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Thermodynamic studies on YBa2Cu3O7−δ and Y2BaCuO5 by solution calorimetry
Mater&Letters Nosh-Holland
15 (1992) 156-161
Thermodynamic studies on YBa2Cu307_s and Y2BaCu05 by solution calorimetry Jin-Ho Choy a, Seong-Gu Kang a, Q.Won Choi a, Duk-Young Jung b and GCrard Demazeau b ’ Department of Chemistry, Seoul National University, Seoul 151-742, South Korea b Laboratoire de Chimie du Solide du CNRS, Upriversitede Bordeaux I, 33405 Talence, France Received 30 July 1992
We have measured calorimetrically the enthalpies of solution of superconducting YBa2Cuj0,_-6 (6=0.09) which has a TccorrSct,of 90 K and semiconducting YZBaCuOS in 1N HCl aqueous solution at 298 K. The standard enthalpies of formation of YBazCu30,_d and Y2BaCuOS have been calculated in combination with supplementary values from the literature as follows: AtNP.(YBa2Cuj06.9L) = - (3027&o) kJ mot-’ and ArH;(Y,BaCuO,) = - (2716&a) kJ mol-’ (a is from the supplementary values and experimental error and is less than 5). An increase in the stability of YBa2Cuj0,_a is expected relative to the parent oxides ( Y203, BaO and CuO) from 300 up to 1 I56 K by applying the entropy value to the calculation of the free energy change of formation for YBazCu@_a, which is the reason why the high-T= superconductor YBaZCu307_-dhas a tendency to decompose above 1200 K.
1. In~duction
of the former are compared with those of the latter.
The advance in superconductor technology resulting from the discovery of YBaZCu307_S with T, of approximately 90 K provided a wide range of research especially on the properties of this phase. It is now well recognized that the crystal structure of this compound is an oxygen-deficient perovskite type. However, despite the success of its structural analyses, little is known about the eqilibrium conditions of fo~ation and the stability of the quaternary compound relative to the initial simple oxides. One scale of this stability is the change in Gibbs energy in the reaction of simple oxides to form the quatemary compound. For this reason we carried out a calorimetric study from which the standard enthalpies of formation of YBazCu306.9, and YzBaCu05 are derived. Referring to the data of heat capacity of YBaZCu307_-6gives us a chance to evaluate the changes of free energy in the formation reaction of YBa2Cu307_-6at room and at higher temperature. Additionally, green YzBaCuOf, often appears in the synthesis of YBa2Cu307_-6as an impurity phase, and the thermodynamic quantities
2. Experimental
156
Superconducting YBa2Cu30,_6 and semiconducting YzBaCuOs were prepared by thermal decomposition of corresponding metal nitrate precursors. In order to obtain the precursors, stoichiometric mixtures of Y203, BafNO,), and Cu metal were dissolved in aqueous solutions of nitric acid. After the heat treatment of 400°C the well-ground samples were pelletized and transferred to a furnace for calcination. The sample was then calcined twice at 900°C in oxygen atmosphere (P,, = I atm) for 25 h and was annealed at 300°C. Y,BaCuOS was prepared at 900’ C in oxygen atmosphere (Po, = 1 atm) for 10 h. The samples were characterized by powder X-ray diffraction with a JEOL DX-GO-2 diffractometer equipped with Ni-filtered Cu Ka radiation (A= 1.5418 A) source. Lattice parameters were determined by weighed least-squares fitting method from 20 values and calibrated by using NaCl as an internal standard. The oxygen content in YBa2Cu307_s was Elsevier Science Publishers B.V.
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MATERIALS LETTERS
estimated by iodometric titration as in a previous report [ 11. The titration was accomplished in the nitrogen atmosphere in order to prevent an unexpected side-reaction owing to ambient gases ( O2 and COZ). Superconducting transition temperature of YBazCu30,_d was determined by electrical resistance and magnetic susceptibility measurements at temperatures from 77 to 300 K. The enthalpies of solution of Y-Ba-Cu oxides were measured in a Parr 1451 solution calorimeter in which 0.5 g of TRIS (tris(hydroxymethyl)aminomethane) was dissolved in 100 ml of 0.1N HCI to evolve 254.76 J/g of TRIS at 25°C. Since the solution reaction is slow, it is possible to distinguish pre-reaction, reaction, and post-reaction periods. The 63% temperature rise method [ 21 was used to determine the temperature rise. Measuring the vertical distance, R, between the two extrapolated lines of the pre- and post-reaction temperature at a point near the middle of the reaction period, AT is estimated at the point where the temperature rise between pre-reaction line and thermogram is equal to 0.63R.
tion). The approximated value could be slightly deviated ( < 5 kJ mol- ’ ) from the empirical molar enthalpy of solution. The application of this approximation to other compounds proves to be reasonable. The reaction scheme to derive AfH,$ of YBa2Cu306.91is given in table 1. The molar enthalpy of formation of YBa2Cu306.91 is obtained as AfH;(YBa2Cu306.9,, =--(3027fa)kJmol-‘, where the uncertainty results from the experimental and auxiliary values. The reaction scheme to derive A&$ (Y,BaCuOS, 298 K) is demonstrated in table 2. The enthalpy of solution of Y2BaCuOS is obtained as -670+2 kJ mol-’ empirically. The enthalpy of solution of Y*BaCuO, has been combined with the reference values, as mentioned in the case of YBa2Cu306.9,, to give the molar enthalpy of formation of Y,BaCuOS, AfH& ( Y2BaCuO,, 298 K) =--(2716ka)
3. Results and discussion YBa2Cu,06.9, and Y,BaCuO, are well characterized by powder X-ray diffraction data, which are compared with previously reported results. Superconducting YBa,Cu,O, _ d shows single-phased orthorhombic structure with the lattice parameters a=3.824 A, b3.887 8, and c= 11.683 A. YBa2Cu@_6 phase of which 6 is determined as 0.09 by redox titration in aqueous media, shows a superconducting transition at Tcjon_setj= 90 K in electrical and diamagnetic measurements. Y,BaCuO, sample has an orthorhombic unit cell with the lattice parametersa=12.273.& b=5.657Aandc=7.131 A, which agree with previous works [ 3,4]. For the enthalpy of solution of YBa2Cu306.9, in IN HCI at 25”C, the value - (789 i 2) kJ mol-’ is estimated in 95Ohconfidence. To calculate the enthalpy of formation of YBazCu306.9,, the enthalpy of solution has to be combined with the auxiliary enthalpies of solution [ 5-101 in the same condition except CuCl,. The molar enthalpy of solution of CuCl, is assumed to be the sum of the molar enthalpies of Cu2+ and Cl- ions ( 1 M aqueous solu-
298 K)
kJmol-‘ .
Based on the standard molar enthalpies of formation at 298 K, after some manipulation, the change of free energy in the following reaction (from the parent binary oxides to the complex oxides) $Y203 +2Ba0+3CuO+
(x/2)0,(g)
-+YRa2Cuj06.5+x,
(1)
has been calculated, as shown in table 3. Using the specific heat values from ref. [ 111 and an unpublished report [ 121 #I, we found that the molar entropy S; ( YBazCu306,9, ) at 298 K is close to 3 16.1 J K- ’ mol- ‘. The free energy change ArCi of reaction (1) for YBa2Cu306.9, (x=0.41 ) at 298 K is estimated to be -492 kJ mol-‘. The negative value of ArG: denotes the stabilization of YBa2Cu306.9, at room temperature. Thermodynamic calculations over a wide range of temperatures can be made with the aid of algebraic equations representing the characteristic properties of the substances being considered here. The values of enthalpy and entropy at high temperature for the reactants in reaction (1 ), are estimated approxiRI Research for the sample 5465. 157
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MATERIALS LETTERS
November 1992
Table 1 Reaction scheme for the enthalpy of formation of YBa2Cu306.91at 298 K, AH*= -~,+~,+~,+LVI4+~~+~~-~,, refers to 1N HCl aqueous solution A&:
Reaction (1) YBa2CuJ06.91(s)+ 13HCl(aqf +{YCl,+2BaClZ+3CuCl,+6.5H,0)(aq)+(0.41/2)0,(g) (2) Y(s)+&-+YClll(aq) (3) (4) (5) (6) (7)
2Ba(s)+2Cl,(g)-+ZBaCl,(aq) 3Cu(s)+3C12(g)-.3CuC12(aq) 6.5H&)+(6.5/2)0&g)+6.5H,O(l) 6.5H~O(l)~(aq)~6.5H~O(aq) 6.5H~(g)~6.5Cl~(g)+(aq)~l3HCl(aq)
(8) Y(s)+ZBa(s)+fCu(s)+
(kJ mol-‘)
-789.1 (exp.) -1168.6 2x 3x 6.5x 6.5x 13x
-864.1 -393.3 -285.85 -0.03 -164.36
(aq)
Ref.
this work [51
[61
1101 171 181 I91
-3027.1
(6.91/2)02(g)-rYBazCu306.gl
Table 2 Reaction scheme for the enthalpy of formation of Y,BaCuO, at 298 K; Al&= -AH, +AHH2+AH~+AHd+AH~+AH~-A&, to IN HCl aqueous solution Reaction (1) Y*BaCuO~(s)+lOHC~(aq) -+{2YCl,+BaCl,+CuC13+ SH,O}(aq) (2) 2Y(s)+3Clz(g)+2YCl,(aq) (3) Ba(s)+Cl&)-+BaCldaq) (4) Cu(s)+Cl2(g)~CuCl2(as) (5) 5H2(g)+aoz(8,-5H,O(l) (6) 5H,O(l)+(aq)-*5H,Otaq) (7) SH,(g)+5Cl2(gl+(aq)-lOHCi(aq) (8) 2Y~s)+Ba(s)~Cu(s)+~O*(g~~Y~BaCuO*(s)
mately from their empirical heat capacity equation [121. In the calculation of the values of enthalpy and entropy at high temperature, it is considered that superconducting YBazCu30,_6 loses oxygen over 670 K [ 141. Oesterreicher and Smith [ 141 reported from thermogravimetric analysis and pressure monitoring in the known volume systems that AS“ for oxygen desorption roughly corresponds to S” (0,) at 298 K. Based on this report, it can be suggested that So (YBaD307_m T) at higher temperature should be close to S” (YBazCu30f_~, 298 K) at room temperature when reaction (1) is considered. 41~0 4H; ( YBa2Cu306.91, 7’) is assumed to remain constant in the value of AfH& (YBazCu306.9,, 298 K), because the enthalpy change of the reaction losing oxygen is close to the formation enthalpy of the equal amount of evolved oxygen. Through the above pro1.58
A&;
(kJ mol-I)
-670.0(exp.) 2X-1168.6 -864.1 - 399.3 5x -285.85 5x 10x
-0.03 - 164.36
(as) refers
Ref.
this work [51 [61 1101 t71 I81 [91
-2716.3
cedure, the free energy changes for reaction ( 1) are evaluated at various temperatures, as shown in fig. 1. &G& becomes zero at 1156 K, implying reaction ( 1) reaches chemical equilib~um. It should be noted that A,G; changes drastically from high to low values according to the oxygen partial pressure because A,G,,,=A,G&+RTln(k,), where k, is the equilibrium constant of reaction ( 1). The calculated temperature represents the limiting value for stable existence of YBa2Cu307_-6at 1 atm oxygen pressure, above which the phase becomes meta@$& In practice, YBa2Cu307_-6 is known to melt incongruently, iu Qther words, it decomposes before melting at high temperature [ 161. The eutecCuO-BaCu02tic temperature upon the YBa2Cu307_-6 triangle was reported [ 161, and it is
Volume
15, number
Table 3 The determination
MATERIALS
3
of temperature
at which A,G&=O in the formation
fY203 ArH; (298 K) S&(298 K)
+
-1905 99
2BaO
November
LETTERS
of YBa2Cu,0,_d
+
3cuo
-548 72
1992
from binary oxides +
-157 42.6
0.2102
+
0 205
YBazCu30b
91
-3027 kJ mol-’ 316JK-‘mol-’
A,H; (YBaZCu306.91) = - 506 W mol-’ A,S;(YBa&ugO,,,) = -47.3 J K-r mol-’ AIG; (YBazCu306.9,) = -492 kJ mol-’
fY203 4H;( 1156K) S;(
+
- 1793 296
1156 K)
2BaO
and entropy
E
3cuo
- 502 137
ArH;(YBa&&O,,,,) = -865 A,S;(YBaZCu906_91)= -748 A,G&(YBazCu90691)=0 ‘) Enthalpy
+
values at any temperature
+
-89 197
0.210*
+
28.8 249
YBazCu30691 - 3027 kJ mol-’ ‘) 316 J K-’ mol-’
a)
kJ mot-’ J K-’ mol-’
are assumed
to be the same as those at 298 K.
-zoo-
a, CT
2
Lc: u
2% ki 2
-300.
-400.
a:, S! LL.
-509 300
500
700
900
Temperature Fig. 1. Change of free energy of formation
of YBazCu,Oc,p,
commonly 1170 k 10 K. Many peritectic reactions occur to be complicated in the temperature region over it. Decomposition of the compound YBaZCuJ0,_-6 has been reported by several workers at around 1200 K [ 18-2 11. They have identified that the decomposition products correspond to oxides;
1100
(K1 from oxides as a function
of temperature.
BaCuO,, YzCu205, YzBaCu05 and so on. Fitzner and Austin Chang [ 22 ] reported a short Letter on thermochemistry by measuring the oxygen potential of heterogeneous oxide mixtures. They concluded that superconducting YBa2Cu307_-6 is metastable at ambient temperature, because the temperature at which 159
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MATERIALS LETTERS
A,G& =O for the reaction that synthesizes YBa2Cu307_-6is lower than that in the green phase. Their schemes have ternary as well as binary oxides. Since they did not consider the stabilities of ternary oxides, BaCu02, Y2Ba205, Y2Cu20s, etc., it is not possible to have a direct insight into the reaction in which stoichiometric binary oxides result in quaternary oxides. Here we report that ArG& (for the reaction of binary oxides) becomes zero at 1156 K, which indicates that the YBa2Cu307_-6phase becomes unstable at rather high temperature. However, in order to explain each decomposition reaction, the entropy values of Y,BaCuOS and other products are needed in order to compare the thermodynamic stability of Y *BaCuO, directly with that of YBa2Cu307 _6 at high temperature. Tretyakov et al. [ 23 ] studied the equilibrium condition for the formation of the compounds Ln&u,O, (Ln=Y, Nb, Er, Dy, Yb, In) in galvanic cells with solid electrolyte ZrOt(Y203) in the temperature range 1173- 1340 K (direct evaluation of free energy change in the solid-state reaction). They have shown that the ternary compound Y2Cu205 reaches the equilibrium condition with the reactant binary oxides at 910 K and becomes thermodynamically stable above this temperature. The value of Gibbs energy change for the formation reaction of Ln,Cu,05 from CuO and LnzOs decreases as the temperature increases, which means that the ternary oxides become stable in proportion to the temperature. On the contrary, the stability of YBazCu@_s is strictly distinguished by a monotonous increase of Gibbs energy change of the formation reaction, from negative to positive value as a function of temperature (fig. 1).
4. Conclusion In the present work, several thermodynamic quantities in oxide reactions have been estimated indirectly, which gave us the s~ndpoi~t of thermal stability of the superconducting phase. We propose an acceptable reason why YBa2Cu307_6 loses stability above 880°C. The thermal decomposition of YBazCu30,_s at high temperature is caused by instability of the phase. It should be noted that 160
November 1992
YBa2Cu307-6 phase is more stable thermodynamically at room temperature than other cuprate compounds ( LnzCuzOS).
This work was supported in part by the Korean Ministry of Science and Technology (MOST).
References [l] J.H. Choy, S.Y. Choi, S.H. Byeon, S.H. Chun, ST. Hong, D.Y. Jung, W.Y. Choe and Y.W. Park, Bull. Korean Chem. Sot. 9 (1988) 289. [2] SC. Canagaratna and J. Witt, J. Chem. Educ. 65 (1988) 126. [3] C. Michel and B. Raveau, J. Solid State Chem. 43 (1982) 73. 141 Y. Kitano, K. Kifune, I. Mukou~, H. Kamimura, J. Sakurai, Y. Komura, K. Hoshino, M. Suzuki, A. Minami, Y. Maeno, M. Kato and T. Fujita, Japan. J. Appl. Phys. 26 (1987) L394. [ 51 F.H. Spedding and J.P. Flynn, J. Am. Chem. Sot. 76 ( 1954) 1474. [ 61 L.R. Moms and C.W. Williams, J. Chem. Thermodynam. 15 (1983) 279. [ 7 ] L.R. Morss and C.W. Williams, I.K. Choi, R. Gens and J. Fuger, J. Chem. The~~ynam. 15 (1983) 1093. [S] E.H.P. Cordfunke and W. Ouweitjes, J. Chem. Thermodynam. 20 (1988) 235. [9] NBS (US) Rept. No. NBSIR 75-968 (1975). [lo] A. Dean, ed., Lange’s handbook of chemistry, 12th Ed. (McGraw-Hill, New York, 1979) p. 9. [ 111 A. Junod, A. Bezinge, D. Eckert, T. Graf and J. Muller, Physica C 152 ( 1988) 495. [ 121 A. Junod, unpublished results. [ 13) R.C. Weast and M.J. Astle, eds., CRC handbook of chemistry and physics, 60th Ed. (CRC Press, Boca Raton, 1979) p. D-61. [ 141 H. Sawada, T. Iwazumi, Y. Saito, Y. Abe, H. Ikeda and R. Yoshizaki, Japan. J. Appl. Phys. 26 (1987) 1054. 151 H. Oesterreicher and M. Smith, Mater. Res. Bull. 22 (1987) 1709. 161 L. Er-Rakho, C. Michel, J. Provost and B. Raveau, J. Solid StateChem.37 (1981) 151. l7 ] K. Gka, K. Nakane, M. Ito, M. Saito and H. Unoki, Japan. J. Appl. Phys. 27 (1988) L1065. 18 J H. Takei, H. Takeya, Y. Iye, T. Tamegai and F. Sakai, Japan. J. Appl. Phys. 26 (1987) L1425. [ 191 K.G. Frase, E.G. Liniger and DR. Clarke, J. Am. Ceram. Sot. 70 (1987) C204.
Volume 15, number 3
MATERIALS LETTERS
[ 201 J. Takeda, H. Kitaguchi, A. Osaka, Y. Miura, K. Takahashi, M. Takano, Y. Ikeda, Y. Bando, N. Yamamoto, Y. Oka and Y. Tomii, Japan. J. Appl. Phys. 26 (1987) L1707. [ 211 R.B. Tripathi, R.K. Kotnala, S.M. Khullar, B.S. Khurana, Satbir Singh, K. Jam, B.V. Reddi, R.C. Gael, K.C. Nagpal, S. Singal and B.K. Das, Solid State Commun. 68 (1988) 319.
November f 992
[22] K. Fitzner and Y. Austin Chang, Extended abstracts, hightemperature superconductors II, eds. B. Batlogg, W.H. Butler, D. Capone and C.W. Chu (Materials Research Society, Pittsburgh, 1988) p. 285. j23 ] Yu.D. Tretyakov, A.R. Kaul and N.V. Makukhin, J. Solid State Chem. 17 (1976) 183.
161
15 (1992) 156-161
Thermodynamic studies on YBa2Cu307_s and Y2BaCu05 by solution calorimetry Jin-Ho Choy a, Seong-Gu Kang a, Q.Won Choi a, Duk-Young Jung b and GCrard Demazeau b ’ Department of Chemistry, Seoul National University, Seoul 151-742, South Korea b Laboratoire de Chimie du Solide du CNRS, Upriversitede Bordeaux I, 33405 Talence, France Received 30 July 1992
We have measured calorimetrically the enthalpies of solution of superconducting YBa2Cuj0,_-6 (6=0.09) which has a TccorrSct,of 90 K and semiconducting YZBaCuOS in 1N HCl aqueous solution at 298 K. The standard enthalpies of formation of YBazCu30,_d and Y2BaCuOS have been calculated in combination with supplementary values from the literature as follows: AtNP.(YBa2Cuj06.9L) = - (3027&o) kJ mot-’ and ArH;(Y,BaCuO,) = - (2716&a) kJ mol-’ (a is from the supplementary values and experimental error and is less than 5). An increase in the stability of YBa2Cuj0,_a is expected relative to the parent oxides ( Y203, BaO and CuO) from 300 up to 1 I56 K by applying the entropy value to the calculation of the free energy change of formation for YBazCu@_a, which is the reason why the high-T= superconductor YBaZCu307_-dhas a tendency to decompose above 1200 K.
1. In~duction
of the former are compared with those of the latter.
The advance in superconductor technology resulting from the discovery of YBaZCu307_S with T, of approximately 90 K provided a wide range of research especially on the properties of this phase. It is now well recognized that the crystal structure of this compound is an oxygen-deficient perovskite type. However, despite the success of its structural analyses, little is known about the eqilibrium conditions of fo~ation and the stability of the quaternary compound relative to the initial simple oxides. One scale of this stability is the change in Gibbs energy in the reaction of simple oxides to form the quatemary compound. For this reason we carried out a calorimetric study from which the standard enthalpies of formation of YBazCu306.9, and YzBaCu05 are derived. Referring to the data of heat capacity of YBaZCu307_-6gives us a chance to evaluate the changes of free energy in the formation reaction of YBa2Cu307_-6at room and at higher temperature. Additionally, green YzBaCuOf, often appears in the synthesis of YBa2Cu307_-6as an impurity phase, and the thermodynamic quantities
2. Experimental
156
Superconducting YBa2Cu30,_6 and semiconducting YzBaCuOs were prepared by thermal decomposition of corresponding metal nitrate precursors. In order to obtain the precursors, stoichiometric mixtures of Y203, BafNO,), and Cu metal were dissolved in aqueous solutions of nitric acid. After the heat treatment of 400°C the well-ground samples were pelletized and transferred to a furnace for calcination. The sample was then calcined twice at 900°C in oxygen atmosphere (P,, = I atm) for 25 h and was annealed at 300°C. Y,BaCuOS was prepared at 900’ C in oxygen atmosphere (Po, = 1 atm) for 10 h. The samples were characterized by powder X-ray diffraction with a JEOL DX-GO-2 diffractometer equipped with Ni-filtered Cu Ka radiation (A= 1.5418 A) source. Lattice parameters were determined by weighed least-squares fitting method from 20 values and calibrated by using NaCl as an internal standard. The oxygen content in YBa2Cu307_s was Elsevier Science Publishers B.V.
Volume i 5, number 3
November 1992
MATERIALS LETTERS
estimated by iodometric titration as in a previous report [ 11. The titration was accomplished in the nitrogen atmosphere in order to prevent an unexpected side-reaction owing to ambient gases ( O2 and COZ). Superconducting transition temperature of YBazCu30,_d was determined by electrical resistance and magnetic susceptibility measurements at temperatures from 77 to 300 K. The enthalpies of solution of Y-Ba-Cu oxides were measured in a Parr 1451 solution calorimeter in which 0.5 g of TRIS (tris(hydroxymethyl)aminomethane) was dissolved in 100 ml of 0.1N HCI to evolve 254.76 J/g of TRIS at 25°C. Since the solution reaction is slow, it is possible to distinguish pre-reaction, reaction, and post-reaction periods. The 63% temperature rise method [ 21 was used to determine the temperature rise. Measuring the vertical distance, R, between the two extrapolated lines of the pre- and post-reaction temperature at a point near the middle of the reaction period, AT is estimated at the point where the temperature rise between pre-reaction line and thermogram is equal to 0.63R.
tion). The approximated value could be slightly deviated ( < 5 kJ mol- ’ ) from the empirical molar enthalpy of solution. The application of this approximation to other compounds proves to be reasonable. The reaction scheme to derive AfH,$ of YBa2Cu306.91is given in table 1. The molar enthalpy of formation of YBa2Cu306.91 is obtained as AfH;(YBa2Cu306.9,, =--(3027fa)kJmol-‘, where the uncertainty results from the experimental and auxiliary values. The reaction scheme to derive A&$ (Y,BaCuOS, 298 K) is demonstrated in table 2. The enthalpy of solution of Y2BaCuOS is obtained as -670+2 kJ mol-’ empirically. The enthalpy of solution of Y*BaCuO, has been combined with the reference values, as mentioned in the case of YBa2Cu306.9,, to give the molar enthalpy of formation of Y,BaCuOS, AfH& ( Y2BaCuO,, 298 K) =--(2716ka)
3. Results and discussion YBa2Cu,06.9, and Y,BaCuO, are well characterized by powder X-ray diffraction data, which are compared with previously reported results. Superconducting YBa,Cu,O, _ d shows single-phased orthorhombic structure with the lattice parameters a=3.824 A, b3.887 8, and c= 11.683 A. YBa2Cu@_6 phase of which 6 is determined as 0.09 by redox titration in aqueous media, shows a superconducting transition at Tcjon_setj= 90 K in electrical and diamagnetic measurements. Y,BaCuO, sample has an orthorhombic unit cell with the lattice parametersa=12.273.& b=5.657Aandc=7.131 A, which agree with previous works [ 3,4]. For the enthalpy of solution of YBa2Cu306.9, in IN HCI at 25”C, the value - (789 i 2) kJ mol-’ is estimated in 95Ohconfidence. To calculate the enthalpy of formation of YBazCu306.9,, the enthalpy of solution has to be combined with the auxiliary enthalpies of solution [ 5-101 in the same condition except CuCl,. The molar enthalpy of solution of CuCl, is assumed to be the sum of the molar enthalpies of Cu2+ and Cl- ions ( 1 M aqueous solu-
298 K)
kJmol-‘ .
Based on the standard molar enthalpies of formation at 298 K, after some manipulation, the change of free energy in the following reaction (from the parent binary oxides to the complex oxides) $Y203 +2Ba0+3CuO+
(x/2)0,(g)
-+YRa2Cuj06.5+x,
(1)
has been calculated, as shown in table 3. Using the specific heat values from ref. [ 111 and an unpublished report [ 121 #I, we found that the molar entropy S; ( YBazCu306,9, ) at 298 K is close to 3 16.1 J K- ’ mol- ‘. The free energy change ArCi of reaction (1) for YBa2Cu306.9, (x=0.41 ) at 298 K is estimated to be -492 kJ mol-‘. The negative value of ArG: denotes the stabilization of YBa2Cu306.9, at room temperature. Thermodynamic calculations over a wide range of temperatures can be made with the aid of algebraic equations representing the characteristic properties of the substances being considered here. The values of enthalpy and entropy at high temperature for the reactants in reaction (1 ), are estimated approxiRI Research for the sample 5465. 157
Volume 15, number 3
MATERIALS LETTERS
November 1992
Table 1 Reaction scheme for the enthalpy of formation of YBa2Cu306.91at 298 K, AH*= -~,+~,+~,+LVI4+~~+~~-~,, refers to 1N HCl aqueous solution A&:
Reaction (1) YBa2CuJ06.91(s)+ 13HCl(aqf +{YCl,+2BaClZ+3CuCl,+6.5H,0)(aq)+(0.41/2)0,(g) (2) Y(s)+&-+YClll(aq) (3) (4) (5) (6) (7)
2Ba(s)+2Cl,(g)-+ZBaCl,(aq) 3Cu(s)+3C12(g)-.3CuC12(aq) 6.5H&)+(6.5/2)0&g)+6.5H,O(l) 6.5H~O(l)~(aq)~6.5H~O(aq) 6.5H~(g)~6.5Cl~(g)+(aq)~l3HCl(aq)
(8) Y(s)+ZBa(s)+fCu(s)+
(kJ mol-‘)
-789.1 (exp.) -1168.6 2x 3x 6.5x 6.5x 13x
-864.1 -393.3 -285.85 -0.03 -164.36
(aq)
Ref.
this work [51
[61
1101 171 181 I91
-3027.1
(6.91/2)02(g)-rYBazCu306.gl
Table 2 Reaction scheme for the enthalpy of formation of Y,BaCuO, at 298 K; Al&= -AH, +AHH2+AH~+AHd+AH~+AH~-A&, to IN HCl aqueous solution Reaction (1) Y*BaCuO~(s)+lOHC~(aq) -+{2YCl,+BaCl,+CuC13+ SH,O}(aq) (2) 2Y(s)+3Clz(g)+2YCl,(aq) (3) Ba(s)+Cl&)-+BaCldaq) (4) Cu(s)+Cl2(g)~CuCl2(as) (5) 5H2(g)+aoz(8,-5H,O(l) (6) 5H,O(l)+(aq)-*5H,Otaq) (7) SH,(g)+5Cl2(gl+(aq)-lOHCi(aq) (8) 2Y~s)+Ba(s)~Cu(s)+~O*(g~~Y~BaCuO*(s)
mately from their empirical heat capacity equation [121. In the calculation of the values of enthalpy and entropy at high temperature, it is considered that superconducting YBazCu30,_6 loses oxygen over 670 K [ 141. Oesterreicher and Smith [ 141 reported from thermogravimetric analysis and pressure monitoring in the known volume systems that AS“ for oxygen desorption roughly corresponds to S” (0,) at 298 K. Based on this report, it can be suggested that So (YBaD307_m T) at higher temperature should be close to S” (YBazCu30f_~, 298 K) at room temperature when reaction (1) is considered. 41~0 4H; ( YBa2Cu306.91, 7’) is assumed to remain constant in the value of AfH& (YBazCu306.9,, 298 K), because the enthalpy change of the reaction losing oxygen is close to the formation enthalpy of the equal amount of evolved oxygen. Through the above pro1.58
A&;
(kJ mol-I)
-670.0(exp.) 2X-1168.6 -864.1 - 399.3 5x -285.85 5x 10x
-0.03 - 164.36
(as) refers
Ref.
this work [51 [61 1101 t71 I81 [91
-2716.3
cedure, the free energy changes for reaction ( 1) are evaluated at various temperatures, as shown in fig. 1. &G& becomes zero at 1156 K, implying reaction ( 1) reaches chemical equilib~um. It should be noted that A,G; changes drastically from high to low values according to the oxygen partial pressure because A,G,,,=A,G&+RTln(k,), where k, is the equilibrium constant of reaction ( 1). The calculated temperature represents the limiting value for stable existence of YBa2Cu307_-6at 1 atm oxygen pressure, above which the phase becomes meta@$& In practice, YBa2Cu307_-6 is known to melt incongruently, iu Qther words, it decomposes before melting at high temperature [ 161. The eutecCuO-BaCu02tic temperature upon the YBa2Cu307_-6 triangle was reported [ 161, and it is
Volume
15, number
Table 3 The determination
MATERIALS
3
of temperature
at which A,G&=O in the formation
fY203 ArH; (298 K) S&(298 K)
+
-1905 99
2BaO
November
LETTERS
of YBa2Cu,0,_d
+
3cuo
-548 72
1992
from binary oxides +
-157 42.6
0.2102
+
0 205
YBazCu30b
91
-3027 kJ mol-’ 316JK-‘mol-’
A,H; (YBaZCu306.91) = - 506 W mol-’ A,S;(YBa&ugO,,,) = -47.3 J K-r mol-’ AIG; (YBazCu306.9,) = -492 kJ mol-’
fY203 4H;( 1156K) S;(
+
- 1793 296
1156 K)
2BaO
and entropy
E
3cuo
- 502 137
ArH;(YBa&&O,,,,) = -865 A,S;(YBaZCu906_91)= -748 A,G&(YBazCu90691)=0 ‘) Enthalpy
+
values at any temperature
+
-89 197
0.210*
+
28.8 249
YBazCu30691 - 3027 kJ mol-’ ‘) 316 J K-’ mol-’
a)
kJ mot-’ J K-’ mol-’
are assumed
to be the same as those at 298 K.
-zoo-
a, CT
2
Lc: u
2% ki 2
-300.
-400.
a:, S! LL.
-509 300
500
700
900
Temperature Fig. 1. Change of free energy of formation
of YBazCu,Oc,p,
commonly 1170 k 10 K. Many peritectic reactions occur to be complicated in the temperature region over it. Decomposition of the compound YBaZCuJ0,_-6 has been reported by several workers at around 1200 K [ 18-2 11. They have identified that the decomposition products correspond to oxides;
1100
(K1 from oxides as a function
of temperature.
BaCuO,, YzCu205, YzBaCu05 and so on. Fitzner and Austin Chang [ 22 ] reported a short Letter on thermochemistry by measuring the oxygen potential of heterogeneous oxide mixtures. They concluded that superconducting YBa2Cu307_-6 is metastable at ambient temperature, because the temperature at which 159
Volume 15, number 3
MATERIALS LETTERS
A,G& =O for the reaction that synthesizes YBa2Cu307_-6is lower than that in the green phase. Their schemes have ternary as well as binary oxides. Since they did not consider the stabilities of ternary oxides, BaCu02, Y2Ba205, Y2Cu20s, etc., it is not possible to have a direct insight into the reaction in which stoichiometric binary oxides result in quaternary oxides. Here we report that ArG& (for the reaction of binary oxides) becomes zero at 1156 K, which indicates that the YBa2Cu307_-6phase becomes unstable at rather high temperature. However, in order to explain each decomposition reaction, the entropy values of Y,BaCuOS and other products are needed in order to compare the thermodynamic stability of Y *BaCuO, directly with that of YBa2Cu307 _6 at high temperature. Tretyakov et al. [ 23 ] studied the equilibrium condition for the formation of the compounds Ln&u,O, (Ln=Y, Nb, Er, Dy, Yb, In) in galvanic cells with solid electrolyte ZrOt(Y203) in the temperature range 1173- 1340 K (direct evaluation of free energy change in the solid-state reaction). They have shown that the ternary compound Y2Cu205 reaches the equilibrium condition with the reactant binary oxides at 910 K and becomes thermodynamically stable above this temperature. The value of Gibbs energy change for the formation reaction of Ln,Cu,05 from CuO and LnzOs decreases as the temperature increases, which means that the ternary oxides become stable in proportion to the temperature. On the contrary, the stability of YBazCu@_s is strictly distinguished by a monotonous increase of Gibbs energy change of the formation reaction, from negative to positive value as a function of temperature (fig. 1).
4. Conclusion In the present work, several thermodynamic quantities in oxide reactions have been estimated indirectly, which gave us the s~ndpoi~t of thermal stability of the superconducting phase. We propose an acceptable reason why YBa2Cu307_6 loses stability above 880°C. The thermal decomposition of YBazCu30,_s at high temperature is caused by instability of the phase. It should be noted that 160
November 1992
YBa2Cu307-6 phase is more stable thermodynamically at room temperature than other cuprate compounds ( LnzCuzOS).
This work was supported in part by the Korean Ministry of Science and Technology (MOST).
References [l] J.H. Choy, S.Y. Choi, S.H. Byeon, S.H. Chun, ST. Hong, D.Y. Jung, W.Y. Choe and Y.W. Park, Bull. Korean Chem. Sot. 9 (1988) 289. [2] SC. Canagaratna and J. Witt, J. Chem. Educ. 65 (1988) 126. [3] C. Michel and B. Raveau, J. Solid State Chem. 43 (1982) 73. 141 Y. Kitano, K. Kifune, I. Mukou~, H. Kamimura, J. Sakurai, Y. Komura, K. Hoshino, M. Suzuki, A. Minami, Y. Maeno, M. Kato and T. Fujita, Japan. J. Appl. Phys. 26 (1987) L394. [ 51 F.H. Spedding and J.P. Flynn, J. Am. Chem. Sot. 76 ( 1954) 1474. [ 61 L.R. Moms and C.W. Williams, J. Chem. Thermodynam. 15 (1983) 279. [ 7 ] L.R. Morss and C.W. Williams, I.K. Choi, R. Gens and J. Fuger, J. Chem. The~~ynam. 15 (1983) 1093. [S] E.H.P. Cordfunke and W. Ouweitjes, J. Chem. Thermodynam. 20 (1988) 235. [9] NBS (US) Rept. No. NBSIR 75-968 (1975). [lo] A. Dean, ed., Lange’s handbook of chemistry, 12th Ed. (McGraw-Hill, New York, 1979) p. 9. [ 111 A. Junod, A. Bezinge, D. Eckert, T. Graf and J. Muller, Physica C 152 ( 1988) 495. [ 121 A. Junod, unpublished results. [ 13) R.C. Weast and M.J. Astle, eds., CRC handbook of chemistry and physics, 60th Ed. (CRC Press, Boca Raton, 1979) p. D-61. [ 141 H. Sawada, T. Iwazumi, Y. Saito, Y. Abe, H. Ikeda and R. Yoshizaki, Japan. J. Appl. Phys. 26 (1987) 1054. 151 H. Oesterreicher and M. Smith, Mater. Res. Bull. 22 (1987) 1709. 161 L. Er-Rakho, C. Michel, J. Provost and B. Raveau, J. Solid StateChem.37 (1981) 151. l7 ] K. Gka, K. Nakane, M. Ito, M. Saito and H. Unoki, Japan. J. Appl. Phys. 27 (1988) L1065. 18 J H. Takei, H. Takeya, Y. Iye, T. Tamegai and F. Sakai, Japan. J. Appl. Phys. 26 (1987) L1425. [ 191 K.G. Frase, E.G. Liniger and DR. Clarke, J. Am. Ceram. Sot. 70 (1987) C204.
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[ 201 J. Takeda, H. Kitaguchi, A. Osaka, Y. Miura, K. Takahashi, M. Takano, Y. Ikeda, Y. Bando, N. Yamamoto, Y. Oka and Y. Tomii, Japan. J. Appl. Phys. 26 (1987) L1707. [ 211 R.B. Tripathi, R.K. Kotnala, S.M. Khullar, B.S. Khurana, Satbir Singh, K. Jam, B.V. Reddi, R.C. Gael, K.C. Nagpal, S. Singal and B.K. Das, Solid State Commun. 68 (1988) 319.
November f 992
[22] K. Fitzner and Y. Austin Chang, Extended abstracts, hightemperature superconductors II, eds. B. Batlogg, W.H. Butler, D. Capone and C.W. Chu (Materials Research Society, Pittsburgh, 1988) p. 285. j23 ] Yu.D. Tretyakov, A.R. Kaul and N.V. Makukhin, J. Solid State Chem. 17 (1976) 183.
161