Subsolidus phase relations in the system Dy2O 3-Rh2O3

Article abstract of DOI:10.1007/s10789-005-0223-9

The Dy₂O₃-Rh₂O₃ system is studied by thermal analysis, x-ray diffraction, and chemical analysis of annealed and quenched samples. The results are used to construct a schematic subsolidus phase diagram of the system. Only one double oxide, DyRhO ₃, is obtained. Some of its physicochemical properties are reported.

Full text of DOI:10.1007/s10789-005-0223-9

Inorganic Materials, Vol. 41, No. 8, 2005, pp. 840–844. Translated from Neorganicheskie Materialy, Vol. 41, No. 8, 2005, pp. 960–965.
Original Russian Text Copyright © 2005 by Skrobot, Ugolkov, Grebenshchikov, Gusarov.
Subsolidus Phase Relations in the System Dy O –Rh O
2
3
2
3
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 December 20, 2004
Abstract—The Dy O –Rh O system is studied by thermal analysis, x-ray diffraction, and chemical analysis
2
3
2
3
of annealed and quenched samples. The results are used to construct a schematic subsolidus phase diagram of
the system. Only one double oxide, DyRhO , is obtained. Some of its physicochemical properties are reported.
3
INTRODUCTION
EXPERIMENTAL
This article presents the continuation of our studies
The starting materials used were 99.9+%-pure
concerned with phase relations in Ln O –Rh O (Ln = Dy O and pure-grade RhCl · 4H O. When exposed to
2
3
2
3
2
3
3
2
rare-earth metal) systems [1–6], which are motivated atmospheric oxygen for a long time at temperatures
by the interesting properties of compounds in these sys- from 650 to 700°C, the latter compound oxidizes to the
tems. First, compounds with the general formula metastable phase α-Rh O , which has the corundum
2
3
LnRhO are used in catalysis and electrochemistry. In (hexagonal) structure [22]. The stable high-temperature
3
particular, LaRhO is a catalyst for the hydrogenation polymorph β-Rh O , which has an orthorhombic
3
2
3
of carbon monoxide to methane, acetaldehyde, and eth- corundum-like structure [23], was prepared by two pro-
anol [7]. In addition, electrolytic cells with LuRhO3 cedures. In one of them, α-Rh O was heat-treated in
2
3
ceramic cathodes achieve powers and photovoltages air at 800–950°C. In the other procedure, fine-particle
sufficient for water photoelectrolysis by solar radiation, rhodium metal was oxidized at 950–1000°C in air or
with no applied potential [8]. Second, both LnRhO and flowing oxygen for at least 20 h. Fine-particle rhodium
3
LnM1 – xRh O (M = 3d transition metal) phases have (rhodium black) was prepared by reducing an RhCl3
x
3
perovskite-like structures and offer a wide variety of solution in HCl with metallic zinc.
electrical and magnetic properties. For example,
In our phase-diagram studies, we used mixtures of
LaNi Rh O is a semiconductor exhibiting Pauli Dy O and α-Rh O in the molar ratios 9 : 1, 4 : 1, 3 : 1,
1
/2
1/2
3
2
3
2
3
paramagnetism in the range 40–300 K and passes into 2 : 1, 3 : 2, 1 : 1, 3 : 5, 1 : 2, 1 : 3, 1 : 4, and 1 : 9 and
a spin glass state below 10 K [9], whereas also an equimolar mixture of Dy O and fine-particle
2
3
LaMn Rh O combines semiconducting and ferro-
1
/2
1/2
3
rhodium.After grinding and homogenization in a jasper
magnetic properties [10]. LaMn0.976Rh0.024O3.15 effec- mortar with ethanol, the mixtures were dried in air and
tively catalyzes three reactions in NO + CO + C H
then fired in corundum crucibles between 600 and
3
6
mixtures: reduction of NO and oxidation of CO and 1600°C in 100°C steps. Some of the mixtures, includ-
C H [11]. Third, some of the mixed oxides in Ln–M– ing the equimolar composition, were fired and
3
6
quenched in 10 to 20°C steps. The firing duration was
Rh–O (M = alkaline-earth metal) systems possess
unusual crystal-chemical, electrical, and magnetic
properties (see, e.g., [12–14]), which suggest that such
systems may contain superconductors.
1
2
1
50 h at 1000°C and 75 h at higher temperatures. Every
0 h, the samples were reground and homogenized. At
000°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 per-
formed in air. The temperature was monitored with
standard Pt/Pt–10% Rh (TPP-0555) or Pt–6% Rh/Pt–
The first report that the Dy–Rh–O system contains a
mixed oxide of composition DyRhO appeared in 1970
3
[
15]. Later, the synthesis, crystal chemistry, and physi-
cochemical properties of this compound have been the
subject of many studies [15–21], but the phase relations
in the Dy–Rh–O system have yet not been investigated.
3
0% Rh (TPR-0679-01) thermocouples. During firing,
the temperature was maintained constant with an accu-
The purpose of this work was to study the phase racy of ±5°C or better using a VRT-3 temperature con-
relations in the Dy O –Rh O system in air at tempera- troller.
2
3
2
3
tures of up to 1600°C and to construct the subsolidus
The phase composition of the starting mixtures and
samples after firing and quenching was determined on
phase diagram of this system.
0
020-1685/05/4108-0840 © 2005 Pleiades Publishing, Inc.

SUBSOLIDUS PHASE RELATIONS IN THE SYSTEM Dy O –Rh O
3
841
2
3
2
a DRON-3 (Ni-filtered CuK radiation) or Siemens D
According to the phase diagram in the figure, the
α
5
00 HS (CuK radiation, graphite monochromator) dif- stable phases in the subsolidus region of the Dy
O –
2 3
α
fractometer. Measurements were made in the range Rh
O
system are Dy
O
(C form), β-Rh O , Rh, and
2
3
2
3
2 3
2
θ = 10°–95° with a scan step of 0.02° and a counting DyRhO . There is also a metastable phase, α-Rh O .
3
2
3
time of 1 s per data point.
The ternary system Dy–Rh–O is monovariant when
Thermal analysis (DTA + TG + DTG) was carried three solid phases are in equilibrium with the gas phase
out in air with a MOM C system (Hungary) at temper- (oxygen). In that case, at a fixed oxygen partial pres-
atures of up to 1200°C and with a SETARAMV-70 sys- sure, three condensed phases coexist only at a certain
tem (France) at temperatures of up to 1600°C. The temperature. In connection with this, three-phase equi-
heating/cooling rate was 7.5 to 10°C/min. As a refer- libria in the phase diagram are represented by horizon-
ence substance, we used alumina powder. The sample tal solid lines. The dashed line in the figure represents
weight was 50 to 150 mg. In addition, thermal analysis the monotropic polymorphic transition
below 1400°C was performed with a Netzsch STA 449
α-Rh O
β-Rh O .
(1)
system (Germany), which simultaneously recorded dif-
ferential scanning calorimetry (DSC) curves and their
derivatives. Powder samples weighing 30.00 ± 0.05 mg
were loaded into a fused corundum crucible, heated in
air from 20 to 1400°C at a rate of 10°C/min, and then
2
3
2
3
The metastable polymorph α-Rh O , which has the
corundum (hexagonal) structure [22], transforms irre-
versibly into the stable polymorph β-Rh O , whose
2
3
2
3
cooled to 100°C at the same rate. As a reference, we orthorhombic structure is also derived from the corun-
used an empty corundum crucible of the same weight. dum structure [23]. The main features of this monotro-
The use of air instead of a reference substance was jus- pic phase transition were studied earlier [21, 27–31].
tified because the sample weight was smaller than the The temperature range and rate of transformation (1)
crucible weight by more than a factor of 6. In addition, depend on the chemical composition, structure, and
this precluded the effects of the state of reference pow- particle size of the starting α-Rh O , which, in turn, are
2 3
der (particle size and tap density) and possible changes determined by the synthesis procedure and conditions.
in its properties in the course of thermal analysis. The For the rhodium oxide synthesized in this work, trans-
DSC curves obtained with no sample also indicated that formation (1) is detected easily after firing α-Rh O
2
3
the use of air as a reference had a positive effect at small above 750°C. Above 1030°C, we observe irreversible
sample weights. This approach was substantiated by thermal dissociation according to the scheme
Speyer [24].
The composition of DyRhO was checked by
gravimetry. We determined the weight of a DyRhO3
sample and then the weight of the thermolysis products,
3
2
3
β-Rh O = 2Rh + -- O .
(2)
2
3
2
Dy O and Rh. To this end, the mixture of Dy O and Equilibrium (2) and some of its thermodynamic fea-
2
3
2
3
tures have been studied in detail [27–33]. Using earlier
data [28, 32], one can evaluate the dissociation temper-
Rh was treated with concentrated nitric acid. The
Dy O fully dissolved, while rhodium metal remained
2
3
ature of β-Rh O in air at normal pressure, i.e., at an
intact, which allowed us to determine its weight and
then calculate the weight of Dy O .
2
3
5
oxygen partial pressure of 0.21 × 10 Pa. The values
2
3
thus obtained, 1034 and 1033°C, agree well with the
Full details of our experiments were described else-
where [3, 5].
1
042°C reported by Jacob and Sriram [33]. According
to the DTA, DSC, and high-temperature x-ray diffrac-
tion (XRD) results obtained in [29–31], the temperature
of equilibrium (2) in air is 1030 ± 5°C.
RESULTS AND DISCUSSION
The figure shows a schematic diagram of subsolidus
At temperatures of up to 1600°C, DyRhO also
3
phase relations in the Dy O –Rh O system as a combi- undergoes only one chemical transformation: revers-
2
3
2
3
nation of two pseudobinary systems, Dy O –Rh and ible thermal dissociation according to the scheme
2
3
Dy O –Rh O , in the Dy–Rh–O ternary. A key feature
2
3
2
3
1
2
3
4
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.
DyRhO = -- Dy O + Rh + -- O .
(3)
3
2
3
2
Phase equilibria in a ternary system can then be repre- The thermal dissociation of dysprosium rhodite shows
sented by a combination of two binaries, which makes up as a large endotherm in the DTA curve and a sharp
it possible to obtain a clear schematic phase diagram in weight loss of 7.58% in the TG curve. This values
two dimensions. This approach to representing phase agrees well with the 7.66% following from scheme (3).
relations was first used by Muan and Gee [25] in studies The weight loss is well seen in the TG curve starting at
of systems containing iron oxides. Schneider et al. [26] 1345°C and in the DTG curve starting at 1320°C. In the
were the first to apply this method to systems contain- DSC and derivative curves, the endothermic event
ing platinum-metal oxides.
begins at 1345°C. After annealing single-phase
INORGANIC MATERIALS Vol. 41 No. 8 2005

8
42
t, °C
SKROBOT et al.
Thus, the Dy O –Rh O system contains only one
2
3
2
3
1
600
mixed oxide: DyRhO . The chemical analysis data for
3
DyRhO agree well (to within 0.2%) with the nominal
composition. Our powder XRD results confirm earlier
reports [15–21] that DyRhO3 crystallizes in the
3
1
1
1
1
1
1
500
400
300
200
100
000
Dy O + Rh
2
3
GdFeO structure (orthorhombically distorted perovs-
1
345
3
kite). Single-phase DyRhO samples consist of homo-
3
DyRhO + Rh
3
geneous, dark brown powder which is insoluble in
water, mineral acids (HCl, HNO , H SO , H PO ), and
3
2
4
3
4
caustic alkalies (NaOH, KOH), as are other LnRHO3
compounds [3–6]. Note that Rh O also has extremely
1
7
030
50
2
3
9
8
7
6
00
00
00
00
Dy2O3 + DyRhO3
DyRhO3 + β-Rh2O3
DyRhO + α-Rh O
high acid and alkali stability.
The proposed subsolidus phase diagram of the
Dy O –Rh O system is similar in topology to the
2
3
2
3
Ln O –Rh O (Ln = La, Ho, Er, Yb, Lu) phase dia-
2
3
2
3
3
2
3
grams [1–6]. These phase diagrams differ only in the
dissociation temperature of LnRhO , which decreases
3
from 1450 to 1275°C in going from LaRhO to LuRhO
3
3
Dy O
20
40
60
80
Rh O
2 3
2
3
[
1–6]. The phase diagram in the figure represents phase
mol %
relations in air at normal pressure, i.e., at an oxygen
5
partial pressure of 0.21 × 10 Pa. Clearly, the dissocia-
Schematic diagram of subsolidus phase relations in the
Dy O –Rh O system; the dashed line represents the irre-
tion temperatures of β-Rh O and DyRhO depend on
2
3
2
3
2
3
3
versible polymorphic phase transition of Rh O .
the oxygen partial pressure. According to Bayer and
Wiedemann [28], increasing the oxygen partial pres-
2
3
3
5
sure from 10 to 0.96 × 10 Pa increases the equilibrium
DyRhO samples above 1350°C, XRD patterns show
3
dissociation temperature of rhodium(III) oxide from
sharp peaks due to large, well-crystallized rhodium par- 900 to 1126°C. For DyRhO , such experimental data
3
ticles and the so-called C-form of Dy O , which has a
2
3
are not available in the literature. As reported by Jacob
cubic structure [34]. In the range 1345–1600°C, these and Waseda [2], the dissociation temperature of the
substances are nonreactive with one another and atmo- analogous perovskite oxide LaRhO rises from 1455 to
3
spheric oxygen. At the same time, after long-term
more than 100 h) firing of a mixture of Dy O and fine-
1
570°C as the oxygen partial pressure is raised from
(
5 5
2
3
0.21 × 10 to 10 Pa. Thus, as pointed out in many stud-
ies [1–6, 27–33, 35–37], the dissociation temperature
of rhodium oxides in such systems depends signifi-
particle rhodium at 1320°C in air, XRD indicated the
formation of DyRhO . Thus, our XRD and thermal
3
analysis results for annealed and quenched samples cantly on oxygen partial pressure, as do their tempera-
confirm scheme (3) and indicate that the temperature of ture and composition stability limits.
this equilibrium is 1345 ± 10°C.
Note that the thermal decomposition of DyRhO by
CONCLUSIONS
3
reaction (3) is similar to that of β-Rh O by reaction (2).
2
3
The present results confirm the close physicochem-
ical similarity between the platinum metal–oxygen and
base metal–platinum metal–oxygen systems and some
inherent features of oxides in such systems [1–6, 18,
21, 26, 29–31, 35–38].
The two oxides dissociate reversibly with the formation
of rhodium metal and oxygen, but the decomposition
temperature of DyRhO is substantially higher than that
3
of Rh O .
2
3
Subsolidus phase relations in binary oxide systems
containing platinum-metal oxides are dominated by the
physicochemical nature of the noble-metal oxide and
depend much weaker on the base-metal oxide.
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 rhodium were
much sharper than those from fine-particle rhodium.
Most simple and double oxides of platinum metals
Fine-particle rhodium can easily be oxidized to single- exhibit polymorphism and/or undergo reversible ther-
phase β-Rh O , while well-crystallized, large-particle mal dissociation. Melting is atypical of both simple and
2
3
double oxides of platinum metals.
rhodium cannot be oxidized to Rh O , even at very long
2
3
(500 h) annealing times in air or flowing oxygen at tem-
Simple oxides of platinum metals decompose
peratures of up to 1000°C. The same was observed for reversibly into the metal and oxygen (of the 15 solid
equilibrium (3). These results indicate that the equili- simple oxides of platinum metals, the only exceptions
bration time for equilibria (2) and (3) depends signifi- are RuO , OsO , and OsO ). Double oxides of platinum
4 4 2
cantly on the particle size of metallic rhodium.
metals typically dissociate into the metal, oxygen, and
INORGANIC MATERIALS Vol. 41 No. 8 2005

SUBSOLIDUS PHASE RELATIONS IN THE SYSTEM Dy O –Rh O
843
2
3
2
3
a base-metal oxide. In most cases, the double oxides
dissociate at higher temperatures than do the corre-
sponding simple oxides. The highest thermal stability is
offered by ruthenium, rhodium, and iridium oxides.
The great majority of the double oxides existing in
base metal–platinum metal–oxygen systems inherit
their physicochemical properties from the correspond-
ing simple oxides of platinum metals. In particular,
there is a strong correlation between the chemical and
thermal stabilities of simple and mixed oxides of plati-
num metals.
LaMn0.976Rh0.024
pp. 197–204.
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ACKNOWLEDGMENTS
Type Sr MRhO (M = Sm, Eu, Tb, Dy, Ho, Er, and Yb),
3
6
We are grateful to V.F. Popova, A.E. Lapshin, and
D.P. Romanov for their assistance in the experimental
work. We are also indebted to M.I. Morozov for provid-
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