J. MATER. CHEM., 1993, 3(10), 1025-1029 1025

Phase Relations in the Systems Cu-0-R,O, (R = Tm, Lu) and Gibbs Energies of Formation of Cu,R,O, Compounds

Tom Mathews and K. Thomas Jacob* Department of Metallurgy, Indian Institute of Science, Bangalore-560 012, India

The phase relations in the systems Cu-0-R,O, (R=Tm, Lu) have been determined at 1273 K by X-ray diffraction, optical microscopy and electron probe microanalysis of samples equilibrated in evacuated quartz ampules and in pure oxygen. Only ternary compounds of the type Cu,R,O, were found to be stable. The standard Gibbs energies of formation of the compounds have been measured using solid-state galvanic cells of the type,

Pt I Cu,O + Cu,R,O, + R,O, 1 1 (Y,O,)ZrO, 1 1 CuO + Cu,O I Pt

in the temperature range 950-1325 K. The standard Gibbs energy changes associated with the formation of Cu,R,O, compounds from their binary component oxides are:

2CuO(s) +Tm,O,(s)+Cu,Tm,O,(s), AGO= (10400- 14.0 T/K) 100 J mol - ' 2CuO(s)+ Lu,O,(s)~Cu,Lu,O,(s), A G O = (10210- 14.4 T/K)& 100 J mol-'

Since the formation is endothermic, the compounds become thermodynamically unstable with respect to compo- nent oxides at low temperatures, Cu,Tm,O, below 743 K and Cu,Lu,O, below 709 K. When the chemical potential of oxygen over the Cu,R,O, compounds is lowered, they decompose according to the reaction,

2Cu2R,O,(s)+2R,O,(s) + 2Cu,O(s) + O,(g)

The equilibrium oxygen potential corresponding to this reaction is obtained from the emf. Oxygen potential diagrams for the CU-0-R,O, systems at 1273 K are presented.

Keywords: Phase diagram ; Thermodynamics ; Free energy; Enthalpy ; Entropy; Rare-earth cuprate

An accurate knowledge of the phase relations in the systems R-Ba- Cu-0 (R = rare-earth element), containing the super- conducting compounds RBa2Cu307 - d (R = Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm and Yb),lP4 R2Ba4C~7015-6 (R=Y, Eu, Gd, Dy, Ho, Er and Yb)sP7 and RBa2Cu4O8 (R=Y, Nd, Sm, Eu, Gd, Dy, Ho, Er and Tm),839 is important for optimizing the processing parameters of these superconducting phases. A prerequisite for understanding phase equilibria in the quatern- ary systems is an adequate definition of phase relations in the bounding binary and ternary subsystems. Since the systems R-Cu-0 constitute one bounding face of the quaternary systems R-Ba-Cu-0, phase relations and thermodynamic properties of the system Cu-O-R203 (R=Tm, Lu) were investigated, as part of a larger programme of research.

The sesquioxides of rare-earth elements Tb, Dy, Ho, Er, Tm, Yb and Lu react with cupric oxide to form isostructural compounds with the general formula CU,R,O,.'~ These com- pounds have the orthorhombic crystal structure, with space group P2,nb."~'2 Tretyakov et have measured the stan- dard Gibbs energy of formation of Cu2R2OS (R = Tb, Dy, Er, Yb) as a function of temperature in the range 1173-1340 K using solid-state cells based on yttria-stabilized zirconia as the solid electrolyte. More recently, phase relations in the systems Cu-Gd-0 and Cu-Ho-0 and thermodynamic properties of CuGd204 and Cu2H0205 have been determined as a function of temperature in the range 973-1350K.'4*'s The literature does not contain any report of studies on thermodynamics and phase relations in the systems Cu-Tm-0 and Cu-Lu-0.

Because of the much greater stability of the sesquioxides of rare-earth elements ( R 2 0 3 ) compared to oxides of alloys or intermetallic compounds of Cu-R systems are expected to be in equilibrium with R203. This has been confirmed by studies on the Cu-Gd-0 and Cu-Ho-0 sys-

Hence alloy-oxide equilibrium was not investigated

in this study. Phase relations in the remaining part of the ternary diagram, i.e. Cu-O-R,03, were established by experi- ment for R=Tm, Lu. Solid-state galvanic cells were then designed to measure the standard Gibbs energies of formation of the stable ternary compounds in the temperature range 950- 1325 K. The electrochemical cells employed for the measurements can be represented as

Pt ICu20 + Cu2Tm20, (1) + Tm,03 1 1 (Y,03)Zr02 I/ C u 2 0 + CuO I Pt

Pt 1 c u 2 0 + Cu,Lu,Os (11) + L u ~ O ~ I / (Y203)Zr02 I / C u 2 0 + CuO 1 Pt

The cells are written such that the right-hand electrode is positive. From the thermodynamic data, oxygen potential diagrams for the systems Cu-0-R203 (R=Tm, Lu) were constructed.

Experimental Materials

The oxide materials used in the preparation of the ternary compounds, obtained from Johnson Matthey, were fine pow- ders of CuO and Cu,O, each of purity 99.99%, and Tm,03 and Lu203 of 99.9% purity. The compounds Cu2Tm205 and Cu2Lu20s were prepared by heating an intimate mixture of their component oxides, CuO-Tm203 and CuO-Lu,03 in the molar ratio 2 : 1 at 1273 K for ca. 30 h in flowing oxygen. The products were identified by X-ray diffraction analysis as single-phase Cu2Tm,0s and Cu2Lu20s. The argon gas, used to flush the reference and working electrodes of the cells, was 99.999% pure. It was further dried by passing over anhydrous magnesium perchlorate and phosphorus pentoxide, and deoxi- dized by passing over copper wool at 675 K. The yttria- stabilized zirconia solid electrolyte tubes, with one flat closed

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1026 J. MATER. CHEM., 1993, VOL. 3

end, were obtained from Corning Glass. The tubes were found to be impervious by testing under vacuum.

Determination of Phase Diagrams

The phase relations in the ternary systems Cu-O-R203 (R= Tm, Lu) were explored by equilibrating mixtures containing copper and oxides at 1273 K for ca. 145 h followed by quenching into liquid nitrogen and phase identification. The fine powders were mixed in an agate mortar and the mixtures were then pelletized in a steel die. The pellets, contained in alumina crucibles, were generally equilibrated inside quartz ampules sealed under vacuum or reduced pressure of argon gas. This was to prevent oxidation of copper and lower oxides. The mixtures containing only oxides in their highest oxidation state were equilibrated under pure oxygen at a pressure of 1.01 x lo5 Pa. The apparatus used for this purpose was similar to that described earlier.'* At the end of the equilibration period the samples were quenched in liquid nitrogen. They were then mounted and examined under an optical micro- scope. The phases present in the equilibrated samples were identified by X-ray diffraction and electron probe microanal- ysis (EPMA). The overall composition of the samples studied in the systems Cu-O-Tm,03 and Cu-O-Lu20, are shown in Fig. 1 and 2. The composition of samples equilibrated in sealed quartz ampules are shown by triangles and those equilibrated in pure oxygen by squares.

EMF Measurement using (Y203)Zr02 Electrolyte

A schematic diagram of the apparatus used for electrochemical studies on cells I and I1 is shown in Fig. 3. The gas phase over the two electrodes was physically separated by the solid electrolyte tube. The open ends of the alumina tube housing the cell and the stabilized-zirconia electrolyte tube emerging at the top of the furnace were closed with water-cooled brass- heads, which had provisions for gas inlets and exits and for platinum leads. The apparatus was designed to reduce the volume of free space over the electrodes. Tubes with small cross-sectional areas were used in constructing the cell. Since the partial pressure of oxygen over the electrodes is appreci- able at high temperature, a small cell volume minimizes the extent of decomposition of oxides at equilibrium. The elec-

Tr

L l

c u 0.2 C U ~ O 0.4 (

'3

1 0.6 0.8

Fig. 2 Phase diagram for the system Cu-O-Lu,03 at 1273 K: A, silica-ampoule; ., flowing O2 gas (1.01 x lo5 Pa)

Pt leads

, r r

A1203 tubes

(Y203)Zr02 tube

cu20 + CUO

Cu20 + Cu2Tm205 + Tm203 or

cu20 + CU2LU*O5 + Lu203

3 3

Pt/Pt-13% Rh thermocouple

0 0.6 0.8

Fig. 1 Phase diagram for the system Cu-0-Tm,O, at 1273 K: A, silica-ampoule; , flowing 0, gas (1.01 x lo5 Pa)

Fig.3 Schematic diagram of the cell arrangement used for emf measurement

trodes of cells I and I1 were initially flushed with argon gas. The argon flow was then cut off and the electrodes were allowed to establish their own equilibrium oxygen partial pressures. Continued flushing with inert gas was found to result in loss of oxygen from the electrodes by entrainment, especially at high temperatures. The cell assembly was housed inside a vertical furnace such that the electrodes were main- tained in the even-temperature (+ 1 K) zone. A foil of stainless steel was wrapped around the outer alumina tube of the cell. The foil was grounded to minimize induced voltage on the platinum leads. All wires used for connections were shielded.

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50

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-

-

The cell temperature was measured by a Pt/Pt-l3%Rh ther- mocouple placed adjacent to the measuring electrode. The thermocouple was calibrated against the melting point of gold. The temperature of the cell was controlled to within + 1 K.

The reference electrodes of cells I and I1 were prepared by compacting an intimate mixture of C u 2 0 and CuO in the molar ratio 1 : 2 inside the solid electrolyte tube, with a platinum lead embedded in the mixture. The working electrode for cell I was prepared by ramming an intimate mixture of Cu20, Tm203 and Cu2Tm20S in the molar ratio 1 : 1 : 2 against the closed end of an alumina tube with a platinum lead embedded in the mixture. The solid electrolyte tube was spring loaded against the electrode mixture. For cell I1 the working electrode was an intimate mixture of Cu20, Lu203 and Cu2Lu205 in the molar ratio 1 : 1 :2.

The emf of the solid-state galvanic cells I and I1 was measured as a function of temperature in the range 950-1 325 K with a high-impedance digital voltmeter (Keithley). The reversibility of the cell was checked by micro- coulometric titration in both directions. A small current (ca. 100 PA) was passed through the cell in either direction for ca. 5 min, using an external potential source. During titration the oxygen potential at each electrode was displaced by an infinitesimal amount. The open-circuit emf of the cell was followed after each titration. It was found that the emf returned to the steady value before the titration in each case in 20-30 min. The cell emf was also found to be reproducible on temperature cycling. The phase composition of the elec- trodes was checked after each experiment by X-ray diffraction analysis. The phase composition of the electrodes was unaltered during the experiment.

Results and Discussion Phase Diagrams

The phase relations in the systems Cu-O-R203 (R=Tm, Lu) at 1273 K, obtained from the analysis of equilibrated samples, are shown in Fig. 1 and 2. The diagrams for the two systems are very similar. Only one ternary compound of type Cu2R205 (R = Tm, Lu) was found to be stable in each system at 1273 K. The structure of the compound Cu2R205 (R=Tm, Lu) is orthorhombic with space group P2,nb."*'2 The compound consists of an orthorhombic framework of R 0 6 (R=Tm, Lu) polyhedra. The copper ions have a highly deformed octahedral coordination. The compounds Cu2R205 (R = Tm, Lu) do not appear to have a large non-stoichiometric range. The maxi- mum variations in the oxide composition of the Cu2R205 compound in equilibrium with different phases, CuO, Cu20, R 2 0 3 and O2 was less than 1 mol% at 1273 K. The mutual solubility between R 2 0 3 (R=Tm, Lu) and the oxides of copper, as determined by EPMA, was also less than 1 mol%. It is interesting to note that the ternary oxide (Cu2R205) is not in equilibrium with metallic copper.

EMF Measurements

The emf of the cells attained steady values in 30-160 min after the cells registered constant temperature. Longer periods were required to obtain steady emf at lower temperatures. At each temperature, the cell emf was constant for long periods, extending up to 30 h in many instances. The variation of the reversible emf of cells I and I1 with temperature is shown in Fig. 4 and 5. The numbers on the graphs show the sequence of measurement. The least-squares regression analysis gives

EI=(-53.9+7.25 x ~ O - ~ T/K)+0.4 mV (1)

I I I I I I I

1000 1100 1200 1300 77K

Fig. 4 Temperature dependence of the emf of cell I

1 50

t 101 I ' I I I I I I I

1000 1100 1200 1300 77K

Fig. 5 Temperature dependence of the emf of cell I1

EII=(-52.9+7.46 xlO-' T/K)f0.3 mV (2) The error limits correspond to twice the standard error estimate. The anodic reaction of cells I and I1 can be written as:

2CuO(s)+ 2e-+Cu20(s)+02- (3) The cathodic reaction of cell I is,

C ~ ~ O ( s ) + T r n ~ O ~ ( s ) + O ~ - - - t C ~ ~ T r n ~ 0 ~ ( ~ ) + 2 e - (4)

The overall virtual reaction corresponding to cell I can be written as

2CuO(s)+ Tm203(s)+Cu2Tm20S(s) ( 5 )

Since the transport number of oxygen ions is greater than 0.99 in the solid electrolyte under the experimental conditions, the emf of cell I is related to the standard Gibbs energy change for reaction ( 5 ) by the Nernst equation:

AG;= -nFEI=(10400-14.0 T/K)+100 J mol-' (6)

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where n=2 is the number of electrons participating in the electrode reaction, F is the Faraday constant, and E is the emf. The uncertainty estimate is based on random and system- atic errors in emf and temperature measurements. Precision of the measurements was evaluated by multiple measurements with different samples. Accuracy was established by making emf measurements on standard materials like NiO and C U , ~ . Eqn. (6) provides the only information now available on the thermodynamic properties of Cu2Tm205.

The cathodic reaction for cell I1 is

Cu,0(s)+Lu203(s)+ 0 2 - + C u 2 L u 2 0 5 ( s ) + 2 e ~ (7)

Combining reactions (3) and (7) gives the virtual cell reaction

2CuO(s) + Lu,O,(s) -+CU2LU,O,(S) (8) From the measured emf of cell 11, the standard Gibbs energy change for reaction (8) is

A G i = -2FE=(10210-14.4 T/K)k100 J mol-' (9)

No other information is available on thermodynamic proper- ties of Cu2Lu205 for comparison. The temperature- independent terms of equations (6) and (9) represent average values for the standard enthalpy change for reactions ( 5 ) and (8), respectively, in the temperature range of measurement. The second, temperature-dependent, terms correspond to entropy changes. The solid-state cells were able to produce thermodynamic data with remarkable accuracy.

The standard Gibbs energies of formation of Cu,R20, (R = Tm, Lu) as a function of temperature are shown in Fig. 6. The values for the two compounds differ only by a small amount. The compound Cu2Lu205 is marginally more stable than Cu,Tm,O, at all temperatures. The data from emf measurements indicate that the enthalpy of formation of the compounds Cu,R,O, (R = Tm, Lu) are positive. The positive enthalpy of formation probably arises from the change from square-planar coordination of Cu2+ ions in CuO to the deformed octahedral coordination in Cu2R205 (R =Tm, Lu). It would be interesting to confirm the enthalpy of formation by direct calorimetric studies. The compounds are entropy stabilized at high temperature. Heat capacity measurements from the lowest possible temperature up to decomposition temperature may provide more insights into the origin of the high entropy of Cu2R205 compounds. The compounds Cu,Tm205 and Cu2Lu205 are thermodynamically unstable

r

-71 -91

\

1 1 I I 1 J

1000 1100 1200 1300 1400 TIK

Fig. 6 Variation of the Gibbs free energies of formation of ( a ) Cu,Tm,O, and ( b ) Cu,Lu,O, from component oxides as a function of temperature

below 743 and 709 I(, respectively, with respect to their component oxides. In view of the slow diffusion of ions at these temperatures, it may be difficult to verify the decompo- sition experimentally. The relatively high entropy of Cu,R,O, (R=Tm, Lu) suggests the possibility of cation mixing in the crystallographically non-equivalent sites in the structure, akin to spinels CuX204 (X = Al, Ga, Cr). 19-" It will be interesting to check for possible site disorder by careful struc- tural studies.

The oxygen partial pressures at the left-hand electrodes of cells I and I1 are defined by the reactions

2Cu20(s)+ 2Tm20,(s)+ O2(g)-+2Cu2Tm2O5(s) (10)

and

2Cu,O(s)+ 2LU203(S)+ O,(g)+2Cu2Lu,O,(s) ( 1 1 )

respectively. The values of chemical potential of oxygen corresponding to reactions (10) and (11) can be obtained by combining the emf data with the free-energy change associated with the decomposition of CuO to C u 2 0 from Jacob and Alcock." The values can be expressed by the equations

Ape, =(- 240,100 + 159.82 T/K)&450 J mol- ' (12)

for the Cu20-Tm203-Cu2Tm20, equilibrium and

Ape, = (- 240,470 + 159.02 T/K) & 450 J rnol - ' (1 3)

for the Cu20-Lu203-Cu2Lu205 equilibrium. The larger uncertainties associated with these equations are caused by the relatively larger error bands on thermodynamic data for C u 2 0 and CuO.

Oxygen Potential Diagrams Using the thermodynamic data obtained in this study for Cu,Tm205 and Cu,Lu,O,, and auxiliary data for Cu20, CuO, T m 2 0 3 and Lu203 from the l i t e r a t ~ r e , ' ~ ~ ' ~ . ' ~ * ~ ~ the partial pressures of oxygen for different three-phase equilibria in the ternary systems Cu-0-Tm203 and Cu-0-Lu,O, were computed at 1273 K. Since 1 rnol of CuO gives 0.5 mol of Cu,O or 1 mol of Cu on reduction, the number of moles of Cu remains unchanged when 1 mol of CuO, 0.5 mol of C u 2 0 or 1 rnol of Cu is chosen as one of the components. Hence, the composition of the ternary mixtures can be expressed in terms of CuO + +Cu20 + Cu and RO1 ., (R = Tm, Lu), in order to facilitate two-dimensional representation of phase equilib- ria. The computed oxygen potential diagrams for the ternary systems Cu-0-Tm203 and Cu-0-Lu203 at 1273 K are shown in Fig. 7 and 8. The diagrams obey the same topological rules as the conventional temperature-composition phase diagrams. Oxygen potential diagrams at other temperatures can be constructed from the thermodynamic data obtained in this study.

When three condensed phases and a gas phase are in equilibrium in a ternary system such as Cu-O-R203 ( R = Tm, Lu), the system is monovariant. At a given temperature, three condensed phases coexist at a unique partial pressure of oxygen. The three-phase equilibria are therefore represented by horizontal lines in the oxygen potential diagram. The diagrams provide values for oxygen potential for different equilibria which are not available from Gibbs triangle rep- resentations of Fig. 1 and 2. However, information on oxygen non-stoichiometry of compounds cannot be displayed in the oxygen potential diagrams. The two methods of representation of phase equilibria, Gibbs triangle and oxygen potential diagrams, are therefore complementary.

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Cu2Tm205 +

CUO

1029

Cu2Tm205 +

Tm2°3

-7

-1 Cu2Tm205 1.28 x 1 O4 Pa +

3.95 x Pa CU + Tm2O3

I 1 I I

cu20 I 3.14 x lo3 pa

CU2LU205 CU2LU205 +

Cu20 + Tm2O3 -2t -6 i CUO 0.2 0.4 0.6 0.8 TmOl,5

+ qTrn/(qTrn -t- ~ C U ) 3cu*o

+ c u

Fig.7 Oxygen potential diagram for the system Cu-0-Tm,O, at 1273 K

1273 K

+ Lu203

1 4 + cupo

2.75 x 1 O3 Pa

-2 t

cupo + Lu203

-6L 4 -1

3.95 x lo-* Pa Cu + Lu203

-71 I I I I J -2

3 cu20

CUO 0.2 0.4 0.6 0.8 LuO1.5 + VLV/(VL" + VCJ + cu

Fig. 8 Oxygen potential diagram for the system Cu-0-Lu,O, at 1273 K

Conclusions The phase diagrams for the systems Cu-O-R203 (R = Tm, Lu) at 1273 K have been determined by identifying coexisting phases in equilibrated samples. Only one ternary compound, Cu2R205, was identified in each system. Since there were no prior thermodynamic investigations on Cu2Tm205 and

Cu2Lu205 reported in the literature, the standard Gibbs energy of formation of both compounds from their component oxides was determined by an electrochemical technique to an accuracy of +lo0 J mol-'. The formation of Cu,Tm,O, and Cu2Lu205 from their component oxides is endothermic. The compounds are entropy stabilized at high temperature. They are thermodynamically unstable at low temperatures with respect to their component oxides, Cu2Tm205 below 743 and Cu2Lu205 below 709 K. Based on the thermodynamic data obtained in this study and auxiliary information from the literature, oxygen potential diagrams have been constructed for the systems Cu-O-R,03 (R = Tm, Lu) at 1273 K.

References

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Paper 3/02835F; Received 18th May, 1993

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