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The problem of nonstoichiometry in the chemical preparation of high-Tc superconducting ceramics
652
Journal of Crystal Growth 85 (1987) 652—655 North-Holland, Amsterdam
THE PROBLEM OF NONSTOICHIOMETRY IN THE CHEMICAL PREPARATION OF HIGH-TC SUPERCONDUCTING CERAMICS A.C. PASTOR and R.C. PASTOR Hughes Research Laboratories, Malibu, California 90265. USA
Received 12 August 1987; manuscript received in final form 17 September 1987
The iiiherent difficulties in prepanng a single-phase oxide material from a system wherein the problem of oxygen nonstoichiometry is resident, as in the case of the high -7~superconducting ceramics, are indicated. In particular, it is shown that thermochemical
analytical studies must accompany phase-diagram studies in order to make an investigation of the phase-equilibrium behavior of such a system complete. The conditions pertaining to oxygen stoichiometry control in the preparation of the copper oxide-based superconductors are described.
It is an interesting observation [1] that the history of the development of superconducting materials runs in parallel with the numerical order of increasing complexity of multicomponent material systems. Thus, superconductivity was first discovered in the unary system, Hg, then in other unary systems shortly thereafter. After that came the binary alloys, such as those of the A15 structure, then ternanes, such as Li—Ti--O. The era of the quaternaries perhaps started with Ba(Pb, Bi)O3, with La1 ~Ba~CuO4 and YBa2Cu3O7 providing its climax. Recently, there have been efforts to search for superconductors with higher transition temperatures among quinary systems that are not too far removed from, but are in fact derivative forms of, YBa2Cu3O7 ~ as is exemplified by the substitution of F for some 02 at higher concentrations than are generally regarded as doping levels of concentrations [2,31. With the unary and the binary-alloy systems there were no particular difficulties in preparing the pure superconducting phase. The problem of preparing the superconducting phase in pure form must have begun with the Li—Ti 0 spinels of the composition, Li1~Ti2±y°4’ y varying within the range, 0.33
redox nature of the atmosphere under which it was prepared. In other words, the question of nonstoichiometry perhaps even the necessity for it had entered the superconductivity scene at that time and deserved careful consideration. In more specific terms, the material system, Li Ti 0, had to be regarded as a ternary system, and, because x could not be placed under as close quantitative control as y, only one out of a possible two concentration variables could be held constant. Therefore, even if the temperature and pressure could be held constant during the preparation of the material, it would contain no less than two phases in its equilibrium state. That is to say, the preparation of a pure solid phase in the Li Ti 0 system had to be recognized as a near thermodynamic impossibility, the unique exception being that rare if not unlikely situation in which one of the two product phases under the preparation conditions, but not the conditions of application of the material, is itself the equilibrant vapor phase. The same problem exists in the preparation of the quaternary superconducting ceramics that are currently in vogue, as it generally would with any N-component system in which only (N 2) concentration variables can be held constant. If V is the number of thermodynamic variables exclusive of the concentration variables of the system, then
0022-0248/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
—
AC. Pastor, R.C. Pastor / Nonstoichiometry in preparation of high-Ti superconducting ceramics
653
the number of equilibrium phases would be (4 V) as per Gibbs phase rule. It should also be noted that pressure is not constant in a gas flow
material in question is written fully to show the existence region of nonstoichiometry, viz., YBa2Cu3O7 —1
system, and that therefore V 1 in the currently standard procedure of sintering of superconducting ceramics at constant temperature in the penultimate step of their preparation, and it follows that the resultant product of such a process would be tnphasic. On the other hand, the product could be biphasic if the same material were sintered in a bomb. In either case the end product that may be obtained from such a system through thermodynamic processing would be polyphasic. Take the specific example of the preparation of the 90 K superconductor, YBa2Cu3O7 At X 0.5 the entire copper the some material in 2~state, and content for x >of0.5 of isthat the Cuwill become monovalent, i.e., Cut If the copper material should be normally conducting at am-
the magnitude of the problem of obtaining phase purity in a polycrystalline macrosystem becomes more apparent. The range of variability of x also emphasizes the need to recognize the quaternary nature of this material system, as did Schwartzman et al. [9], who started with a tetrahedron with its vertices at Y—Ba Cu 0 as the all-inclusive compositional diagram for the system and subsequently whittled it down to a subtetrahedron enclosing those cornpositions with superconductor relevance. To be consistent with the stoichiometry considerations that have just been outlined, thepreparer compositional range of interest to the materials should be narrowed down to the tetrahedron with its vertices at yo 1 ~ BaO CuO 0. (The set of vertices,because Y015 itBaO CuO CuO15, not aphases good choice cannot account for isvapor that participate in the phase equilibria that come into play during materials preparation. Furthermore, the stability of the phase CuO 1 or Cu203, has not been established.) If now one takes into consideration that each point in the compositional tetrahedron must be characterized by values of the thermodynamic variables, pressure and temperature, before the phase composition corresponding to that point is determined, then it becomes clear that while phase diagrams are indeed essential for
—
=
~.
=
bient temperatures, as this material has been shown 2thosted to be, would the presence the Cu On the lattice make it of an Cu~in n-type conductor. contrary, it has been reported in the literature that the normal conductivity of this material [5], as
~.
~,
well as that of the lower-1~superconductor in the La Ba—Cu 0 system [6], is p-type. Therefore, x 0.5 may be taken as the upper limit of the range of interest of x, as far as the chemical preparation of the material is concerned, and the formula of the 90 K superconductor may be more explicitly written as =
Y(III)Ba(II)2Cu(II)2+2~Cu(III)1
2~O7
At x 0, one third of the copper 3~state. content of It YBa2Cu3O7 would that be in Cu accepted should also be noted the the currently orthorhombic unit cell [7] for the structure of this material, as well as its recently proposed revision [8] is based on this zero value of x or on the nominal formula, YBa 2Cu 307 (rather than YBa2Cu3O9, which does retain the perovskite stoichiometry but cannot be justified by a charge balance). At x 1,state, the entire content 3~ and thecopper Y3~would be would be in the by Cu02 The latter situation is not 10-coordinated favorable in terms of ion size considerations. The lower limit of the range of interest of x may then be taken as 1. Thus, when the formula of the =
—
.
an understanding of such complex systems, phase-diagram considerations alone cannot constitute abehavior sufficient phase-equilibrium andanalysis have toof be their supplemented by thermochemical analytical considerations, such as those described by Gallagher et al. [10]. The final oxidation step in the preparation of high-7~superconducting oxides is a heterogeneous process and occurs at the solid—gas interface. The long periods required for this oxidation do not seem to be the consequence of solid state diffusion because oxidation rateThe persists materials with the highslow specific surface. rate in limitation stems from the dissociation of the oxidant in accordance with the reaction, 0 2(g)
=
2 0(g).
(1)
AC. Pastor, R.C. Pastor / Nonstoichiometry in preparation of high-Ta superconducting ceramics
654
This dissociation requires an input of 120 kcal mol Even with the 02 vibration frequency at 1014 s the mean lifetime of 02 in eq. (1) is 5 x i0~ years. The dissociation must therefore proceed in the adsorbed state. Regardless of whether dissociation takes place in the gaseous or adsorbed state, it is favored by an increase in temperature. However, an upper
[13], the proximity of which makes the oxide ion more negatively ionic. The corresponding oxides for M = Cu show the greatest stability in Group lB. At n = 0 the crossover temperature is above 1400°C,and at n 1 it is 1100°C [12]. The observed value for n = 2 is lower than 400°C [14]. This last value refers to the stability limit of Cu2O3(c). When the latter is
limit on the oxidation temperature is imposed by considerations of thermal stability of the oxidized state, M~l+l, the symbol M representing the polyvalent metal component in the multi-oxide and n (an integer> 0) being its starting valence, Consider the oxidation of M~(c)to M~~’(c),
paired with the oxide of a highly electropositive element such as K (as in KCuO2), the crossover point is placed at just above 500°C [15]. This crossover temperature appears to be close to the realizable limit since potassium is a very electropositive element (i.e., has low electronegativity). Indeed it seems to have been the common experience that 500°C is the upper limit to the oxidation temperature of the various formulations of copper-based high-7~superconducting oxides. From the foregoing it may be concluded of the copper-based that lattice energy, or the energy oxides released in the the oxide transformation (via the Born Haber cycle) from (g) to (c), is more than ample to compensate for the energy deficit between the ionization potential (from M’1 to M~1)and the electron affinity (from 0 to 0 ), guaranteeing the stabilization of various valencies of the copper content of the mixed-oxide material. However, a limitation to the temperature of processing of the material must be observed in order to preserve the oxygen stoichiometry of the desired oxidation macrostate of the multivalent constituent, copper. It is common knowledge that H 20 degrades the superconducting phase, yet air has been considered only for the oxidizing action of its 20% 02 content [16,17] but not for the reducing action of its H20 content. The well-known addition of H20 to the lattice oxide anion is catalyzed by oxide vacancies, V0, as in the reaction
~,
M’~(c)+ 0(c)
=
M~i±I(c) + 0 (c),
(2)
where 0 (c) is the oxide 02 (c) plus a hole. The Born Haber cycle applied to eq. (2) shows that the terms crucial to 1(g), the oxidation are the ionizathe electron affinity of tion potential of M’ 0(g), and the lattice energy to form {M’~1(c)+ 0 (c)} from (M”~1(g)+ 0 (g)}. For Cu2~(g) ionization requires an investment of 850 kcal, and for Ag2~(g) 800 kcal. Electron capture by 0(g) will release only 34 kcal. Therefore, the forward progress of eq. (2) hinges on the energy released in the formation of the lattice, i.e., from (g) to (c). That value is greater than 800 kcal for M3~in the oxides, as is suggested by the trend, 550 kcal for M1 + and 800 kcal for M2~[11]. The instability of M~~’(c),or the tendency towards the reversal of eq. (2), is measurable —
—
—
—
through the amount of 02(g) liberated. The large volume change accompanying the forward direction of eq. (2) means i~S< 0; the reverse direction is favored by an increase in temperature. The constraint on the crossover temperature, the ternperature at which P(02) (A = 1 atm) becomes severe as the atomic weight of M and its valence n increase. Thus, for M Ag and n = 0, P(02) 1 atm at 200°C, but there is no crossover temperature for either n 1, for which P(02) = 4 X 1013 atm at 25°C,or n = 2, for which P(02) = 8 X 1032 atm at 25°C existence of Au[12]. In the case of M = Au the 20 is questionable and Au2O3 is not stable unless it is paired oxide of a highly electropositive metal, with e.g., the as in KAuO 2 —
—
—
—
—‘
(c) + V0 + H20(g) 2 OH (c). (3) OH has such a low oxidation potential that its electronic charge readily transfers to a hole, which may take the form either 0 (02 plus a hole) 3~(Cu2~ plus ofa hole): or Cu 3~(c) 2 OH (c) 2~(c) + 2 Cu +2 V 2 Cu 0 + H2 I + 02 1. (4) 02
—
—
A.C. Pastor, R. C. Pastor / Nonstoichiometry in preparation ofhigh- T, superconducting ceramics
It can only be hoped that the reaction rate of (3) + (4) was sufficiently small to be neglected in the previous studies. A possible solution to the phase purity problem would be to avoid the equilibrium state altogether. This implies that the end product of the preparation process would be dependent on both the overall composition of the material system and the thermochemical pathway through which that systern is guided. Then the phase composition of the end product, which itself need not be in an equilibrium state, would be history-dependent. In such a dynamic process the homogenization of the solid oxide mixture to the molecular level before the sintering operation would be essential, and any phase transition that would result in phase partitioning subsequent that would have to be avoided.
[4] D.C. Johnston, H. Prakash, W.H. Zachariasen and R. Viswanathan, Mater. Res. Bull. 8 (1973) 777. [5] Duan Hong-Mm, Wang Xie-Mei, Lin Shu-Yuan and
Zhang Dian-Lin (Institute of Physics, Chinese Academy of Sciences, Beijing, China), to be published. [61 J.T. Chen, C.J. McEwan, L.E. Wenger and E.M. Logothetis, Phys. Rev. B35 (1987) 7124. [7] See, e.g., J.E. Greedan, A.H. O’Reilly and C.V. Stager, Phys. Rev. B35 (1987) 8770. [8] A. Reller, J.G. Bednorz and K.A. Muller, to be published. [9] A.F. Schwartzman, D.C. Paine and R. Sinclair, Department of Materials Science and Engineering, Stanford University, preprint. [10] P.K. Gallagher, H.M. O’Bryan, S.A. Sunshine and D.W. Murphy, Mater. Res. Bull. 22 (1987) 995. [11] M.C. Ball and A.H. Norburv. Physical Data for Inorganic 1974). Thermodynamic Properties of [12] Chemists C.E. Wicks(Longman, and F.E. Block, 65 Elements
[13]
References [1] K.C. Lim, Hughes Research Laboratories, private cornmunication. [2] C. Politis, reported at the Spring Meeting of the Materials Research Society in Anaheim, CA, 1987. [3] S.R. Ovshinsky, R.T. Young, D.D. Allred, G. DeMaggio and G.A. van der Leeden, Phys. Rev. Letters 58 (1987) 2579.
655
[14] [15]
[16] [17]
Their Oxides, Halides, Carbides, and
Nitrides, Bulletin 605, Bureau of Mines (US Government Printing Office, Washington, DC, 1963). J. Kleinberg, Ed., Treatise on Inorganic Chemistry, Vol. II (Elsevier, Amsterdam, 1956) pp. 416—418. J. Kleinberg, Unfamiliar Oxidation States and Their Stabilization (University of Kansas Press, 1950) p. 60. G. Brauer, Ed., Handbook of Preparative Inorganic Chemistry, Vol. II, 2nd ed. (Academic Press, New York, 1965) p. 1014. P.K. Gallagher, Advan. Cerarn. Mater. 2 (1987) 632. H.M. O’Bryan and P.K. Gallagher, Advan. Cerarn. Mater. 2 (1987) 640.
Journal of Crystal Growth 85 (1987) 652—655 North-Holland, Amsterdam
THE PROBLEM OF NONSTOICHIOMETRY IN THE CHEMICAL PREPARATION OF HIGH-TC SUPERCONDUCTING CERAMICS A.C. PASTOR and R.C. PASTOR Hughes Research Laboratories, Malibu, California 90265. USA
Received 12 August 1987; manuscript received in final form 17 September 1987
The iiiherent difficulties in prepanng a single-phase oxide material from a system wherein the problem of oxygen nonstoichiometry is resident, as in the case of the high -7~superconducting ceramics, are indicated. In particular, it is shown that thermochemical
analytical studies must accompany phase-diagram studies in order to make an investigation of the phase-equilibrium behavior of such a system complete. The conditions pertaining to oxygen stoichiometry control in the preparation of the copper oxide-based superconductors are described.
It is an interesting observation [1] that the history of the development of superconducting materials runs in parallel with the numerical order of increasing complexity of multicomponent material systems. Thus, superconductivity was first discovered in the unary system, Hg, then in other unary systems shortly thereafter. After that came the binary alloys, such as those of the A15 structure, then ternanes, such as Li—Ti--O. The era of the quaternaries perhaps started with Ba(Pb, Bi)O3, with La1 ~Ba~CuO4 and YBa2Cu3O7 providing its climax. Recently, there have been efforts to search for superconductors with higher transition temperatures among quinary systems that are not too far removed from, but are in fact derivative forms of, YBa2Cu3O7 ~ as is exemplified by the substitution of F for some 02 at higher concentrations than are generally regarded as doping levels of concentrations [2,31. With the unary and the binary-alloy systems there were no particular difficulties in preparing the pure superconducting phase. The problem of preparing the superconducting phase in pure form must have begun with the Li—Ti 0 spinels of the composition, Li1~Ti2±y°4’ y varying within the range, 0.33
redox nature of the atmosphere under which it was prepared. In other words, the question of nonstoichiometry perhaps even the necessity for it had entered the superconductivity scene at that time and deserved careful consideration. In more specific terms, the material system, Li Ti 0, had to be regarded as a ternary system, and, because x could not be placed under as close quantitative control as y, only one out of a possible two concentration variables could be held constant. Therefore, even if the temperature and pressure could be held constant during the preparation of the material, it would contain no less than two phases in its equilibrium state. That is to say, the preparation of a pure solid phase in the Li Ti 0 system had to be recognized as a near thermodynamic impossibility, the unique exception being that rare if not unlikely situation in which one of the two product phases under the preparation conditions, but not the conditions of application of the material, is itself the equilibrant vapor phase. The same problem exists in the preparation of the quaternary superconducting ceramics that are currently in vogue, as it generally would with any N-component system in which only (N 2) concentration variables can be held constant. If V is the number of thermodynamic variables exclusive of the concentration variables of the system, then
0022-0248/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
—
AC. Pastor, R.C. Pastor / Nonstoichiometry in preparation of high-Ti superconducting ceramics
653
the number of equilibrium phases would be (4 V) as per Gibbs phase rule. It should also be noted that pressure is not constant in a gas flow
material in question is written fully to show the existence region of nonstoichiometry, viz., YBa2Cu3O7 —1
system, and that therefore V 1 in the currently standard procedure of sintering of superconducting ceramics at constant temperature in the penultimate step of their preparation, and it follows that the resultant product of such a process would be tnphasic. On the other hand, the product could be biphasic if the same material were sintered in a bomb. In either case the end product that may be obtained from such a system through thermodynamic processing would be polyphasic. Take the specific example of the preparation of the 90 K superconductor, YBa2Cu3O7 At X 0.5 the entire copper the some material in 2~state, and content for x >of0.5 of isthat the Cuwill become monovalent, i.e., Cut If the copper material should be normally conducting at am-
the magnitude of the problem of obtaining phase purity in a polycrystalline macrosystem becomes more apparent. The range of variability of x also emphasizes the need to recognize the quaternary nature of this material system, as did Schwartzman et al. [9], who started with a tetrahedron with its vertices at Y—Ba Cu 0 as the all-inclusive compositional diagram for the system and subsequently whittled it down to a subtetrahedron enclosing those cornpositions with superconductor relevance. To be consistent with the stoichiometry considerations that have just been outlined, thepreparer compositional range of interest to the materials should be narrowed down to the tetrahedron with its vertices at yo 1 ~ BaO CuO 0. (The set of vertices,because Y015 itBaO CuO CuO15, not aphases good choice cannot account for isvapor that participate in the phase equilibria that come into play during materials preparation. Furthermore, the stability of the phase CuO 1 or Cu203, has not been established.) If now one takes into consideration that each point in the compositional tetrahedron must be characterized by values of the thermodynamic variables, pressure and temperature, before the phase composition corresponding to that point is determined, then it becomes clear that while phase diagrams are indeed essential for
—
=
~.
=
bient temperatures, as this material has been shown 2thosted to be, would the presence the Cu On the lattice make it of an Cu~in n-type conductor. contrary, it has been reported in the literature that the normal conductivity of this material [5], as
~.
~,
well as that of the lower-1~superconductor in the La Ba—Cu 0 system [6], is p-type. Therefore, x 0.5 may be taken as the upper limit of the range of interest of x, as far as the chemical preparation of the material is concerned, and the formula of the 90 K superconductor may be more explicitly written as =
Y(III)Ba(II)2Cu(II)2+2~Cu(III)1
2~O7
At x 0, one third of the copper 3~state. content of It YBa2Cu3O7 would that be in Cu accepted should also be noted the the currently orthorhombic unit cell [7] for the structure of this material, as well as its recently proposed revision [8] is based on this zero value of x or on the nominal formula, YBa 2Cu 307 (rather than YBa2Cu3O9, which does retain the perovskite stoichiometry but cannot be justified by a charge balance). At x 1,state, the entire content 3~ and thecopper Y3~would be would be in the by Cu02 The latter situation is not 10-coordinated favorable in terms of ion size considerations. The lower limit of the range of interest of x may then be taken as 1. Thus, when the formula of the =
—
.
an understanding of such complex systems, phase-diagram considerations alone cannot constitute abehavior sufficient phase-equilibrium andanalysis have toof be their supplemented by thermochemical analytical considerations, such as those described by Gallagher et al. [10]. The final oxidation step in the preparation of high-7~superconducting oxides is a heterogeneous process and occurs at the solid—gas interface. The long periods required for this oxidation do not seem to be the consequence of solid state diffusion because oxidation rateThe persists materials with the highslow specific surface. rate in limitation stems from the dissociation of the oxidant in accordance with the reaction, 0 2(g)
=
2 0(g).
(1)
AC. Pastor, R.C. Pastor / Nonstoichiometry in preparation of high-Ta superconducting ceramics
654
This dissociation requires an input of 120 kcal mol Even with the 02 vibration frequency at 1014 s the mean lifetime of 02 in eq. (1) is 5 x i0~ years. The dissociation must therefore proceed in the adsorbed state. Regardless of whether dissociation takes place in the gaseous or adsorbed state, it is favored by an increase in temperature. However, an upper
[13], the proximity of which makes the oxide ion more negatively ionic. The corresponding oxides for M = Cu show the greatest stability in Group lB. At n = 0 the crossover temperature is above 1400°C,and at n 1 it is 1100°C [12]. The observed value for n = 2 is lower than 400°C [14]. This last value refers to the stability limit of Cu2O3(c). When the latter is
limit on the oxidation temperature is imposed by considerations of thermal stability of the oxidized state, M~l+l, the symbol M representing the polyvalent metal component in the multi-oxide and n (an integer> 0) being its starting valence, Consider the oxidation of M~(c)to M~~’(c),
paired with the oxide of a highly electropositive element such as K (as in KCuO2), the crossover point is placed at just above 500°C [15]. This crossover temperature appears to be close to the realizable limit since potassium is a very electropositive element (i.e., has low electronegativity). Indeed it seems to have been the common experience that 500°C is the upper limit to the oxidation temperature of the various formulations of copper-based high-7~superconducting oxides. From the foregoing it may be concluded of the copper-based that lattice energy, or the energy oxides released in the the oxide transformation (via the Born Haber cycle) from (g) to (c), is more than ample to compensate for the energy deficit between the ionization potential (from M’1 to M~1)and the electron affinity (from 0 to 0 ), guaranteeing the stabilization of various valencies of the copper content of the mixed-oxide material. However, a limitation to the temperature of processing of the material must be observed in order to preserve the oxygen stoichiometry of the desired oxidation macrostate of the multivalent constituent, copper. It is common knowledge that H 20 degrades the superconducting phase, yet air has been considered only for the oxidizing action of its 20% 02 content [16,17] but not for the reducing action of its H20 content. The well-known addition of H20 to the lattice oxide anion is catalyzed by oxide vacancies, V0, as in the reaction
~,
M’~(c)+ 0(c)
=
M~i±I(c) + 0 (c),
(2)
where 0 (c) is the oxide 02 (c) plus a hole. The Born Haber cycle applied to eq. (2) shows that the terms crucial to 1(g), the oxidation are the ionizathe electron affinity of tion potential of M’ 0(g), and the lattice energy to form {M’~1(c)+ 0 (c)} from (M”~1(g)+ 0 (g)}. For Cu2~(g) ionization requires an investment of 850 kcal, and for Ag2~(g) 800 kcal. Electron capture by 0(g) will release only 34 kcal. Therefore, the forward progress of eq. (2) hinges on the energy released in the formation of the lattice, i.e., from (g) to (c). That value is greater than 800 kcal for M3~in the oxides, as is suggested by the trend, 550 kcal for M1 + and 800 kcal for M2~[11]. The instability of M~~’(c),or the tendency towards the reversal of eq. (2), is measurable —
—
—
—
through the amount of 02(g) liberated. The large volume change accompanying the forward direction of eq. (2) means i~S< 0; the reverse direction is favored by an increase in temperature. The constraint on the crossover temperature, the ternperature at which P(02) (A = 1 atm) becomes severe as the atomic weight of M and its valence n increase. Thus, for M Ag and n = 0, P(02) 1 atm at 200°C, but there is no crossover temperature for either n 1, for which P(02) = 4 X 1013 atm at 25°C,or n = 2, for which P(02) = 8 X 1032 atm at 25°C existence of Au[12]. In the case of M = Au the 20 is questionable and Au2O3 is not stable unless it is paired oxide of a highly electropositive metal, with e.g., the as in KAuO 2 —
—
—
—
—‘
(c) + V0 + H20(g) 2 OH (c). (3) OH has such a low oxidation potential that its electronic charge readily transfers to a hole, which may take the form either 0 (02 plus a hole) 3~(Cu2~ plus ofa hole): or Cu 3~(c) 2 OH (c) 2~(c) + 2 Cu +2 V 2 Cu 0 + H2 I + 02 1. (4) 02
—
—
A.C. Pastor, R. C. Pastor / Nonstoichiometry in preparation ofhigh- T, superconducting ceramics
It can only be hoped that the reaction rate of (3) + (4) was sufficiently small to be neglected in the previous studies. A possible solution to the phase purity problem would be to avoid the equilibrium state altogether. This implies that the end product of the preparation process would be dependent on both the overall composition of the material system and the thermochemical pathway through which that systern is guided. Then the phase composition of the end product, which itself need not be in an equilibrium state, would be history-dependent. In such a dynamic process the homogenization of the solid oxide mixture to the molecular level before the sintering operation would be essential, and any phase transition that would result in phase partitioning subsequent that would have to be avoided.
[4] D.C. Johnston, H. Prakash, W.H. Zachariasen and R. Viswanathan, Mater. Res. Bull. 8 (1973) 777. [5] Duan Hong-Mm, Wang Xie-Mei, Lin Shu-Yuan and
Zhang Dian-Lin (Institute of Physics, Chinese Academy of Sciences, Beijing, China), to be published. [61 J.T. Chen, C.J. McEwan, L.E. Wenger and E.M. Logothetis, Phys. Rev. B35 (1987) 7124. [7] See, e.g., J.E. Greedan, A.H. O’Reilly and C.V. Stager, Phys. Rev. B35 (1987) 8770. [8] A. Reller, J.G. Bednorz and K.A. Muller, to be published. [9] A.F. Schwartzman, D.C. Paine and R. Sinclair, Department of Materials Science and Engineering, Stanford University, preprint. [10] P.K. Gallagher, H.M. O’Bryan, S.A. Sunshine and D.W. Murphy, Mater. Res. Bull. 22 (1987) 995. [11] M.C. Ball and A.H. Norburv. Physical Data for Inorganic 1974). Thermodynamic Properties of [12] Chemists C.E. Wicks(Longman, and F.E. Block, 65 Elements
[13]
References [1] K.C. Lim, Hughes Research Laboratories, private cornmunication. [2] C. Politis, reported at the Spring Meeting of the Materials Research Society in Anaheim, CA, 1987. [3] S.R. Ovshinsky, R.T. Young, D.D. Allred, G. DeMaggio and G.A. van der Leeden, Phys. Rev. Letters 58 (1987) 2579.
655
[14] [15]
[16] [17]
Their Oxides, Halides, Carbides, and
Nitrides, Bulletin 605, Bureau of Mines (US Government Printing Office, Washington, DC, 1963). J. Kleinberg, Ed., Treatise on Inorganic Chemistry, Vol. II (Elsevier, Amsterdam, 1956) pp. 416—418. J. Kleinberg, Unfamiliar Oxidation States and Their Stabilization (University of Kansas Press, 1950) p. 60. G. Brauer, Ed., Handbook of Preparative Inorganic Chemistry, Vol. II, 2nd ed. (Academic Press, New York, 1965) p. 1014. P.K. Gallagher, Advan. Cerarn. Mater. 2 (1987) 632. H.M. O’Bryan and P.K. Gallagher, Advan. Cerarn. Mater. 2 (1987) 640.