Subsolidus phase relations in the system Tm2O 3-Rh2O3

Article abstract of DOI:10.1134/S002016850712014X

The Tm₂O₃-Rh₂O₃ system has been studied by x-ray diffraction, thermal analysis, and chemical analysis of annealed and quenched samples. The results have been used to construct a schematic subsolidus phase diagram of the system. Only one double oxide, TmRhO₃, has been identified. Some of its physicochemical properties are reported.

Full text of DOI:10.1134/S002016850712014X

ISSN 0020-1685, Inorganic Materials, 2007, Vol. 43, No. 12, pp. 1326–1329. © Pleiades Publishing, Inc., 2007.
Original Russian Text © V.N. Skrobot, V.L. Ugolkov, R.G. Grebenshchikov, V.V. Gusarov, 2007, published in Neorganicheskie Materialy, 2007, Vol. 43, No. 12, pp. 1472–1476.
Subsolidus Phase Relations in the System Tm2O3–Rh2O3
V. N. Skrobot, V. L. Ugolkov, R. G. Grebenshchikov, and V. V. Gusarov
Grebenshchikov Institute of Silicate Chemistry, Russian Academy
of Sciences, ul. Odoevskogo 24/2, St. Petersburg, 199155 Russia
e-mail: skrobot@mail.ru
Received January 24, 2007
Abstract—The Tm2O3–Rh2O3 system has been studied by x-ray diffraction, thermal analysis, and chemical
analysis of annealed and quenched samples. The results have been used to construct a schematic subsolidus
phase diagram of the system. Only one double oxide, TmRhO3, has been identified. Some of its physicochem-
ical properties are reported.
DOI: 10.1134/S002016850712014X
INTRODUCTION
In our phase-diagram studies, we used mixtures of
the low-temperature phase α-Rh2O3 and cubic Tm2O3
(C-phase, Tl2O3 fluorite-like structure [20]). The
Tm2O3 was precalcined at 900°C for 3 h in order to
remove the adsorbed water vapor and carbon dioxide.
The Tm2O3 : Rh2O3 molar ratio was 9 : 1, 4 : 1, 3 : 1, 2 : 1,
3 : 2, 1 : 1, 3 : 5, 1 : 2, 1 : 3, 1 : 4, and 1 : 9. In addition,
we used an equimolar mixture of Tm2O3 and fine-parti-
cle rhodium. After grinding and homogenization in a
jasper mortar with ethanol, the mixtures were dried in
air and then fired in corundum crucibles between 600
and 1600°C at 100°C intervals. Some of the mixtures,
including the equimolar composition, were fired and
quenched at 10 to 20°C intervals. The firing duration
was 150 h at temperatures of up to 1000°C and 75 h at
higher temperatures. Every 20 h, the samples were
homogenized by grinding in a jasper mortar. At 1000°C
and lower temperatures, we used an SNOL-I4 electric
furnace. At higher temperatures, the samples were fired
in a Pt–30% Rh wound quenching electric furnace. The
firing and quenching steps were performed in air. The
temperature was monitored with standard Pt/Pt–10%
Rh (TPP-0555) or Pt–6% Rh/Pt–30% Rh (TPR-0679-
01) thermocouples. During firing, the temperature was
maintained constant with an accuracy of 10°C or
better.
Oxide systems containing rare-earth elements (Ln)
and platinum metals (M) have been intensely studied in
recent decades because new oxide phases found in
some of the Ln–M–O systems offer unique electrical,
magnetic, and catalytic properties (see, e.g., [1–3]).
This article presents the continuation of studies con-
cerned with the Ln–Rh–O systems [4–10]. Our main
purpose was to investigate in detail the subsolidus
phase relations in the Tm2O3–Rh2O3 system in air at
temperatures of up to 1600°C and to construct the cor-
responding phase diagram.
Shannon [11] reported as early as 1970 that the Tm–
Rh–O system contains an oxide compound of composi-
tion TmRhO3 with an orthorhombically distorted per-
ovskite structure. The synthesis, crystal chemistry, and
some physicochemical properties of this compound
have been studied extensively [12–17], but the phase
relations in the Tm–Rh–O system remain unexplored.
EXPERIMENTAL
The starting materials used were 99.9+%-pure
Tm2O3 and pure-grade RhCl3 · 4H2O. When exposed to
atmospheric oxygen at temperatures from 650 to
700°C, the latter compound oxidizes to the metastable
phase α-Rh2O3, which has the corundum (hexagonal)
structure [18]. The stable high-temperature polymorph
β-Rh2O3, which has an orthorhombic corundum-like
structure [19], was prepared by two procedures. In one
of them, the metastable phase α-Rh2O3 was heat-
treated in air at temperatures from 800 to 950°C. In the
other procedure, fine-particle rhodium metal was oxi-
dized at 950–1000°C in air or flowing oxygen for at
least 20 h. The fine-particle rhodium (rhodium black)
The composition of TmRhO3 was checked by
gravimetry. We determined the weight of a TmRhO3
sample and then the weight of the solid products of its
thermolysis, Tm2O3 and Rh. To this end, the mixture of
Tm2O3 and Rh was treated with concentrated nitric
acid. The thulium oxide fully dissolved, while the rhod-
ium metal remained intact, which allowed us to deter-
mine its weight and then calculate the weight of the
Tm2O3.
X-ray diffraction (XRD), thermal analysis (DTA +
was prepared by reducing a rhodium trichloride solu- TG), and other characterization details were described
tion in hydrochloric acid by metallic zinc.
elsewhere [8–10].
1326

SUBSOLIDUS PHASE RELATIONS IN THE SYSTEM
1327
RESULTS AND DISCUSSION
1500
1400
1300
1200
1100
1000
900
t, °C
The figure shows a schematic diagram of subsolidus
phase relations in the Tm2O3–Rh2O3 system, repre-
sented as a combination of two pseudobinary systems,
Tm2O3–Rh and Tm2O3–Rh2O3, in the Tm–Rh–O ter-
nary. A key feature of this representation is that the
reversible dissociation of solid oxides is thought of as a
phase transition, but the gas phase (oxygen) is left out
of consideration. Phase equilibria in a ternary system
can then be represented by a combination of two bina-
ries, which makes it possible to obtain a clear schematic
phase diagram in two dimensions. This approach to
representing phase relations was first used by Muan and
Gee [21] in studies of systems containing iron oxides.
Later, Schneider et al. [22] utilized this method to illus-
trate phase equilibria in systems containing platinum-
metal oxides.According to the phase diagram in the fig-
ure, there are four stable phases in the subsolidus region
of the Tm2O3–Rh2O3 system: Tm2O3 (C-phase, Tl2O3
fluorite-like structure), β-Rh2O3, Rh, and TmRhO3.
There is also a metastable phase, α-Rh2O3. The ternary
system Tm–Rh–O is monovariant when three solid
phases are in equilibrium with the gas phase (oxygen).
In that case, at a fixed oxygen partial pressure, three
condensed phases coexist only at a certain temperature.
In connection with this, the three-phase equilibria in the
phase diagram are represented by horizontal solid lines.
The dashed line in the figure represents the monotropic
polymorphic transition of rhodium(III) oxide:
Tm2O3 + Rh
1305
TmRhO3 + Rh
1030
750
Tm2O3 + TmRhO3
TmRhO3 + β-Rh2O3
TmRhO3 + α-Rh2O3
800
700
600
Tm2O3 10 20 30 40 50 60 70 80 90 Rh2O3
mol %
Schematic diagram of subsolidus phase relations in the
Tm O –Rh O system; the dashed line represents the irre-
2
3
2 3
versible polymorphic transition of the metastable phase
α-Rh O (trigonal corundum structure) to the stable orthor-
2
3
hombic corundum-like phase β-Rh O .
2
3
thus obtained, 1034 and 1033°C, respectively, agree
well with the 1042°C calculated from data reported by
Jacob et al. [29]. According to the thermal analysis and
high-temperature XRD results obtained in [4–10, 25–
27], the temperature of equilibrium (2) in air is 1030
5°C. At temperatures from 20 to 1600°C, TmRhO3 also
undergoes only one chemical transformation: revers-
ible thermal dissociation according to the scheme
α-Rh2O3
β-Rh2O3.
(1)
The metastable polymorph α-Rh2O3, which has the
corundum (hexagonal) structure [18], transforms irre-
versibly into the stable polymorph β-Rh2O3, whose
orthorhombic structure is also derived from the corun-
dum structure [19]. The main features of this monotro-
pic phase transition were studied earlier [6–10, 23–27].
The temperature range and rate of transformation (1)
were shown to depend on the impurity composition,
structural perfection, and particle size of the starting
α-Rh2O3, which, in turn, are determined in large part by
1
3
4
--
--
TmRhO3 = Tm2O3 + Rh + O2.
(3)
2
In thermal analysis, the dissociation of thulium
rhodite shows up starting at 1305°C, as a large endot-
herm in the DTA curve and a sharp weight loss of
the nature of the rhodium-containing reagent and the 7.37% in the TG curve. This value agrees well with the
procedure used to synthesize the metastable polymorph 7.50% following from scheme (3). After annealing sin-
α-Rh2O3. For the α-Rh2O3 oxide synthesized in this gle-phase TmRhO3 samples at temperatures above
work, transformation (1) and β-Rh2O3 formation are 1310°C, their XRD patterns show sharp peaks due to
large, well-crystallized rhodium particles and the
C-form of Tm2O3 [20]. In the range 1310–1600°C,
these substances are nonreactive with one another and
atmospheric oxygen. At the same time, after long-term
(more than 100 h) firing of a mixture of Tm2O3 and
fine-particle rhodium at 1200–1320°C in air, XRD indi-
cated the formation of TmRhO3. Thus, our XRD and
thermal analysis results for annealed and quenched
samples confirm scheme (3) and indicate that the tem-
perature of this equilibrium is 1305 10°C.
detected easily after firing the α-Rh2O3 in the range
750°C < t ≤ 1030°C. Above 1030°C, we observe irre-
versible thermal dissociation according to the scheme
3
2
--
β-Rh2O3 = 2Rh + O2.
(2)
Equilibrium (2) and some of its thermodynamic fea-
tures have been studied in sufficient detail [24, 28, 29].
Using earlier experimental data [24, 28], we performed
thermodynamic calculations of the dissociation tem-
perature of β-Rh2O3 in air at normal pressure, i.e., at an
Note that the thermal decomposition of TmRhO3 by
reaction (3) is similar to that of β-Rh2O3 by reaction (2).
oxygen partial pressure of 0.21 × 105 Pa. The values
INORGANIC MATERIALS Vol. 43 No. 12 2007

1328
SKROBOT et al.
The two oxides dissociate reversibly with the formation dissociation temperature of the analogous perovskite
of rhodium metal and oxygen, but the decomposition oxide LaRhO3 rises from 1455 to 1570°C as the oxygen
partial pressure is raised from 0.21 × 105 to 105 Pa.
Clearly, the same would be expected for TmRhO3.
Thus, as pointed out in many studies [4–10, 13, 23–32],
the dissociation temperature of the rhodium oxides in
such systems depends significantly on the oxygen par-
tial pressure, as do their temperature and composition
stability limits.
temperature of the double oxide TmRhO3 (1305°C) is
substantially higher than that of Rh2O3 (1030°C).
Also of importance are the following findings: After
heating rhodium(III) oxide to above 1030°C, XRD
indicated the presence of well-crystallized, large-parti-
cle rhodium: the diffraction peaks from the rhodium
were much sharper than those from fine-particle rhod-
ium. Fine-particle rhodium can easily be oxidized to
β-Rh2O3 at temperatures from 900 to 1000°C, whereas
well-crystallized, large-particle rhodium cannot be
fully oxidized to Rh2O3 in this temperature range, even
at very long (500 h) annealing times in air or flowing
oxygen. The same applies to equilibrium (3). These
results indicate that the time needed for the system to
reach equilibria (2) and (3) through the reverse reac-
tions depends significantly on the particle size of metal-
lic rhodium. It seems likely that the rate of these reac-
tions is limited by oxygen transport through the rhod-
ium oxide or mixed oxide layer on the metal surface.
Thus, the Tm2O3–Rh2O3 system contains only one
double oxide: TmRhO3. The chemical analysis data for
TmRhO3 agree well (to within 0.4%) with the nominal
composition. Phase-pure TmRhO3 samples consist of
homogeneous, light brown powder insoluble in water,
mineral acids (hydrochloric, nitric, sulfuric, and phos-
phoric), and alkalies (NaOH, KOH). Note that rhod-
ium(III) oxide also has high acid and alkali stability.
CONCLUSIONS
The key features of the subsolidus phase relations in
the Tm2O3–Rh2O3 system in air are the irreversible
polymorphic transition of the metastable trigonal (hex-
agonal) phase α-Rh2O3 (corundum structure) to the sta-
ble orthorhombic corundum-like phase β-Rh2O3 above
750°C and two similar equilibria, (2) and (3), at 1030
and 1305°C, respectively. TmRhO3, the only mixed
oxide in the system studied, has an orthorhombically
distorted perovskite structure (GdFeO3 type) and is
insoluble in mineral acids and alkalies.
The present results, in combination with earlier data
[4–10, 12–17, 23–33], attest to a close physicochemical
similarity between the Rh–O and Ln–Rh–O systems.
The double oxides with the general formula LnRhO3 in
the Ln2O3–Rh2O3 systems inherit their physicochemi-
cal properties from Rh2O3. In particular, there is, as a
rule, correlation between the high chemical stability of
Rh2O3 and LnRhO3, and they are similar in thermal
behavior: reversible thermal dissociation with the for-
mation of rhodium metal and oxygen.
Our XRD results confirm earlier reports [11–17]
that TmRhO3 crystallizes in the GdFeO3 structure
(orthorhombically distorted perovskite).
The proposed subsolidus phase diagram of the
Tm2O3–Rh2O3 system is similar in topology to the
Ln2O3–Rh2O3 (Ln = Dy, Ho, Er, Yb, Lu) phase dia-
grams [6–10]. The subsolidus phase diagrams of these
Ln2O3–Rh2O3 systems only differ in the dissociation
temperature of LnRhO3, which decreases from 1345 to
1275°C in going from DyRhO3 to LuRhO3 [6–10].
Accordingly, the horizontal representing the equilib-
rium
ACKNOWLEDGMENTS
We are grateful to D.P. Romanov and S.K. Kuchaeva
for their assistance in the experimental work. We are
also indebted to M.I. Morozov for providing us with
preprints of reports needed for our work.
This work was supported in part by the Russian
Foundation for Basic Research, project no. 06-03-
32780a.
1
2
3
4
--
--
LnRhO3 = Ln2O3 + Rh + O2
REFERENCES
1. Khalifah, P., Nelson, K.D., Jin, R., et al., Non-Fermi-
Liquid Behavior in La4Ru6O19, Nature, 2001, vol. 411,
no. 6838, pp. 669–671.
2. Nishimine, H., Wakeshima, M., and Hinatsu, Y., Crystal
Structures, Magnetic and Thermal Properties of Ln3IrO7
(Ln = Pr, Nd, Sm, and Eu), J. Solid State Chem., 2004,
vol. 177, no. 3, pp. 739–744.
3. Pena, M.A. and Fierro, J.L.G., Chemical Structures and
Performance of Perovskite Oxides, Chem. Rev. (Wash-
ington, D. C.), 2001, vol. 101, no. 7, pp. 1981–2017.
4. Shevyakov, A.M., Skrobot, V.N., and Fedorov, N.F.,
Phase Equilibria in the La2O3–Rh2O3 System, Dokl.
Akad. Nauk SSSR, 1978, vol. 241, no. 2, pp. 423–425.
shifts to lower temperatures.
The phase diagram in the figure represents phase
relations in air at normal pressure, i.e., at an oxygen
partial pressure of 0.21 × 105 Pa. Clearly, the dissocia-
tion temperatures of β-Rh2O3 and TmRhO3 depend on
the oxygen partial pressure. According to Bayer and
Wiedemann [24], increasing the oxygen partial pres-
sure from 1000 to 0.96 × 105 Pa increases the equilib-
rium dissociation temperature of the binary oxide
Rh2O3 from 900 to 1126°C. For the mixed oxide
TmRhO3, such experimental data are not available in
the literature. As reported by Jacob and Waseda [5], the
INORGANIC MATERIALS Vol. 43 No. 12 2007

SUBSOLIDUS PHASE RELATIONS IN THE SYSTEM
1329
5. Jacob, K.T. and Waseda, Y., Phase Relations in the Sys- 20. Glushkova, V.B., Polimorfizm okislov redkozemel’nykh
tem La–Rh–O and Thermodynamic Properties of
LaRhO3, J. Am. Ceram. Soc., 1995, vol. 78, no. 2,
pp. 440–444.
elementov (Polymorphism of Rare-Earth Oxides), Len-
ingrad: Nauka, 1967.
21. Muan, A. and Gee, C.L., Phase Equilibrium Studies in
the System Iron Oxide–Al2O3 in Air and at 1 atm. O2
Pressure, J. Am. Ceram. Soc., 1956, vol. 39, no. 6,
pp. 207–214.
6. Skrobot, V.N. and Grebenshchikov, R.G., Phase Rela-
tions in the Yb2O3–Rh2O3 System, Zh. Prikl. Khim.
(S.-Peterburg), 1999, vol. 72, no. 8, pp. 1247–1250.
22. Schneider, S.J., Waring, J.L., and Tressler, R.E., Phase
Relations between Iridium and the Sesquioxides in Air,
J. Res. Natl. Bur. Stand., Sect. A, 1965, vol. 69, no. 3,
pp. 245–254.
7. Skrobot, V.N. and Grebenshchikov, R.G., Phase Rela-
tions in the Lu2O3–Rh2O3 System, Zh. Prikl. Khim.
(S.-Peterburg), 2000, vol. 73, no. 11, pp. 1767–1769.
8. Skrobot, V.N., Subsolidus Phase Relations in the System
Ho2O3–Rh2O3, Zh. Neorg. Khim., 2002, vol. 47, no. 12,
pp. 2041–2045.
23. Muller, O. and Roy, R., Formation and Stability of the
Platinum and Rhodium Oxides at High Oxygen Pres-
sures and Structures of Pt3O4, β-PtO2, and RhO2,
J. Less-Common Met., 1968, vol. 16, no. 2, pp. 129–146.
9. Skrobot, V.N., Kuchaeva, S.K., Romanov, D.P., and
Grebenshchikov, R.G., Subsolidus Phase Relations in
the System Er2O3–Rh2O3, Zh. Prikl. Khim. (S.-Peter-
burg), 2003, vol. 76, no. 7, pp. 1063–1066.
24. Bayer, G. and Wiedemann, H.G., Bilding und ther-
mische Stabilitat von Rhodium-Oxiden, Thermochim.
Acta, 1976, vol. 15, no. 2, pp. 213–226.
10. Skrobot, V.N., Ugolkov, V.L., Grebenshchikov, R.G.,
and Gusarov, V.V., Subsolidus Phase Relations in the
System Dy2O3–Rh2O3, Neorg. Mater., 2005, vol. 41,
no. 8, pp. 960–965 [Inorg. Mater. (Engl. Transl.),
vol. 41, no. 8, pp. 840–844].
25. Skrobot, V.N., Romanov, D.P., Kuchaeva, S.K., and
Grebenshchikov, R.G., X-ray Powder Diffraction Study
of the System Rh–O, Int. Conf. on Powder Diffraction
and Crystal Chemistry, St. Petersburg, 1994, p. 81.
26. Skrobot, V.N. and Grebenshchikov, R.G., Phase Rela-
tions in the System CaO–Rh2O3, Zh. Neorg. Khim.,
1989, vol. 34, no. 8, pp. 2127–2130.
11. Shannon, R.D., Cell Dimensions of Rare Earth Orthor-
hodites, Acta Crystallogr., Sect. B: Struct. Crystallogr.
Cryst. Chem., 1970, vol. 26, no. 4, pp. 447–449.
27. Skrobot, V.N. and Grebenshchikov, R.G., Phase Rela-
tions in the System MgO–Rh2O3–Rh, Zh. Neorg. Khim.,
1997, vol. 42, no. 11, pp. 1908–1911.
12. Shaplygin, I.S. and Lazarev, V.B., Physicochemical
Properties of Orthorhombic MRhO3 Perovskites, Vse-
soyuznoe soveshchanie po khimii tverdogo tela (All-
Union Conf. on Solid-State Chemistry), Sverdlovsk–
Pervoural’sk, 1975, part 2, p. 68.
28. Schmahl, N.G. and Minzl, E., Die thermische Zerset-
zung des Rhodium (III)-Oxids und deren Beeinflussung
durch Legierungsbildung mit Platin und Palladium,
Z. Phys. Chem., 1964, vol. 41, nos. 1–2, pp. 78–96.
13. Lazarev, V.B. and Shaplygin, I.S., Electrical Properties
of Platinum Metal–Base Metal Mixed Oxides, Zh.
Neorg. Khim., 1978, vol. 23, no. 2, pp. 291–303.
29. Jacob, K.T., Uda, T., Okabe, T.H., and Waseda, Y., The
Standard Enthalpy and Entropy of Formation of
Rh2O3—A Third-Law Optimization, High Temp. Mater.
Processes, 2000, vol. 19, no. 1, pp. 11–16.
14. Lazarev, V.B. and Shaplygin, I.S., Electrical Conductiv-
ity of Platinum Metal–Base Metal Double Oxides, Izv.
Akad. Nauk SSSR, Neorg. Mater., 1978, vol. 14, no. 11,
pp. 1942–1949.
30. Skrobot, V.N. and Grebenshchikov, R.G., Phase Rela-
tions in Oxide Systems Containing Platinum Metals,
Mezhdunarodnaya konferentsiya po khimii tverdogo tela
(Int. Conf. on Solid-State Chemistry), Odessa, 1990,
part 2, p. 86.
15. Lazarev, V.B. and Shaplygin, I.S., Electrical Conductiv-
ity of Platinum Metal–Nonplatinum Metal Double
Oxides, Mater. Res. Bull., 1978, vol. 13, no. 3, pp. 229–
235.
31. Skrobot, V.N. and Grebenshchikov, R.G., Phase Rela-
tions in Oxide Systems Containing Platinum-Metal
Oxides, Mezhdunarodnaya konferentsiya po vysokotem-
peraturnoi khimii silikatov i oksidov (Int. Conf. on High-
Temperature Chemistry of Silicates and Oxides), St.
Petersburg, 1998, p. 101.
16. Shaplygin, I.S., Prosychev, I.I., and Lazarev, V.B.,
Chemistry and Properties of Rhodium Oxide Com-
pounds, Zh. Neorg. Khim., 1986, vol. 31, no. 11,
pp. 2870–2875.
17. Taniguchi, T., Iizuka, W., Nagata, Y., et al., Magnetic
Properties of RRhO3 (R = Rare Earth), J. Alloys Compd.,
2003, vol. 350, nos. 1–2, pp. 24–29.
32. Skrobot, V.N., Kuchaeva, S.K., Romanov, D.P., and
Grebenshchikov, R.G., Thermal Stability of Platinum-
Metal Oxides, Mezhdunarodnaya konferentsiya po
vysokotemperaturnoi khimii silikatov i oksidov (Int.
Conf. on High-Temperature Chemistry of Silicates and
Oxides), St. Petersburg, 2002, p. 115.
18. Coey, J.M.D., The Crystal Structure of Rh2O3, Acta
Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem.,
1970, vol. 26, no. 11, pp. 1876–1877.
19. Biesterbos, J.W.M. and Hornstra, J., The Crystal Struc-
ture of the High-Temperature, Low-Pressure Form of
Rh2O3, J. Less-Common Met., 1973, vol. 30, no. 1,
pp. 121–125.
33. Shevyakov, A.M., Skrobot, V.N., and Fedorov, N.F.,
Synthesis of Mixed Oxides in Systems Based on Plati-
num-Metal and Rare-Earth Oxides, Dokl. Akad. Nauk
SSSR, 1977, vol. 235, no. 2, pp. 372–375.
INORGANIC MATERIALS Vol. 43 No. 12 2007