- Email: [email protected]
Subsolidus phase relationships in the SrO-CaO-CuO-O and SrO-Y2o3-CuO-O systems
Material Chemistry and Physics, 22
SUBSOLIDUS
(1989) 523-529
PHASE RELATIONSHIPS
IN THE
523
SrO-CaO-CuO-0 and SrO-Y$3-CuO-0
SYSTEMS
M.
VALLINO,
D.
MAZZA,
F.
ABBATTISTA,
C.
BRISI
and M. LUCCO-BORLERA
Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Turin (Italy)
Received December 7,1988; accepted January 11.1989
ABSTRACT The solid-state equilibrium relationships were investigated in the SrO-CaOCuO-0 and SrO-Y%03-CuO-0 systems at 900°C in air. The first system is characterized by three wide solid solution series. One stretches from the compound Sr%Cu03 to the compound CazCuO3. The other ones originate from the compounds SrCu02 and Srl4Cu24041 respectively, by substitution of calcium for strontium up to a limit of 70 at.% in the first case and of slightly more than 50 at.% in the
second. In this system a ternary phase is also present, whose composition lies between (SrD,88CaU_l%)CuO2and (SrC_85CaC.l5)CuO%. In the system SrO-Y%03-CuO-0 no proper ternary phases originate. In the compounds Srl4Cu24041 yttrium can sustitute for strontium up to a limit of about 30 at.%.
INTRODUCTION
In the last two years several new materials with high T, superconductivity have been discovered. All these substances are complex metal oxides always containing copper and one or more alkaline earth metals, joined with different elements such rare-earth, bismuth, thallium,etc. After an initial period, during which research was restricted to the investigation of the physical properties of single solids of composition and structure often not precisely determined, the requirement of systematic studies starting from the knowledge of the various oxide systems (firstly the binary ones and then the ternary and quaternary) grew more and more necessary. On the other hand the results obtained by studying these systems,besidesthe present great interest for superconductingmaterials, also form a patrimony of knowledge very important for other possible future applications. 0254-0584/89/33.50
0 Elsevier Sequoia/Printed
in The Netherlands
With this aim we are working throught a research program of new phases and structures in different systems. In this paper we report the results of a study of the solid state equilibrium relationships in the systems SrO-CaO-CuO-0 and SrO-Y203-CuO-0 at 900°C. The binary system CaO-CuO was studied by Gadalla and White 111, who evidenced only one intermediate compound Ca2CuO3, already reported by Schmahl and Minzl [21. In a further communication Teske and Miiller-Buschbaum[3] described a second compound CaCu203 which is stable only above 950°C I41 and therefore it is extraneous to our investigations,which refer to 900°C. In the SrO-CuO system the existence of the compounds Sr2CuO3 [5] and SrCu02 [6] has been known for a long time. In a very recent paper, McCarron et al. [7] describe the structural features of a phase to which the theoretical composition Srl4Cu24041 was assigned. It was obtained by melting mixtures of Sr02 , CuO and CaC03. This phase contains some trivalent copper, therefore it is extraneous to the true binary system SrO-CuO. It is nevertheless stable in the presence of air at 900°C and so it is to be considered in our study of phase relationships. The compounds Sr2Cu03 and Ca2Cu03 are isomorphous, their structure deriving from the perovskite layered structure of the K2NiF4 type by withdrawing two oxygen atoms per unit cell from the positions 0 l/2 0
and l/2 0 l/2. The Cu2+
ions result in a planar fourfold coordination with oxygen, whilst at the same time the lattice transforms its symmetry from tetragonal (layered perovskites A2B04) to orthorhombic. In the system CaO-SrO a complete solid solution forms, as evidenced by X-ray diffraction by Hubert and Wagener [81. The binary system SrO-Y203 was investigated by Tresvyatskii et a1.[9]. Only one intermediate compound forms in the temperature range of our interest, namely SrY204, with a structure related to that of monocalcium ferrite. In the CUO-Y2O3 system only one compound was evidenced in the literature, with formula Y2Cu2O5 [lo].
EXPERIMKNTAL The solids examined in the course of the present research were obtained starting from mixtures of SrC03, CaC03 or Y2O3 and CuO. After a short heat treatment at 900°C to decompose at least partially the carbonates, the solids were pressed into tablets and fired in air in silver vessels, with intermediate grindings, for about 100 hours at 900°C. The samples with molar ratios (CaO+SrO)/CuOhigher than two were fired at 1050°C in platinum vessels. After the heat treatment the samples were quickly cooled to room temperature and analysed both analytically and by X-ray diffraction.
525
RESULTS AND DISCUSSION System SrO-CaO-CuO-0 The equilibrium relationships valid for samples air-quenched from 900°C are shown schematically in the diagram of Fig.1.
It has to be noted that this is
not a true ternary diagram, as it concerns phases containing trivalent copper as well, whose composition can not be expressed by the relative amount of the three corner-compounds.
Co0
Fig. 1. Subsolidus phase relationships at 900°C of the Sr-Ca-Cu-0 system.
The isostructural compounds Sr2Cu05 and Ca2CuOS are connected by a complete solid solution line. The unit cell parameter are nearly linearly dependent on the composition, as shown in Fig.2. The region characterized by a molar ratio, alkaline earth oxide/copper oxide higher than two ("1 is occupied by a biphasic field in which the solid solutions (Sr,Ca)2Cu05 are in equilibrium with the series of mixed crystals stretching from CaO to SrO. Due to the higher basicity of SrO the joints connecting the two series of solid solutions are strongly biased toward CaO.
(") The firing in this region was carried at lOSO'C, as already noticed.
526
EO-
4.0 -
_
a,
-
~
3.5+
3.0
I
sr,cuo,
25
I 50 mol. %
I 75
c+CUO~
Fig.2. Lattice parameters for (Sr,Ca)2Cu03 solid solution. The unit cell is reoriented in order to better show its relationships with the perovskite-layered structure of K2NiP4-type.
As a matter of fact, for example, the equimolar solid solution between Sr2CuO3 and Ca2CuO3 results in equilibrium with a term of the CaO-SrO solid solution containing 5% of SrO. Considering now the solids with a molar ratio (SrO+CaO)/CuO= 1, we find a continous solid solution starting from SrCu02 by gradual substitution of Ca for Sr up to the composition (SrC_3OCaC.T9)CuO2for which the unit cell dimensions were measured as aoa3.394, boE3.853 and c0=16.02 A.
They can be compared with
ao=3.565, bo=3.912 and ~~16.33 h for the pure compound SrCuO2 [6]. The limit solid solution results in equilibrium with the ternary phase recently reported by Siegrist et al.[ll]. who assign to it the formula (Ca9,86SrC.l4&02.
This phase shows a structure deriving from a simple perovskitic
lattice, with oxygen vacancies. Our iodometric titrations confirmed that copper is present in this phase only in its bivalent state. It shows however a small compositional field, by varying the calcium atomic percent, on the (Ca+Sr) total amount, from 85 to 88%. This is confirmed by a corresponding variation in the unit cell constant from aor3.867 and c0=3.218 to a0=3.864 and c-=3.210 A.
527
Sr Y2 04 Fig. 3. Subsolidus phase relatioships at 900°C of the Sr-Y-Cu-0 system.
Further increasing the CaO percent one enters a triphasic field in which the phase (CaC_ggSrC.l2)Cu02coexists with CuO and the (SrO.C2CaC.gg)2Cu03term of the solid solutions (Sr,Ca)2CuO3. The region of the system with molar ratio. alkaline-earth oxide/copper oxide, lower than one is characterized by the presence of
the compound Srl4Cu24041. It
is a phase of orthorhombic symmetry with a complex structure, based on a stacking of 0~02 and Sr2Cu203 groups, repeated along c with two different periodicities, of 2.749 and 3.931 A, which can not find a perfectly congruent superstructure. The reported formula has so to be considered as indicative only. By firing in air at 9OO"C, oxide mixtures with atomic ratio Sr/Cu equal to 7/12 we obtained anyhow practically monophasic solids, while even small variations from such a value caused the formation of CuO or SrCu02 (").
The iodome-
tric analysis of the samples confirmed for copper an average oxidation number slightly lower than 2.25. In the phase Srl4Cu24041, as evidenced also by McCarron et a1.[71, it is possible to substitute strontium by calcium up to a percent slightly higher than
(") Our research, practically concluded before we knew of the paper of McCarron et a1.[71, brought us initially to fix for the named phase a Sr/Cu ratio equal to 417, very near to 7112. Other literature references to SrCu203 [12] and Sr2Cu30, [13] are likely to be considered as approximate compositions of the same phase Srl4Cu24041.
50% atomic. The limit solid solution resulted in equilibrium with copper oxide and with the term (Sr0,6gCaOV32)Cu02of the solid solution series deriving SrCu02. With still higher CaO content two other triphasic regions follow, separated by narrow biphasic zones. The first one is limited by copper oxide, the phase (SrO.g5CaO.l5)Cu02and the limit solid solution (Sr,Ca)Cu02 ; the second one by CuO, (Srg.l2Cao_gg)CuO2and nearly unsubstituted Ca2Cu03.
System SrO-YoO?-CuO-0 The solid state compatibility relationships are shown in the diagram of Pig.3, referring to samples quenched in air from 900°C. In this case also we are not dealing with a true ternary system, as again phases arise containing Cu in an oxidation state higher than two. The diagram is relatively simple: the compound SrY204 is in equilibrium with both Sr2CuO3 and SrCu02 and with Y2Cu2O5. None of the above compounds gives rise to noticeable solid state solubility. The situation is different for what concerns the
Srl4Cu24041 phase. In this case the yttrium atoms can substitute
for a maximum of about 30 4 of strontium atoms (">. This phenomenon is allowed clearly by the presence of trivalent copper in the starting composition: the substitution of one Y3+ ion for one Sr2+ is therefore accompanied by the contemporary oxidation of one Cu2+ ion to Cu3+, without variations in the total amount of oxygen and possible collapse of the structure. This process is confirmed by the data of chemical analysis which yielded for copper in the limit solid solution an average oxidation number of t2.07 (which corresponds to the formula Srg,gY4_2Cu240,41). The limit solid solution is in equilibrium with both Y2Cu2O5 and SrY204. Its unit cell parameters were measured as ao~11.333, bo312.927 and co3.947 A (the cell parameter corresponding to the periodicity of cz3.9 A is only measurable on the powder diffraction patterns).
(') Recent research [14] on a single crystal of the series (Sr,Y)14Cu24041, obtained by crystallizationof liquid at temperatures surely higher than that (9OOOC) to which our data are referring, showed that the limit of solid solubility corresponds to the notation SrgY&u24041, with about 43 at.2 of Sr substituted by yttrium.
529
REFERENCES 1
A.M.M.
Gadalla and Y.White, Trans. Brit. Ceram. Sot., 65
(1966)181.
2
N.G.Schmahl and E.Minzl, Z. physik. Chew
3
C.L.Teske and H. Miiller-Buschbaum. Z. anorg. ally. Chemie.370 (1969) 134.
47 (1965) 358.
4
JCPDS Intern. Centre Diffract. Data 34-284 (1984)
5
C.L.Teske and H. Miiller-Buschbaum, Z. anorg. allg. Chemie,371 (1969) 325.
6
C.L.Teske and H. Miiller-Buschbawn, Z. anorg. ally. Chemie,379 (1970) 234.
7
E.M. McCarron, M.A. Subramanian, Y.C. Calabrese and R.L. Harlow, Mat. Res. Bull.,23(1988) 1355.
8
H.Hubert and S.Wagener, Z. techn. Phys.,23 (1942) 1.
9
S.G.Tresvyatskii,L.M.Lopato, A.E.Kushchevskii and A.V.Shevchenko, Izv. Akad. Nauk SSSR, Neorg. Mater.,7 (1971) 1808.
10 JCPDS Inter. Centre Diffract. Data 33-511 (1983). 11 T.Siegrist. S.M.Zahurak, D.W.Murphy and R.S.Roth, Nature 334
(1988) 231.
12 J.B.Torrance, Y.Tokura and A.Nazzal, Chemtronics,2 (1987) 120. 13 J.Hahn, T.O.Mason, S.J.Hwu and K.R. Poeppelmeier, Chemtronics,2 (1987) 126. 14 T.Siegrist, L.F.Schneemeyr, S.A. Sunshine, J.V. Waszczak and R.S.Roth Mat. Res. Bull., 23 (1988) 1429.
SUBSOLIDUS
(1989) 523-529
PHASE RELATIONSHIPS
IN THE
523
SrO-CaO-CuO-0 and SrO-Y$3-CuO-0
SYSTEMS
M.
VALLINO,
D.
MAZZA,
F.
ABBATTISTA,
C.
BRISI
and M. LUCCO-BORLERA
Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Turin (Italy)
Received December 7,1988; accepted January 11.1989
ABSTRACT The solid-state equilibrium relationships were investigated in the SrO-CaOCuO-0 and SrO-Y%03-CuO-0 systems at 900°C in air. The first system is characterized by three wide solid solution series. One stretches from the compound Sr%Cu03 to the compound CazCuO3. The other ones originate from the compounds SrCu02 and Srl4Cu24041 respectively, by substitution of calcium for strontium up to a limit of 70 at.% in the first case and of slightly more than 50 at.% in the
second. In this system a ternary phase is also present, whose composition lies between (SrD,88CaU_l%)CuO2and (SrC_85CaC.l5)CuO%. In the system SrO-Y%03-CuO-0 no proper ternary phases originate. In the compounds Srl4Cu24041 yttrium can sustitute for strontium up to a limit of about 30 at.%.
INTRODUCTION
In the last two years several new materials with high T, superconductivity have been discovered. All these substances are complex metal oxides always containing copper and one or more alkaline earth metals, joined with different elements such rare-earth, bismuth, thallium,etc. After an initial period, during which research was restricted to the investigation of the physical properties of single solids of composition and structure often not precisely determined, the requirement of systematic studies starting from the knowledge of the various oxide systems (firstly the binary ones and then the ternary and quaternary) grew more and more necessary. On the other hand the results obtained by studying these systems,besidesthe present great interest for superconductingmaterials, also form a patrimony of knowledge very important for other possible future applications. 0254-0584/89/33.50
0 Elsevier Sequoia/Printed
in The Netherlands
With this aim we are working throught a research program of new phases and structures in different systems. In this paper we report the results of a study of the solid state equilibrium relationships in the systems SrO-CaO-CuO-0 and SrO-Y203-CuO-0 at 900°C. The binary system CaO-CuO was studied by Gadalla and White 111, who evidenced only one intermediate compound Ca2CuO3, already reported by Schmahl and Minzl [21. In a further communication Teske and Miiller-Buschbaum[3] described a second compound CaCu203 which is stable only above 950°C I41 and therefore it is extraneous to our investigations,which refer to 900°C. In the SrO-CuO system the existence of the compounds Sr2CuO3 [5] and SrCu02 [6] has been known for a long time. In a very recent paper, McCarron et al. [7] describe the structural features of a phase to which the theoretical composition Srl4Cu24041 was assigned. It was obtained by melting mixtures of Sr02 , CuO and CaC03. This phase contains some trivalent copper, therefore it is extraneous to the true binary system SrO-CuO. It is nevertheless stable in the presence of air at 900°C and so it is to be considered in our study of phase relationships. The compounds Sr2Cu03 and Ca2Cu03 are isomorphous, their structure deriving from the perovskite layered structure of the K2NiF4 type by withdrawing two oxygen atoms per unit cell from the positions 0 l/2 0
and l/2 0 l/2. The Cu2+
ions result in a planar fourfold coordination with oxygen, whilst at the same time the lattice transforms its symmetry from tetragonal (layered perovskites A2B04) to orthorhombic. In the system CaO-SrO a complete solid solution forms, as evidenced by X-ray diffraction by Hubert and Wagener [81. The binary system SrO-Y203 was investigated by Tresvyatskii et a1.[9]. Only one intermediate compound forms in the temperature range of our interest, namely SrY204, with a structure related to that of monocalcium ferrite. In the CUO-Y2O3 system only one compound was evidenced in the literature, with formula Y2Cu2O5 [lo].
EXPERIMKNTAL The solids examined in the course of the present research were obtained starting from mixtures of SrC03, CaC03 or Y2O3 and CuO. After a short heat treatment at 900°C to decompose at least partially the carbonates, the solids were pressed into tablets and fired in air in silver vessels, with intermediate grindings, for about 100 hours at 900°C. The samples with molar ratios (CaO+SrO)/CuOhigher than two were fired at 1050°C in platinum vessels. After the heat treatment the samples were quickly cooled to room temperature and analysed both analytically and by X-ray diffraction.
525
RESULTS AND DISCUSSION System SrO-CaO-CuO-0 The equilibrium relationships valid for samples air-quenched from 900°C are shown schematically in the diagram of Fig.1.
It has to be noted that this is
not a true ternary diagram, as it concerns phases containing trivalent copper as well, whose composition can not be expressed by the relative amount of the three corner-compounds.
Co0
Fig. 1. Subsolidus phase relationships at 900°C of the Sr-Ca-Cu-0 system.
The isostructural compounds Sr2Cu05 and Ca2CuOS are connected by a complete solid solution line. The unit cell parameter are nearly linearly dependent on the composition, as shown in Fig.2. The region characterized by a molar ratio, alkaline earth oxide/copper oxide higher than two ("1 is occupied by a biphasic field in which the solid solutions (Sr,Ca)2Cu05 are in equilibrium with the series of mixed crystals stretching from CaO to SrO. Due to the higher basicity of SrO the joints connecting the two series of solid solutions are strongly biased toward CaO.
(") The firing in this region was carried at lOSO'C, as already noticed.
526
EO-
4.0 -
_
a,
-
~
3.5+
3.0
I
sr,cuo,
25
I 50 mol. %
I 75
c+CUO~
Fig.2. Lattice parameters for (Sr,Ca)2Cu03 solid solution. The unit cell is reoriented in order to better show its relationships with the perovskite-layered structure of K2NiP4-type.
As a matter of fact, for example, the equimolar solid solution between Sr2CuO3 and Ca2CuO3 results in equilibrium with a term of the CaO-SrO solid solution containing 5% of SrO. Considering now the solids with a molar ratio (SrO+CaO)/CuO= 1, we find a continous solid solution starting from SrCu02 by gradual substitution of Ca for Sr up to the composition (SrC_3OCaC.T9)CuO2for which the unit cell dimensions were measured as aoa3.394, boE3.853 and c0=16.02 A.
They can be compared with
ao=3.565, bo=3.912 and ~~16.33 h for the pure compound SrCuO2 [6]. The limit solid solution results in equilibrium with the ternary phase recently reported by Siegrist et al.[ll]. who assign to it the formula (Ca9,86SrC.l4&02.
This phase shows a structure deriving from a simple perovskitic
lattice, with oxygen vacancies. Our iodometric titrations confirmed that copper is present in this phase only in its bivalent state. It shows however a small compositional field, by varying the calcium atomic percent, on the (Ca+Sr) total amount, from 85 to 88%. This is confirmed by a corresponding variation in the unit cell constant from aor3.867 and c0=3.218 to a0=3.864 and c-=3.210 A.
527
Sr Y2 04 Fig. 3. Subsolidus phase relatioships at 900°C of the Sr-Y-Cu-0 system.
Further increasing the CaO percent one enters a triphasic field in which the phase (CaC_ggSrC.l2)Cu02coexists with CuO and the (SrO.C2CaC.gg)2Cu03term of the solid solutions (Sr,Ca)2CuO3. The region of the system with molar ratio. alkaline-earth oxide/copper oxide, lower than one is characterized by the presence of
the compound Srl4Cu24041. It
is a phase of orthorhombic symmetry with a complex structure, based on a stacking of 0~02 and Sr2Cu203 groups, repeated along c with two different periodicities, of 2.749 and 3.931 A, which can not find a perfectly congruent superstructure. The reported formula has so to be considered as indicative only. By firing in air at 9OO"C, oxide mixtures with atomic ratio Sr/Cu equal to 7/12 we obtained anyhow practically monophasic solids, while even small variations from such a value caused the formation of CuO or SrCu02 (").
The iodome-
tric analysis of the samples confirmed for copper an average oxidation number slightly lower than 2.25. In the phase Srl4Cu24041, as evidenced also by McCarron et a1.[71, it is possible to substitute strontium by calcium up to a percent slightly higher than
(") Our research, practically concluded before we knew of the paper of McCarron et a1.[71, brought us initially to fix for the named phase a Sr/Cu ratio equal to 417, very near to 7112. Other literature references to SrCu203 [12] and Sr2Cu30, [13] are likely to be considered as approximate compositions of the same phase Srl4Cu24041.
50% atomic. The limit solid solution resulted in equilibrium with copper oxide and with the term (Sr0,6gCaOV32)Cu02of the solid solution series deriving SrCu02. With still higher CaO content two other triphasic regions follow, separated by narrow biphasic zones. The first one is limited by copper oxide, the phase (SrO.g5CaO.l5)Cu02and the limit solid solution (Sr,Ca)Cu02 ; the second one by CuO, (Srg.l2Cao_gg)CuO2and nearly unsubstituted Ca2Cu03.
System SrO-YoO?-CuO-0 The solid state compatibility relationships are shown in the diagram of Pig.3, referring to samples quenched in air from 900°C. In this case also we are not dealing with a true ternary system, as again phases arise containing Cu in an oxidation state higher than two. The diagram is relatively simple: the compound SrY204 is in equilibrium with both Sr2CuO3 and SrCu02 and with Y2Cu2O5. None of the above compounds gives rise to noticeable solid state solubility. The situation is different for what concerns the
Srl4Cu24041 phase. In this case the yttrium atoms can substitute
for a maximum of about 30 4 of strontium atoms (">. This phenomenon is allowed clearly by the presence of trivalent copper in the starting composition: the substitution of one Y3+ ion for one Sr2+ is therefore accompanied by the contemporary oxidation of one Cu2+ ion to Cu3+, without variations in the total amount of oxygen and possible collapse of the structure. This process is confirmed by the data of chemical analysis which yielded for copper in the limit solid solution an average oxidation number of t2.07 (which corresponds to the formula Srg,gY4_2Cu240,41). The limit solid solution is in equilibrium with both Y2Cu2O5 and SrY204. Its unit cell parameters were measured as ao~11.333, bo312.927 and co3.947 A (the cell parameter corresponding to the periodicity of cz3.9 A is only measurable on the powder diffraction patterns).
(') Recent research [14] on a single crystal of the series (Sr,Y)14Cu24041, obtained by crystallizationof liquid at temperatures surely higher than that (9OOOC) to which our data are referring, showed that the limit of solid solubility corresponds to the notation SrgY&u24041, with about 43 at.2 of Sr substituted by yttrium.
529
REFERENCES 1
A.M.M.
Gadalla and Y.White, Trans. Brit. Ceram. Sot., 65
(1966)181.
2
N.G.Schmahl and E.Minzl, Z. physik. Chew
3
C.L.Teske and H. Miiller-Buschbaum. Z. anorg. ally. Chemie.370 (1969) 134.
47 (1965) 358.
4
JCPDS Intern. Centre Diffract. Data 34-284 (1984)
5
C.L.Teske and H. Miiller-Buschbaum, Z. anorg. allg. Chemie,371 (1969) 325.
6
C.L.Teske and H. Miiller-Buschbawn, Z. anorg. ally. Chemie,379 (1970) 234.
7
E.M. McCarron, M.A. Subramanian, Y.C. Calabrese and R.L. Harlow, Mat. Res. Bull.,23(1988) 1355.
8
H.Hubert and S.Wagener, Z. techn. Phys.,23 (1942) 1.
9
S.G.Tresvyatskii,L.M.Lopato, A.E.Kushchevskii and A.V.Shevchenko, Izv. Akad. Nauk SSSR, Neorg. Mater.,7 (1971) 1808.
10 JCPDS Inter. Centre Diffract. Data 33-511 (1983). 11 T.Siegrist. S.M.Zahurak, D.W.Murphy and R.S.Roth, Nature 334
(1988) 231.
12 J.B.Torrance, Y.Tokura and A.Nazzal, Chemtronics,2 (1987) 120. 13 J.Hahn, T.O.Mason, S.J.Hwu and K.R. Poeppelmeier, Chemtronics,2 (1987) 126. 14 T.Siegrist, L.F.Schneemeyr, S.A. Sunshine, J.V. Waszczak and R.S.Roth Mat. Res. Bull., 23 (1988) 1429.