chemicalcomposition of the solid phases, small portions of the selected
solid phases were completely dissolved in concentrated nitricacid, and the Ln concentration was determined by ICP-MS(ELAN DRC II, PerkinElmer) to calculate the molar ratio of
Ln included in each solid phase. The chemical composition of
the solid phases agreed well with the mixing ratio of the stock
solutions used for sample preparation.

Moniruzzaman & Kobayashi
of the sample bottles were tightly closed, and the sample bottles were placed in a temperature chamber (ETTAS E0450B,
AS ONE co.) at the controlled aging temperature (Tage) of
60 °C. The bottles were occasionally shaken by hand for a few
minutes during aging up to 4 weeks. After aging at 60 °C, thesample bottles were removed from the temperature chamberand cooled to 25 °C for a few days. The pHc of the sample
solutions was within the 8.2 ± 0.3 range, and the solid phases
were separated by centrifugation at 5,000 rpm (H-103N,
Kokusan) for 5 min and dried in a vacuum desiccator. The
diffraction patterns of the solid phases were recorded by XRD.
3. Results and discussion
3.1 Characterization of the synthesized binary and ternary mixed oxides. Figure 1 shows the XRD patterns of thesynthesized (La,Nd) 2O3 mixed oxides at different mixing
ratios of La and Nd stock solutions ranging from La:Nd = 9:1
to 1:9 during sample preparation, together with those of pure
La2O3 and Nd2O3. As observed in the XRD patterns, the peaks
corresponding to La2O3 slightly shifted towards the right sidewith increasing mixing ratio of Nd to pure Nd 2O3. It is notethat the observed peak positions of Nd2O3 were slightly higher
than those of the reference (Nd 2O3: ICSD No. 26867). The
XRD pattern of the 5:5 mixture of pure La2O3 and Nd2O3 without any treatment is also presented in Figure 1. Separated
peaks were observed for La2O3 and Nd 2O3, indicating that theresolution of the present measurement was sufficient to distinguish the peaks of two different pure oxides. Therefore, it was
assumed that complete solid solutions of (La,Nd)2O3 wereformed using the polymeric steric entrapment method at1000 °C.
Figure 2 shows the XRD patterns of the synthesized(La,Eu)2O3 at different mixing ratios of La and Eu stock solutions ranging from La:Eu = 9:1 to 1:9, together with those ofpure La2O3, Eu2O3, and their mixture without heating. In contrast to (La,Nd)2O3, the peaks corresponding to both cubic andmonoclinic Eu 2O3 disappeared only at an La:Eu mixing ratioof 9:1, and separately appeared at an La:Eu mixing ratio of 7:3
to pure Eu 2O3, suggesting that the solid solution limit of Eu to
La2O3 was between 10 and 30% in molar ratio under the investigated conditions. On the other hand, the solid solution limitof La to Eu 2O3 was considered to be less than 10% in molar

La2O3+Nd2O3
Nd2O3

Relative intensity

16

La:Nd=1:9
La:Nd=3:7
La:Nd=4:6
La:Nd=5:5
La:Nd=7:3
La:Nd=9:1
La2O3

2.3 Immersion experiment. For the immersion experiment, 80 mL of a 0.1 M NaClO 4 solution containing 0.01 M
HEPES as pH buffer was prepared in sample bottles, and the
pHc was adjusted to pH = 8.2 using 0.1 M HClO 4 and 0.1 M
NaOH. As an initial solid phase, approximately each 130 mgof synthesized oxide solid solution or mixture of pure oxides
was then placed in the sample bottles. The maximum Ln concentration would reach about 0.01 M in each bottle if the added
solid phase was completely dissolved. Subsequently, the caps

10

20

30

40

50

60

2 Cu-K
Figure 1. XRD patterns of the synthesized (La,Nd)2O3 at different
mixing ratios ranging from La:Nd =9:1 to 1:9, along with that of a
mixture of La2O3 and Nd 2O3 without heating. XRD patterns of pure
La2O3 and Nd 2O3 and their reference peak positions (●: La2O3: ICSD
No. 1010278, ■: Nd 2O3: ICSD No. 26867) are also presented for comparison.

J. Nucl. Radiochem. Sci., Vol. 21, 2021

Phase transformation of mixed lanthanide oxides in an aqueous solution

Eu2O3+Tm2O3

Eu2O3

Tm2O3

Relative intensity

La2O3+Eu2O3

Relative intensity

La:Eu=1:9
La:Eu=3:7
La:Eu=4:6

La:Eu=5:5

Eu:Tm=1:9
Eu:Tm=3:7
Eu:Tm=4:6
Eu:Tm=5:5
Eu:Tm=7:3

La:Eu=7:3

Eu:Tm=9:1

La:Eu=9:1

Eu2O3

La2O3

20

30
40
2 Cu-K

50

10

60

Figure 2. XRD patterns of the synthesized (La,Eu)2O3 at different
mixing ratios ranging from La:Eu = 9:1 to 1:9, along with that of a
mixture of La2O3 and Eu 2O3 without heating. XRD patterns of pure
La2O3 and Eu 2O3 and their reference peak positions (●: La2O3: ICSD
No. 1010278, ♦: Eu 2O3 (cubic): ICSD No. 27997, ▼: Eu 2O3 (monoclinic): ICSD No. 8066) are also presented for comparison.

ratio, as a small peak at 29.8° corresponding to La 2O3 wasalready found in (La,Eu)2O3 at a La:Eu mixing ratio of 1:9.
It was reported that Eu 2O3 exhibits a stable cubic phase up
to approximately 620 °C that transforms into a monoclinic
phase.22,23 The cubic phase observed in the XRD pattern ofpure Eu2O3 in the present study may have been formed during
furnace cooling after heating at 1000 °C. It is interesting tonote that diffraction peaks corresponding to monoclinic Eu2O3
were observed in the solid phases with La:Eu mixing ratios of
3:7, 4:6, and 5:5. In previous studies, the inclusion of anotherrare earth oxide such as Y2O3 to Eu 2O3 was observed to affectthe phase change temperature.23 The appearance of the monoclinic phase for (La,Eu)2O3 at certain mixing ratios shown in
Figure 2 can be attributed to the inclusion of La in the Eu 2O3phase.
The XRD patterns of the synthesized (Eu,Tm)2O3 at different mixing ratios of Eu and Tm stock solutions ranging from
Eu:Tm = 9:1 to 1:9 are shown in Figure 3, together with thoseof pure Eu 2O3, Tm 2O3, and their mixture without heating. At
an Eu:Tm mixing ratio of 9:1, the peaks corresponding to
Tm 2O3 were not observed in the corresponding XRD pattern,suggesting the formation of a (Eu,Tm)2O3 solid solution, whilea small shoulder peak was observed for Eu:Tm = 7:3. In contrast, the peaks of Eu2O3 did not appear at Eu:Tm = 1:9 and 3:7,
while the peaks corresponding to Tm 2 O3 slightly shifted
towards the left side, indicating that the solid solution limit of
Eu to Tm 2O3 was higher than that of Tm to Eu2O3. At Eu:Tm
ratios of 5:5 to 7:3, separated peaks corresponding to bothcubic Eu 2O3 and Tm 2O3 were observed in the diffraction patterns. As described above, the cubic phase of Eu 2O3 was considered to derive from its monoclinic phase during furnace
cooling, while the cubic phase of Tm 2O3 was stable up to
approximately 2000 °C.22 This may be the reason for the separation of the diffraction peaks of the Eu 2O3 and Tm 2O3 cubic
phases.
Figure 4 shows the XRD patterns of the synthesized(La,Nd,Eu) 2 O 3 at different La:Nd:Eu mixing ratios of
4.75:4.75:0.5, 4.5:4.5:1.0, and 3.5:3.5:3.0, along with a mixtureof pure oxides (La 2O3+Nd 2O3+Eu 2O3) without any heat treatment. For the binary systems, the synthesized (La,Nd)2O3 was
a complete solid solution, while (La,Eu)2O3 showed trigonal,
cubic, and monoclinic phases depending on the La and Eu

20

30
40
2 Cu-K

50

60

Figure 3. XRD patterns of the synthesized (Eu,Tm)2O3 at different
mixing ratios ranging from Eu:Tm = 9:1 to 1:9, along with that of a
mixture of Eu 2O3 and Tm 2O3 without heating. XRD patterns of pureEu 2O3 and Tm 2O3 and their reference peak positions (♦: Eu 2O3: ICSD
No. 27997, ▲: Tm 2O3: ICSD No. 33657) are also presented for comparison.

La2O3+ Nd2O3+
Eu2O3

Relative intensity

10

17

Eu2O3
La:Nd:Eu=
3.5:3.5:3.0
La:Nd:Eu=
4.5:4.5:1.0
La:Nd:Eu=
4.75:4.75:0.5
Nd2O3
La2O3

10

20

30
40
2 Cu-K

50

60

Figure 4. XRD patterns of the synthesized (La,Nd,Eu)2O3 at different mixing ratios as La:Nd:Eu = 4.75:4.75:0.5, 4.5:4.5:1.0 and3.5:3.5:3.0 along with that of a mixture of La2O3, Nd 2O3, and Eu 2O3
without heating. The reference peak positions (●: La2O3: ICSD No.
1010278, ■: Nd 2O3: ICSD No. 26867, ♦:Eu 2O3: ICSD No. 27997,▼:
Eu 2O3: ICSD No. 8056) are presented for comparison.

mixing ratios. In the case of ternary systems with a low Eu
mixing ratio such as La:Nd:Eu = 4.75:4.75:0.5 and 4.5:4.5:1.0, asingle phase corresponding to the trigonal phase was observed.
The intermediate peak positions between those of pure La2O3
and Nd 2O3 suggested the formation of a solid solution, which
agrees with the observation for the (La,Nd)2O3 binary system.
As shown in Figure 2 for (La,Eu) 2O3 binary systems, theinclusion of 30% Eu resulted in the appearance of Eu2O3 withcubic and monoclinic phases, which was also found in the
XR D patter ns of the ter nar y system with La:Nd:Eu =3.5:3.5:3.0.
Figure 5 shows the XRD patterns of the synthesized(La,Eu,Tm)2O3 at different mixing ratios including La:Eu:Tm =0.5:4.75:4.75, 1.0:4.5:4:5, 3.0:3.5:3.5, and 4.5:4.5:1.0, along witha mixture of pure oxides (La 2O3+Eu 2O3+Tm 2O3) without any
heat treatment. For the (Eu,Tm)2O3 binary system, separated

J. Nucl. Radiochem. Sci., Vol. 21, 2021

Moniruzzaman & Kobayashi

cubic phases occur red at a Eu:Tm mixing ratio of 5:5.
Similarly, peaks corresponding to cubic Eu 2O3 and Tm 2O3
were observed separately at all mixing ratios for the investigated ternary systems. The peaks corresponding to trigonal
La2O3 were not confirmed for lower La inclusions at La:Eu:Tm
mixing ratios of 0.5:4.75:4.75 and 1.0:4.5:4:5 as well as those
appearing at La:Eu:Tm = 3.0:3.5:3.5 and 4.5:4.5:1.0, althoughthese peaks were observed for 10% La inclusion in the binary(La,Eu)2O3 systems. At La:Eu:Tm = 3.0:3.5:3.5 and 4.5:4.5:1.0,
peaks corresponding to monoclinic Eu 2O3 were found, which

La2O3+ Eu2O3+
Tm2O3

Relative intensity

La2O3
La:Eu:Tm=
4.5:4.5:1.0
La:Eu:Tm=
3.0:3.5:3.5
La:Eu:Tm=
1.0:4.5:4.5
La:Eu:Tm=
0.5:4.75:4.75
Tm2O3
Eu2O3

10

40
30
2 Cu-K

20

50

60

Figure 5. XRD patterns of the synthesized (La,Eu,Tm)2O3 at different mixing ratios as La:Eu:Tm = 0.5:4.75:4.75, 1.0:4.5:4.5, 3.0:3.5:3.5
and 4.5:4.5:1.0 along with that of a mixture of La 2O3, Eu 2O3, andTm 2O3 without heating. The reference peak positions (●: La 2O3:ICSD No. 1010278, ♦: Eu 2O3: ICSD No. 27997, ▼: Eu 2O3 ICSD No.
8056, ▲: Tm 2O3: ICSD No. 33657) are presented for comparison.

11.1

(a)

La2O3 ICSD 1010278
Nd2O3 ICSD 26867
La2O3 this study
Nd2O3 this study

3.95
La:Eu:Tm =3.0:3.5:3.5

La:Eu:Tm =4.5:4.5:1.0

3.90
La:Nd:Eu =4.75.:4.75:0.50

3.85

La:Nd:Eu =La:Nd:Eu = 4.5:4.5:1.0
3.5:3.5:3.0(La,Nd)2O3(La,Eu)2O3(La,Nd,Eu)2O3(La,Eu,Tm)2O3

Eu2O3 ICSD 27997
Tm2O3 ICSD 33657
Eu2O3 this study
Tm2O3 this study

La:Eu:Tm =1.0:4.5:4.5

10.9
La:Eu:Tm =3.0:3.5:3.5

10.8
10.7

La:Eu:Tm =0.50:4.75:4.75

10.6

La:Eu:Tm =1.0:4.5:4.5
Tm2O3-based(Eu,Tm)2O3(La,Eu,Tm)2O3

10.5

1.0

0.8

0.8

0.6

0.4
in (La,Nd) 2O3
in (La,Eu) 2O3
in (La,Nd,Eu) 2O3
in (La,Eu,Tm) 2O3

0.2

0.0

0.6

0.4

Tm2O3-basedin (Eu,Tm) 2O3
in (La,Eu,Tm)2O3

0.2

0.4
0.6
La ratio

0.8

1.0

14.3

La:Nd:Eu =3.5:3.5:3.0

14.2

0.8

0.6

0.4

0.2
in (La,Eu,Tm) 2O3
in (La,Eu) 2O3
in (La,Nd,Eu) 2O3

0.0

0.0
0.0

La:Eu:Tm =4.5:4.5:1.0

14.4

14.1
1.0

Eu2O3-basedin (La,Eu) 2O3
in (Eu,Tm) 2O3
in (La,Eu,Tm)2O3

0.2

Eu2O3 ICSD 8056

(La,Eu)2O3(La,Nd,Eu)2O3(La,Eu,Tm)2O3

wt% of monoclinic phase

1.0

(c)

Eu2O3-based(La,Eu)2O3(Eu,Tm)2O3(La,Eu,Tm)2O3

10.4

wt% of cubic phase

wt% of trigonal phase

3.80

14.5

(b)

11.0
Lattice parameter, a (Å)

Lattice parameter, a (Å)

4.00

agreed with the observations for the (La,Eu)2O3 binary systemat La:Eu = 5:5. Thus, the trends of the XRD patterns for the(La,Nd,Eu)2O3 and (La,Eu,Tm)2O3 ternary solid phases could
be suitably explained based on the combination of those of thebinary systems.
For further analysis, the lattice parameters of the synthesized solid solutions were obtained from Rietveld refinement,
as summarized in Tables A1-A5 in the Appendix. Figure 6aillustrates the obtained lattice parameter a and weight fractionof the trigonal phase found in the binary and ternary systems
as a function of the La mixing ratio. As pure La2O3 and Nd2O3
possess the same crystal structure of the trigonal phase with a(P-3m1) space group, the replacement of Nd with La continuously increased the lattice parameter a, which could be attributed to the smaller radius of the Nd ion compared with that of
the La ion. The lattice parameter of the trigonal phase foundin (La,Eu) 2O3 slightly decreased compared to that of pure
La 2O3 with decreasing La mixing ratios. The low values
observed at La ratios of 0.3, 0.4, and 0.5 are possibly due to theappearance of the Eu2O3 monoclinic phase (C2/m) under these
conditions. As shown in Figure 6b, La hardly incorporated
into the Eu2O3 cubic phase (Ia-3), but it did into the monoclinic
phase. The La ratio of the (La,Eu)2O3 trigonal phase can therefore be expected to be lower than that of the mixing La ratio
during sample preparation. The lattice parameters of(La,Nd,Eu)2O3 plotted in Figure 6a agreed well with the trend
observed for (La,Nd)2O3, which was likely due to the limited
amount of Eu. In the case of the ternary (La,Eu,Tm)2O3 system, Eu incorporated into Tm 2O3, as shown in Figure 6b; consequently, the Eu ratio decreased, while the La ratio increasedin the (La,Eu)2O3 trigonal phase. This may compensate for thedecrease in the La ratio due to the appearance of the monoclinic phase.
The lattice parameter of the cubic phase (Ia-3) of (La,Eu)2O3
shown in Figure 6b was slightly larger than that of pure Eu2O3
at an Eu ratio of 0.9, while being almost constant at a lower Eu

Lattice parameter, a (Å)

18

0.0

0.2

0.4
0.6
Eu ratio

0.8

1.0

0.0

0.2

0.4
0.6
Eu ratio

0.8

1.0

Figure 6. Lattice parameter a (upper) and weight fraction (lower) of the synthesized (a) trigonal phase (b) cubic phase, and (c) monoclinic phaseof the binary and ternary systems as a function of the molar mixing ratio. Broken lines in the figures represent the eye guides to show the trend
for the binary systems.

J. Nucl. Radiochem. Sci., Vol. 21, 2021

Phase transformation of mixed lanthanide oxides in an aqueous solution

3.2 Characterization of the solid phases after contact
with an aqueous solution. The XRD patterns of the solid
phases after contact of the synthesized (La,Nd)2O3 with anaqueous sample solution at 60 °C for 4 weeks are presented in
Figure 7. The peaks corresponding to the (La,Nd)2O3 solid
solution shown in Figure 1 disappeared, and new peaks continuously shifted rightward in correspondence of mixing ratios
of La and Nd stock solutions varying from La:Nd = 9:1 to 1:9.
In the previous studies, it was described that pure La2O3 andNd 2O3 are converted to La(OH)3 and Nd(OH)3 upon contact
with aqueous sample solutions.18,19,24 By comparing the peak
positions in Figure 7 to those for La(OH)3 and Nd(OH)3, it was
assumed that the complete solid solution of (La,Nd)2O3 wastransformed into a complete solid solution of (La,Nd)(OH)3.
However, it is interesting to note that the mixture of La2O3 andNd 2O3 without any heat treatment resulted in the formation of
La(OH)3 and Nd(OH)3, respectively, as shown in Figure 7.
Figure 8 shows the XRD patterns of the solid phases after
contact of the synthesized (La,Eu)2O3 with an aqueous solution. Depending on the mixing ratios of the La and Eu stock
solutions, the synthesized (La,Eu)2O3 was found to occur as a
mixture of an La 2O3- and Eu 2O3-based solid solution before
contacting aqueous phase as shown in Figure 2. However,after aging in the sample solutions, the peaks corresponding toboth oxide-based solid solutions disappeared, and new peaks
continuously shifted rightward with the La:Eu ratio varying
from 9:1 to 1:9. It was therefore assumed that for the synthesized (La,Eu)2O3, the mixture of the two oxide-based solid
solutions was transformed to a complete solid solution of(La,Eu)(OH)3. In the pure system, Eu 2O3 remained stable inaqueous solution at 25 °C and was converted to Eu(OH)3 after
aging in aqueous solution at 90 °C.19 Figure 8 shows that the
mixture of La 2 O3 and Eu 2 O3 without any heat treatment
resulted in the formation of La(OH)3 and Eu(OH)3 after aging
at 60 °C. As some of the main peak positions of Eu 2O3 overlapped those of La(OH)3 and Eu(OH)3, the amount of remaining Eu 2O3 was determined in the Rietveld refinement andshown in the Table A2. The results suggested that Eu 2O3 waspartly transformed to Eu(OH)3 after aging at 60 °C. Based on
the reported thermodynamic constants, the solubility productof Eu(OH)3 was calculated to be slightly lower than that of

La2O3+Nd2O3

Relative intensity

La:Nd=1:9

La:Nd=3:7

La:Nd=4:6

La:Nd=5:5

La:Nd=7:3

La:Nd=9:1

10

20

30

40

50

60

2 Cu-K
Figure 7. XRD patterns of (La,Nd)2O3 after contacting the aqueous
sample solution at 60 °C for 4 weeks at different mixing ratios ranging from La:Nd = 9:1 to 1:9, along with that of a mixture of La 2O3
and Nd 2O3. The reference peak positions ○: La(OH)3 (ICSD No.
31584) and □: Nd(OH)3 (ICSD No. 398) are presented for comparison. The mixture of oxides was identified as La(OH)3+ Nd(OH)3after the aging period.

La2O3+ Eu2O3
La:Eu=1:9

Relative intensity

ratio. As observed in the XRD patterns, the inclusion of 10%
La exceeded the solid solution limit of La into Eu2O3, supporting the obser ved trend of the lattice parameters. For(Eu,Tm)2O3, two cubic phases, i.e., an Eu2O3- and Tm2O3-basedsolid solution, were observed. As shown in Figure 6b, the lattice parameters for both Eu2O3- and Tm2O3-based cubic phases
slightly increased with increasing Eu ratios. Upon heating at
1000 °C, Eu 2O3 can exist as a monoclinic phase and be converted to the cubic phase upon furnace cooling. In contrast toLa, the co-existing Tm seemed to have no significant effect onmaintaining the monoclinic phase, while potentially causing
some scattering in the increasing lattice parameter trend. The
lattice parameters of the Eu 2O3-based cubic phase of ternary(La,Eu,Tm)2O3 were plotted between those of (La,Eu)2O3 and(Eu,Tm)2O3, while those of the Tm 2O3-based cubic phase wererather close to those of (Eu,Tm)2O3. As shown in Figure 6c, Laseemed to be easily incorporated into the Eu 2O3 monoclinic
phase, which was converted to the Eu 2O3 cubic phase. This
could be why the lattice parameters of the Eu 2O3-based cubic
phase of (La,Eu,Tm)2O3 showed higher values than those of(Eu,Tm)2O3.
The lattice parameter a and weight fraction of the monoclinic phase (C2/m) are shown in Figugre 6c, respectively. As
discussed above, the lattice parameters of the binary and ternary systems were larger than those of the pure Eu2O3 monoclinic phase, suggesting a relatively good incorporation of La
into the Eu2O3 monoclinic phase.

19

La:Eu=3:7

La:Eu=4:6

La:Eu=5:5

La:Eu=7:3

La:Eu=9:1

10

20

30
40
2 Cu-K

50

60

Figure 8. XRD patterns of (La,Eu)2O3 after contacting the aqueous
sample solution at 60°C for 4 weeks at different mixing ratios ranging from La:Eu = 9:1 to 1:9, along with that of a mixture of La 2O3
and Eu 2O3. The reference peak positions ○: La(OH)3 ( ICSD No.
31584) , ◊: Eu(OH)3 (ICSD No. 200488) and ♦: Eu 2O3 (ICSD No.
27997) are presented as references. The mixture of oxides was identified as La(OH)3+Eu 2O3+Eu(OH)3 after the aging period.

Eu 2O3 at 25 to 90 °C, 5,6,18,19 indicating the transformation of
Eu2O3 to Eu(OH)3 proceeds thermodynamically via dissolution
and reprecipitation. However, in the present study weobserved that the solid phases after contact of (La,Eu)2O3 andLa 2O3+ Eu 2O3 with the aqueous solution were different fromeach other as shown in Figure 8. Therefore, the temperaturedependences of the transformation of Eu 2O3 to Eu(OH)3 may
not be related in the dissolution and reprecipitation processes.
Figure 9 shows the XRD patterns of the solid phases after
contact of the synthesized (Eu,Tm)2O3 with an aqueous solution. The Tm 2 O3 -based solid solution remained stable at
Eu:Tm ratios varying from 1:9 to 4:6, after aging in an aqueous
sample solution at 60 °C, and no peak due to Eu 2O3 or Eu(OH)3
was observed. The peaks corresponding to Eu(OH)3 appeared

20

J. Nucl. Radiochem. Sci., Vol. 21, 2021

Moniruzzaman & Kobayashi

La2O3+Eu2O3+Tm2O3

Eu2O3 + Tm2O3

La:Eu:Tm=
4.5:4.5:1.0

Relative intensity

Relative intensity

Eu:Tm=1:9

Eu:Tm=3:7
Eu:Tm=4:6

Eu:Tm=5:5

La:Eu:Tm=
3.0:3.5:3.5

La:Eu:Tm=
1.0:4.5:4.5

Eu:Tm=7:3

La:Eu:Tm=
0.5:4.75:4.75
Eu:Tm=9:1

10

20

30

40
2 Cu-K

50

60

La2O3+
Nd2O3+
Eu2O3

Relative intensity

La:Nd:Eu=
3.5:3.5:3.0

La:Nd:Eu=
4.5:4.5:1.0

La:Nd:Eu=
4.75:4.75:0.5

20

30

40

20

30

40

50

60

2 Cu-K

Figure 9. XRD patterns of (Eu,Tm)2O3 after contacting the aqueous
sample solution at 60°C for 4 weeks at different mixing ratios ranging from Eu:Tm = 9:1 to 1:9, along with that of a mixture of Eu 2O3
and Tm 2O3. The reference peak positions ◊: Eu(OH)3 (ICSD No.
200488), ♦: Eu 2O3 (ICSD No. 27997), and ▲: Tm 2O3 (ICSD No.
33657) are presented for comparison. The mixture of oxides was
identified as Eu 2O3+Eu(OH)3+Tm 2O3 after the aging period.

10

10

50

60

2 Cu-K
Figure 10. XRD patterns of (La,Nd,Eu)2O3 after contacting theaqueous sample solution at 60 °C for 4 weeks at different mixing
ratios as La:Nd:Eu = 4.75:4.75:0.5, 4.5:4.5:1.0 and 3.5:3.5:3.0 alongwith that of a mixture of La 2O3, Nd 2O3, and Eu 2O3 without heating.
XRD pattern for reference peak positions ○: La(OH)3 (ICSD No.
31584), □: Nd(OH)3 (ICSD No. 398), ◊: Eu(OH)3 (ICSD No. 200488),
and ♦: Eu 2O3 (ICSD No. 27997) are presented for comparison. The
m i x t u r e of ox ide s wa s ide nt i f ie d a s L a (OH ) 3 +Nd (OH ) 3 +Eu(OH)3+Eu 2O3 after the aging period.

at Eu:Tm = 5:5 to 9:1, supporting the transformation of Eu2O3
to Eu(OH)3. At a mixing ratio of Eu:Tm = 9:1, only the peaksof Eu(OH)3 were observed, suggesting that the incorporationof Tm into Eu(OH)3 took place. In Figure 9, the XRD patternof the solid phase after contact with a mixture of Eu 2O3 andTm 2O3 revealed that Eu 2O3 partly transformed to Eu(OH)3,
while Tm2O3 remained stable.
The XRD patterns of the solid phases after contact of thesynthesized (La,Nd,Eu)2O3 with an aqueous sample solution at
60 °C for 4 weeks are presented in Figure 10. Although thepeaks corresponding to Eu 2O3 were separately observed at a
30% Eu mixing ratio before contact with an aqueous solution,

Figure 11. XRD patterns of (La,Eu,Tm)2O3 after contacting theaqueous sample solution at 60 °C for 4 weeks at different mixing
ratios as La:Eu:Tm = 0.5:4.75:4.75, 1.0:4.5:4.5, 3.0:3.5:3.5 and4.5:4.5:1.0, along with that of a mixture of La2O3, Eu 2O3 and Tm 2O3
without heating. XRD pattern for reference peak positions ○:La(OH)3 (ICSD No. 31584), ◊: Eu(OH)3 (ICSD No. 200488), ♦: Eu 2O3(ICSD No. 27997), and ▲: Tm 2O3 (ICSD No. 33657) are presented
for compa r ison. T he m i xt u re of ox ides was ident if ied as
La(OH)3+Eu 2O3+Tm 2O3 after the aging period.

these peaks disappeared after contact with the aqueous solution. Newly observed peaks with small rightward shifts, i.e.
higher 2θ values, depending on the mixing ratios suggested
the formation of a single (La,Nd,Eu)(OH)3 solid solution at all
mixing ratios. A similar behavior was observed for the binary
systems, where (La,Nd)2O3 and (La,Eu)2O3 were transformed
to complete solid solutions of (La,Nd)(OH)3 and (La,Eu)(OH)3after contact with an aqueous solution at 60 °C. The mixtureof pure La 2 O3, Nd 2 O3, and Eu 2 O3 without heat treatment
resulted in the formation of separate hydroxides, i.e., La(OH)3,
Nd(OH)3, Eu(OH)3, and Eu 2O3 after contact with an aqueous
solution as shown in Table A4.
Figure 11 shows the XRD patterns of the solid phases after
contact of the synthesized (La,Eu,Tm)2O3 with an aqueous
solution at 60 °C for 4 weeks. At an La:Eu:Tm mixing ratio of
0.5:4.75:4.75, the peaks corresponding to the Eu2O3 cubic phase
disappeared, while those of the Tm 2O3 cubic phase remained.
The newly observed peaks were assigned to the hexagonalphase of Eu(OH)3. As no separated peaks for the La(OH)3 hexagonal phase were confirmed, it was assumed that La wasincorporated into the Eu(OH)3 phase to form a solid solution,
as observed for the La-Eu binary system. The peaks corresponding to the (La,Eu)(OH)3 hexagonal phase continuously
shifted leftward, i.e. lower 2θ values, with increasing La mixing ratios for the ternary system. As observed for the(Eu,Tm)2O3 binary system after contact with an aqueous solution, the solid solution limit of Tm to Eu(OH)3 fall between 10
and 30%. The peaks of Tm 2O3 disappeared at an La:Eu:Tm
mixing ratio of 4.5:4.5:1.0, indicating that Tm was incorporated into the (La,Eu)(OH)3 hexagonal phase. It is also interesting to note that the mixture of La 2O3+Eu 2O3+Tm 2O3 after
contact with an aqueous phase showed a consistent behavior,undergoing transformation into a mixture of La(OH)3, Eu 2O3,
and Tm2O3 as shown in Table A5. Since the lattice parametersof Eu(OH)3 were not determined in reasonable ranges due totheir limited contributions, Eu(OH)3 was excluded in the analysis.
The lattice parameters for the solid phases after contact of
the sample solutions are summarized in Tables A1-A5 andplotted in Figure 12 as a function of the mixing ratio. Figure

J. Nucl. Radiochem. Sci., Vol. 21, 2021

Phase transformation of mixed lanthanide oxides in an aqueous solution(a)Lattice parameter, a (Å)

6.55

10.70
La:Nd:Eu =4.75.:4.75:0.50
La(OH)3 ICSD 31584
Nd(OH)3 ICSD 31586 La:Nd:Eu =Eu(OH)3 ICSD 200488 4.5:4.5:1.0
La(OH)3 this study
Nd(OH)3 this study
Eu(OH)3 this study

6.50

La:Nd:Eu =3.5:3.5:3.0

6.45

La:Eu:Tm =3.0:3.5:3.5

6.40
La:Eu:Tm =0.5:4.75:4.75

6.35

La:Eu:Tm =4.5:4.5:1.0

La:Eu:Tm =1.0:4.5:4.5(La,Nd)(OH)3(La,Eu)(OH)3(La,Nd,Eu)(OH)3(La,Eu,Tm)(OH)3

10.60

La:Eu:Tm =0.50:4.75:4.75
La:Eu:Tm =1.0:4.5:4.5

La:Eu:Tm =3.0:3.5:3.5

10.55

La:Eu:Tm =4.5:4.5:1.0

10.50

10.45
Tm2O3 ICSD 33657
Tm2O3 this study

6.30

10.40

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4
in (La,Nd)(OH)3

0.2

(b)

10.65
Lattice parameter, a (Å)

6.60

in (La,Eu)(OH)3in (La,Nd,Eu)(OH)3in (La,Eu,Tm)(OH)3

0.0

21

Tm2O3-based(Eu,Tm)2O3(La,Eu,Tm)2O3

0.2

Tm2O3-basedin (Eu,Tm)2O3
in (La,Eu,Tm) 2O3

0.0

0.0

0.2

0.4
0.6
La ratio

0.8

1.0

0.0

0.2

0.4
0.6
Eu ratio

0.8

1.0

Figure 12. Lattice parameter a (upper) and weight fraction (lower) of the synthesized (a) hexagonal phase and (b) cubic phase of the binary andternary systems as a function of the molar mixing ratios. Broken lines in the figures represent the eye guides to show the trend of the binary system.

TABLE 1: Summary of the solid phase samples before and after the immersion experiment.
SystemSubsystem

Mixing molar ratio(La:Nd, La:Eu, Eu:Tm)

Crystal phasea)
Before immersion

After immersion(La,Nd)2O3

La:Nd = 9:1~1:9

trigonal (P-3m1) single SS

hexagonal (P63/m), single
SS

La2O3+Nd2O3

La:Nd = 5:5

two separated trigonal (P-3m1)

two separated hexagonal(P63/m)(La,Eu)2O3

La:Eu = 9:1~1:9

trigonal (P-3m1) SS, cubic (Ia-3)
hexagonal (P63/m) single SS
SS, monoclinic (C12/m1) SS

La2O3+Eu2O3

La:Eu = 5:5

trigonal (P-3m1), cubic (Ia-3)

cubic (Ia-3), two separated
hexagonal (P63/m)(Eu,Tm)2O3

Eu:Tm = 9:1~1:9

two separated cubic (Ia-3) SS

hexagonal (P63/m) SS, cubic(Ia-3) SS

Eu2O3+Tm2O3

Eu:Tm = 5:5

two separated cubic (Ia-3)

hexagonal (P63/m), two separated cubic (Ia-3)(La,Nd,Eu)2O3

La:Nd:Eu =4.75:4.75:0.5~3.5:3.5:3.0

trigonal (P-3m1) SS, cubic (Ia-3)
hexagonal (P63/m) single SS
SS, monoclinic (C12/m1) SS

La2O3+Nd2O3+Eu2O3

La:Nd:Eu = 5:5:5

two separated trigonal (P-3m1), three separated hexagonal
cubic (Ia-3)(P63/m), cubic (Ia-3)(La,Eu,Tm)2O3

La:Eu:Tm =0.5:4.75:4.75~4.5:4.5:1

trigonal (P-3m1) SS, cubic (Ia-3) hexagonal (P63/m) SS, cubic
SS, monoclinic (C12/m1) SS(Ia-3) SS

La2O3+Eu2O3+Tm2O3

La:Eu:Tm = 5:5:5

trigonal (P-3m1), two separated hexagonal (P63/m), two sepcubic (Ia-3)
arated cubic (Ia-3)

BinaryTernarya) SS in the table stands for solid solution.

22

J. Nucl. Radiochem. Sci., Vol. 21, 2021

12a shows the lattice parameter a and weight fraction of thehexagonal phase (P63/m) as a function of the La ratio. For
both (La,Nd)(OH)3 and (La,Eu)(OH)3, the lattice parameter
gradually increased compared to those of pure Nd(OH)3 andEu(OH)3 with increasing La ratios, supporting the formation of
a complete solid solution of (La,Nd)(OH)3 and (La,Eu)(OH)3.
The ternary system (La,Nd,Eu)2O3 also resulted in the formation of a complete solid solution of the hexagonal (La,Nd,Eu)(OH)3 phase. The lattice parameter determined for La:Nd:Eu= 4.75:4.75:0.5 was found to follow the trend of binary (La,Nd)(OH) 3 and approach that of (La,Eu)(OH) 3, as the Eu ratio
increased up to La:Nd:Eu = 3.5:3.5:3.0. The lattice parameterof the hexagonal phase for (La,Eu,Tm)(OH)3 was rather scattered; however, it seemed to be along that of the binary (La,Eu)(OH)3. As discussed in the previous section, the low value of
La:Eu:Tm = 4.5:4.5:1.0 may be due to the incorporation of Tm
into the hexagonal phase.
Figure 12b shows the lattice parameter a and weight fraction of the Tm2O3-based cubic phase (Ia-3) as a function of theEu ratio. By comparing the lattice parameters to those before
contact with an aqueous solution in Figure 6b, hardly any
change in values was noticed after aging at 60 °C for 4 weeks.
Therefore, the Tm 2O3-based cubic phase was considered to bestable, and no significant change appeared to occur in its bulk
phase.
It should be noted that the lattice parameters for the solid
phases after contact of La 2O3 + Nd 2O3 with an aqueous solution matched well those of pure La(OH)3 and Nd(OH)3, asshown in Figure 12a. This confirmed that only the (La,Nd)2O3 solid solution was transformed into the (La, Nd)(OH)3
solid solution, while the separated La 2O3 + Nd 2O3 was transformed to separated La(OH)3 + Nd(OH)3. Therefore, it wassuggested that the transformation of La 2 O 3 or Nd 2 O 3 to
La(OH)3 or Nd(OH)3 under the investigated conditions proceeded via solid phase reactions, such as the incorporation of
H2O from the surface, that led to the conversion of the oxides
to hydroxides. The low La and Nd solubilities within the neutral pH region might lead to a limited pathway of dissolution/
reprecipitation to form a solid solution.
The determined lattice parameters were in good agreementwith those obtained for the La-Eu binary system. These
results indicate that the solid phases of the investigated ternary
systems can be suitably explained based on a combination ofbinary systems.
Table 1 summarizes the obtained solid phases before andafter contacting the aqueous phase.
4. Conclusion
Binary and ternary mixed lanthanide oxides in various proportions were synthesized and studied before and after contact
with an aqueous solution using powder XRD. The XRD patterns of (La,Nd)2O3 confirmed the formation of single complete solid solutions with a trigonal phase at all mixing ratios,
whereas (La,Eu)2O3 and (Eu,Tm)2O3 consisted of mixtures ofdifferent solid solutions. The formation of ternary oxides suchas (La,Nd,Eu)2O3 and (La,Eu,Tm)2O3 could be well explained
based on the combination of binary (La,Nd)2O3, (La,Eu)2O3,
and (Eu,Tm)2O3 phases. After contact with an aqueous phaseat neutral pH and 60 °C for 4 weeks, it was found that
(La,Nd)2O3 and (La,Eu)2O3 were transformed into complete
sol id solut ion s of ( L a , Nd)(OH ) 3 a nd ( L a , Eu)(OH ) 3 .
Interestingly, a mixture of La2O3 and Nd 2O3 without any heat
treatment was transformed to separate La(OH)3 and Nd(OH)3.
A f t e r c o nt a c t w it h a n a q u e o u s p h a s e , t h e t e r n a r y(La,Nd,Eu)2O3 solid solution was completely transformed intoa single solid solution of (La,Nd,Eu)(OH)3. (La,Eu,Tm)2O3
was also converted to (La,Eu,Tm)(OH)3. The trend of the solid
phase transformation of the ternary phases, including the lat-

Moniruzzaman & Kobayashi
tice parameters tendency, was well interpreted using a combination of binary systems. Since the safety assessment ofradioactive waste disposal requires a reliable prediction ofradionuclides solubilities, it is important to characterize thesolubility-controlling solid phase in aqueous systems. The
present study on the transformation of binary and ternary lanthanide oxides in an aqueous solution after aging at an elevated temperature would be a basis for the characterization of
polyvalent actinide oxide solid phases, and subsequently more
complexed multicomponent solid phases in aqueous systems
relevant to the radioactive waste disposal.
Acknowledgements
The authors gratefully acknowledge the financial support
f rom the Japan Societ y for the Promotion of Science
KAKENHI (Grant No. 20H02665).
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Appendix

TABLE A1. Summary of the Rietveld analysis of the La-Nd binary solid phases before and after contact with the sample solution.
Mixing ratio
Weight fraction
Crystal phaseof crystal phase
La/La+Nd
before the solubility experiment
1
1.00
La2O3
LN91
0.9
1.00
LN73
0.7
1.00
LN55
0.5
1.00
LN46
0.4
1.00
Trigonal
P-3m1(164)
LN37
0.3
1.00
LN19
0.1
1.00
0
1.00
Nd2O3
0.49
La2O3 +
0.5
Nd2O3
0.51
after the solubility experiment
LN91
0.9
1.00
LN73
0.7
1.00
LN55
0.5
1.00
LN46
0.4
1.00
Hexagonal
P63/m(176)
LN37
0.3
1.00
LN19
0.1
1.00
1.00
La2O3 +
0.5
Nd2O3
1.00
Samplec(Å)

Rwp

R-factor
Rp

3.93997(9)3.93374(13)3.91685(17)3.89332(19)3.8839(3)3.8529(3)3.83675(15)3.83310(6)3.93909(4)3.83220(4)6.13380(14)6.1282(2)6.1065(3)6.0764(4)6.0640(6)6.0226(5)6.0049(3)6.00381(10)6.13361(7)6.00089(7)

0.165
0.232
0.163
0.166
0.184
0.179
0.184
0.130

0.106
0.163
0.118
0.121
0.137
0.130
0.127
0.089

2.183
3.160
2.327
2.548
2.881
2.833
3.067
2.139

0.06540.04881.037

6.5333(7)6.5333(7)6.4904(2)6.4904(2)6.4749(6)6.4749(6)6.4564(10) 6.4564(10)6.4406(7)6.4406(7)6.42609(17) 6.42609(17)6.5216(6)6.5216(6)6.4152(6)6.4152(6)3.8492(4)3.81315(15)3.7940(4)3.7793(6)3.7614(5)3.74253(13)3.8494(4)3.7315(4)

0.258
0.114
0.171
0.181
0.188
0.101

0.195
0.090
0.131
0.136
0.135
0.075

3.183
1.756
2.277
2.384
2.888
1.439

0.110

0.083

1.545

a(Å)

Lattice parameterb(Å)

3.93997(9)3.93374(13)3.91685(17)3.89332(19)3.8839(3)3.8529(3)3.83675(15)3.83310(6)3.93909(4)3.83220(4)S24

J. Nucl. Radiochem. Sci., Vol. 21, 2021

Moniruzzaman & Kobayashi

TABLE A2. Summary of the Rietveld analysis of the La-Eu binary solid phases before and after contact with the sample solution.
Mixing ratio
Crystal phase
La/La+Eu
before the solubility experiment
Trigonal
La2O31
P-3m1(164)
Trigonal
LE91
0.9
P-3m1(164)
Trigonal
P-3m1(164)LE73
0.7
Cubic Ia-3(206)
Trigonal
P-3m1(164)LE55
0.5
Cubic Ia-3(206)
Monoclinic
C12/m1(12)Trigonal
P-3m1(164)
LE46
0.4
Cubic Ia-3(206)
Monoclinic
C12/m1(12)Trigonal
P-3m1(164)LE37
0.3
Cubic Ia-3(206)
Monoclinic
C12/m1(12)Trigonal (P3m1(164))
LE19
0.1
Cubic Ia-3(206)
Monoclinic
C12/m1(12)1
Cubic Ia-3(206)
Eu2O3
Trigonal (PLa2O3 +3m1(164))
0.5
Eu2O3
Cubic Ia-3(206)after the solubility experiment
LE91
0.9
LE73
0.7
LE55
0.5
Hexagonal
P63/m(176)
LE46
0.4
LE37
0.3
LE19
0.1
Sample

La2O3 +
Eu2O3

0.5

Hexagonal
P63/m(176)
Cubic Ia-3(206)

Lattice parameter
b(Å)

c(Å)

Rwp

R-factor
Rp

S

3.93997(9)6.13380(14)

0.165

0.106

2.183

3.92971(14)3.92971(14)6.1225(3)

0.178

0.120

2.771

0.79

3.9272(4)3.9272(4)6.1184(7)

0.132

0.089

2.448

0.21

10.924(4)10.924(4)10.924(4)0.31

3.8678(5)3.8678(5)6.0540(9)0.06

10.884(2)10.884(2)10.884(2)

0.056

0.039

1.137

0.63

14.373(2)3.6989(5)8.9968(13)0.173.8688(13)3.8688(13)6.060(2)0.20

10.883(2)10.883(2)10.883(2)

0.055

0.037

1.191

0.64

14.354(3)3.6893(8)8.9785(18)0.20

3.8691(11)3.8691(11)6.062(3)0.26

10.895(3)10.895(3)10.895(3)

0.121

0.092

2.471

0.54

14.310(6)3.6668(16)8.939(4)0.23

3.9306(8)3.9306(8)6.1184(16)0.39

10.8915(14)10.8915(14)10.8915(14)

0.111

0.077

2.392

0.38

14.171(12)3.700(3)8.947(9)1.00

10.8755(2)10.8755(2)10.8755(2)

0.074

0.047

2.011

0.45

3.94036(12) 3.94036(12)6.1357(2)

0.088

0.051

1.801

0.55

10.8761(4)10.8761(4)10.8761(4)

1.00
1.00
1.00
1.00
1.00
1.00
0.54
0.11
0.35

6.5143(5)6.4937(6)6.4260(7)6.4334(5)6.4028(5)6.3762(12)6.5372(7)6.361(3)11.333(7)6.5143(5)6.4937(6)6.4260(7)6.4334(5)6.4028(5)6.3762(12)6.5372(7)6.361(3)11.333(7)3.8379(3)3.8236(4)3.7427(4)3.7391(3)3.7097(3)3.6827(7)3.8573(5)3.6615(16)11.333(7)

0.182
0.102
0.094
0.049
0.046
0.073

0.133
0.080
0.069
0.037
0.036
0.053

2.538
1.752
1.792
1.131
1.096
1.524

0.074

0.057

1.455

Weight fractionof crystal phase

a(Å)

1.00

3.93997(9)1.00Phase transformation of mixed lanthanide oxides in an aqueous solution

J. Nucl. Radiochem. Sci., Vol. 21, 2021

25

TABLe A3. Summary of the Rietveld analysis of the Eu-Tm binary solid phases before and after contact with the sample solution.
Mixing ratio
Crystal phase
Eu/Eu+Tm
before the solubility experiment
Eu2O3
1
Cubic Ia-3(206)
ET91
0.9
Cubic Ia-3(206)
Cubic Ia-3(206)ET73
0.7
Cubic Ia-3(206)
Cubic Ia-3(206)
ET55
0.5
Cubic Ia-3(206)
Cubic Ia-3(206)
ET46
0.4
Cubic Ia-3(206)
Cubic Ia-3(206)
ET37
0.3
Cubic Ia-3(206)ET19
0.1
Cubic Ia-3(206)0
Cubic Ia-3(206)
Tm2O3
Cubic Ia-3(206)
Eu2O3 +
0.5
Tm2O3
Cubic Ia-3(206)after the solubility experiment
Hexagonal
ET91
0.9
P63/m(176)
Hexagonal
P63/m(176)ET73
0.7
Cubic Ia-3(206)
Hexagonal
P63/m(176)
ET55
0.5
Cubic Ia-3(206)
ET46
0.4
Cubic Ia-3(206)
ET37
0.3
Cubic Ia-3(206)ET19
0.1
Cubic Ia-3(206)
Hexagonal
P63/m(176)
Eu2O3 +
0.5
Cubic Ia-3(206)
Tm2O3
Cubic Ia-3(206)
SampleWeight fractionof crystal phase
1.00
1.00
0.62
0.38
0.49
0.51
0.20
0.80
0.34
0.66
1.00
1.00
0.39
0.61

a(Å)

Lattice parameter
b(Å)

c(Å)

10.8755(2)10.8755(2)10.8755(2)10.8495(4)10.8495(4)10.8495(4)10.8346(11) 10.8346(11) 10.8346(11)10.6453(10) 10.6453(10) 10.6453(10)10.8297(8)10.8297(8)10.8297(8)10.5819(5)10.5819(5)10.5819(5)10.768(3)10.768(3)10.768(3)10.5891(10) 10.5891(10) 10.5891(10)10.7093(15) 10.7093(15) 10.7093(15)10.6054(14) 10.6054(14) 10.6054(14)10.5296(4)10.5296(4)10.5296(4)10.49207(17) 10.49207(17) 10.49207(17)10.8777(3)10.8777(3)10.8777(3)10.49316(14) 10.49316(14) 10.49316(14)1.00

6.3452(3)6.3452(3)3.63678(19)0.38

6.3478(6)6.3478(6)3.6308(5)0.62

10.6695(11)10.6695(11)10.6695(11)0.13

6.319(3)6.319(3)3.674(2)

0.87
1.00
1.00
1.00

10.5641(10)10.5675(8)10.5469(7)10.5383(6)10.5641(10)10.5675(8)10.5469(7)10.5383(6)10.5641(10)10.5675(8)10.5469(7)10.5383(6)0.23

6.336(7)6.336(7)3.299(6)

0.45
0.32

10.944(6)10.4988(4)10.944(6)10.4988(4)10.944(6)10.4988(4)Rwp

R-factor
Rp

S

0.074
0.067

0.047
0.041

2.011
1.699

0.051

0.034

1.182

0.063

0.043

1.425

0.078

0.056

1.529

0.085

0.058

1.772

0.080
0.052

0.058
0.039

1.830
1.637

0.052

0.037

1.485

0.045

0.034

1.122

0.053

0.039

1.447

0.047

0.035

1.168

0.064
0.054
0.106

0.046
0.041
0.078

1.211
1.295
2.419

0.058

0.045

1.639

26

J. Nucl. Radiochem. Sci., Vol. 21, 2021

Moniruzzaman & Kobayashi

TABLE A4. Summary of the Rietveld analysis of the La-Nd-Eu ternary solid phases before and after contact with the sample
solution.
Mixing ratio
La
Nd
Eu
before the solubility experiment
SampleLNE-1

0.475 0.475 0.05

LNE-2

0.45

LNE-3

La2O3 +
Nd2O3 +
Eu2O3

0.35

0.33

0.45

0.35

0.33

0.1

0.3

0.33

Crystal phase

Trigonal
P-3m1(164)
Trigonal
P-3m1(164)
Cubic Ia-3(206)
Trigonal
P-3m1(164)
Cubic Ia-3(206)
Monoclinic
C12/m1(12)Trigonal
P-3m1(164)
Trigonal
P-3m1(164)
Cubic Ia-3(206)

Lattice parameter
b(Å)

c(Å)

R-factor
Rwp RpS

3.88810(17)6.0758(3)0.137 0.096 1.906

0.99

3.87403(17) 3.87403(17)6.0561(3)0.01

11.308(10)11.308(10)11.308(10)0.463.8543(3)3.8543(3)6.0359(6)0.03

10.974(4)10.974(4)10.974(4)0.51

14.3294(15)3.6894(4)8.9723(10)0.323.93890(13) 3.93890(13)6.1337(3)0.27

3.83288(18)3.83288(18)6.0020(3)0.41

10.8705(2)10.8705(2)10.8705(2)1.00

6.4687(3)6.4687(3)3.78469(17) 0.119 0.090 1.795

1.00

6.4585(2)6.4585(2)3.77479(14) 0.099 0.076 1.629

1.00

6.4280(3)6.4280(3)3.7401(2)0.35

6.5344(8)6.5344(8)3.8622(5)0.23

6.4335(11)6.4335(11)3.7468(8)0.216.424(2)6.424(2)3.6813(13)0.22

10.83(3)10.83(3)10.83(3)Weight fractionof crystal phase

a(Å)

1.00

3.88810(17)0.084 0.061 1.246

0.055 0.041 0.974

0.129 0.089 2.517

after the solubility experiment
LNE-1

0.475 0.475 0.05

LNE-2

0.45

0.45

0.1

LNE-3

0.35

0.35

0.3

La2O3 +
Nd2O3 +
Eu2O3

0.33

0.33

0.33

Hexagonal
P63/m(176)
Hexagonal
P63/m(176)
Hexagonal
P63/m(176)
Hexagonal
P63/m(176)
Hexagonal
P63/m(176)
Hexagonal
P63/m(176)
Cubic Ia-3(206)0.100 0.072 1.717

0.231 0.260 1.062

J. Nucl. Radiochem. Sci., Vol. 21, 2021

Phase transformation of mixed lanthanide oxides in an aqueous solution

27

TABLE A5. Summary of the Rietveld analysis of the La-Eu-Tm ternary solid phases before and after contact with the sample
solution.
Mixing ratio
La
Eu
Tm
before the solubility experiment
SampleLET-1

0.05 0.475 0.475

LET-2

0.1

0.45

0.45

LET-3

0.3

0.35

0.35

LET-4

La2O3 +
Eu2O3 +
Tm2O3

0.45

0.33

0.45

0.33

0.1

0.33

Crystal phase

Cubic Ia-3(206)
Cubic Ia-3(206)
Cubic Ia-3(206)
Cubic Ia-3(206)
Trigonal
P-3m1(164)
Cubic Ia-3(206)
Cubic Ia-3(206)
Trigonal
P-3m1(164)
Cubic Ia-3(206)
Monoclinic
C12/m1(12)Trigonal
P-3m1(164)
Cubic Ia-3(206)
Cubic Ia-3(206)

Lattice parameterb(Å)

Weight fractionof crystal phase

a(Å)

0.62
0.38
0.56
0.44

10.8312(15)10.6367(11)10.8455(13)10.6111(12)10.8312(15)10.6367(11)10.8455(13)10.6111(12)0.30

3.9027(15)3.9027(15)6.113(3)

0.16
0.54

10.867(2)10.632(2)10.867(2)10.632(2)10.867(2)10.632(2)0.143.9016(11)3.9016(11)6.095(2)0.19

10.813(2)10.813(2)10.813(2)0.67

14.377(6)3.6821(13)8.976(4)0.26

3.94100(13)3.94100(13)6.1367(2)

0.34
0.40

10.8782(4)10.4994(2)10.8782(4)10.4994(2)10.8782(4)10.4994(2)0.23

6.396(4)6.396(4)3.582(2)0.77

10.618(2)10.618(2)10.618(2)0.35

6.3443(8)6.3443(8)3.6280(5)0.65

10.5832(12)10.5832(12)10.5832(12)0.38

6.444(2)6.444(2)3.7175(15)0.62

10.575(2)10.575(2)10.575(2)0.98

6.4199(7)6.4199(7)3.7211(4)0.02

10.552(3)10.552(3)10.552(3)0.376.5306(5)6.5306(5)3.8572(4)

0.16
0.48

10.8750(15) 10.8750(15) 10.8750(15)10.4857(2) 10.4857(2) 10.4857(2)c(Å)

R-factor
Rwp RpS

10.8312(15)0.052 0.033 1.264
10.6367(11)10.8455(13)0.047 0.033 1.111
10.6111(12)0.105 0.073 2.282

0.103 0.066 1.847

0.099 0.059 2.293

after the solubility experiment
LET-1

LET-2

LET-3

LET-4

La2O3 +
Eu O +
Tm2O3

0.05 0.475 0.475

0.1

0.3

0.45

0.33

0.45

0.35

0.45

0.33

0.45

0.35

0.1

0.33

Hexagonal
P63/m(176)
Cubic Ia-3(206)
Hexagonal
P63/m(176)
Cubic Ia-3(206)
Hexagonal
P63/m(176)
Cubic Ia-3(206)
Hexagonal
P63/m(176)
Cubic Ia-3(206)
Hexagonal
P63/m(176)
Cubic Ia-3(206)
Cubic Ia-3(206)

0.062 0.045 0.984

0.043 0.034 1.060

0.118 0.083 2.554

0.063 0.048 1.457

0.096 0.070 2.207