Phase Equilibria and Crystal Chemistry in Portions of the System SrO-CaO-Bi2O3-CuO, Part IV— The System CaO-Bi2O3-CuO

New data are presented on the phase equilibria and crystal chemistry of the binary systems CaO-Bi2O3 and CaO-CuO and the ternary CaO-Bi2O3-CuO. Symmetry data and unit cell dimensions based on single crystal and powder x-ray diffraction measurements are reported for several of the binary CaO-Bi2O3 phases, including corrected compositions for Ca4Bi6O13 and Ca2Bi2O5. The ternary system contains no new ternary phases which can be formed in air at ~700–900 °C.


Introduction
The discovery of superconductivity in cuprates by Bednorz and Miiller [1], and its confirmation by Takagi et al. [2] as being due to the phase La2-;cBaiCu04, led to a world-wide search for other compounds with higher TcS. Identification of the superconducting phase Ba2YCu306+i [3], with a critical temperature Tc -90 K [4], has resulted in hundreds of published reports on the properties of this and related phases.
Phases with still higher TcS were found in the systems SrO-CaO-Bi203-CuO and BaO-CaO-ThOa-CuO [5,6]. These phases belong mostly to a homologous series A2Ca"-iB2Cu"02«+4 (A=Sr, Ba; B = Bi, Tl). In the Bi*^ containing systems a phase with « =2 and Tc -80 K is easily prepared. The exact single-phase region of this phase is not well known, and a structure determination has not been completed because of very strong incommensurate diffraction that is apparently due to a modulation of the Bi positions. Higher n (and higher Tc) phases have not been prepared as single-phase bulk specimens (without PbO). We undertook a comprehensive study of phase equilibria and crystal chemistry in the four component system SrO-CaO-Bi203-CuO in the hope that such a study will define the optimum processing parameters for reproducible synthesis of samples with useful properties.
A prerequisite to understanding the phase equilibria in the four component system is adequate definition of the phase relations in the boundary binary and ternary systems. The ternary system SrO-CaO-CuO was the first to be investigated [7,8], followed by the ternary system SrO-BiaOs-CuO and its binary subsystems [9,10,11,12]. Preliminary versions have been published of the systems CaO-Bi203-CuO and SrO-CaO-BijOs [13], and the details of the system SrO-CaO-Bi203 will appear in the near future [14]. The experimental details. phase relations, and crystal chemistry of the binary CaO-BizOs and the ternary system CaO-BizOs-CuO are the subject of this publication.
In the following discussion of phase equilibria and crystal chemistry, the oxides under consideration will always be given in the order of decreasing ionic radius, largest first, e.g., CaO:l/2Bi203:CuO. The notation l/aBizOa is used so as to keep the metal ratios the same as the oxide ratios. The "shorthand" notation is used to designate the phases with C = CaO, B = l/2Bi203 and Cu = CuO. Thus compositions may be listed simply by numerical ratio e.g., the formula Ca4Bi60i3 can be written as C2B3 or simply 2:3.

Experimental Procedures
In general, about 3.5 g specimens of various compositions in binary and ternary combinations were prepared from CaC03, Bi203 and CuO. Neutron activation analyses of the starting materials indicated that the following impurities (in jxg/g) were present: in CuO-3.9Cr, 2.8Ba, 28Fe, 410Zn, 0.09CO, 1.9Ag, 0.03EU, 14Sb; in Bi203-2.1Cr, 0.0002SC, 26Fe, 21Zn, O.6C0, 0.5Ag, O.OOOSEu, 0.2Sb; in CaCOj-l.lCr, 6Ba, 160Sr, O.OOOlSc, 5Fe, 14Zn, 0.14CO, O.OlAg, O.OOOSEu, 0.02Sb. The constituent chemicals were weighed on an analytical balance to the nearest 0.0001 g and mixed either dry or with acetone in an agate mortar and pestle. The weighed specimen was pressed into a loose pellet in a stainless steel dye and fired on an MgO single crystal plate, or on Au foil, or on a small sacrificial pellet of its own composition. The pellets were then calcined several times at various temperatures from ~600 to 850 °C, with grinding and repelletizing between each heat treatment. Duration of each heat treatment was generally about 16-20 h. For the final examination a small portion of the calcined specimen was refired at the desired temperature (1-8 times), generally overnight, either as a small pellet or in a small 3 mm diameter Au tube, either sealed or unsealed. Too many heat treatments in the Au tube generally resulted in noticeable loss of Cu and/or Hi.
When phase relations involving partial melting were investigated, specimens were contained in 3 mm diameter Au or Pt tubes and heated in a vertical quench furnace. This furnace was heated by six MoSi2 hairpin heating elements with a vertical 4 in diameter Zr02 tube and a 1 in diameter AI2O3 tube acting as insulators. The temperature was measured separately from the controller at a point within approximately 1 cm of the specimen by a Pt/90Ptl0Rh thermocouple, calibrated against the melting points of NaCl (800.5 "C) and Au (1063 "C). After the appropriate heat treatment, the specimen was quenched by being dropped into a Ni crucible, which was cooled by He flowing through a copper tube immersed in liquid N2.
In order to approach equilibrium phase boundaries by different synthesis routes, many specimens were prepared from pre-made compounds or two phase mixtures as well as from end members. These were weighed, mixed, and ground in the same way as for the previously described specimens. Also, some specimens were: 1) annealed at temperature (Ti) and analyzed by x-ray powder diffraction; 2) annealed at a higher or lower temperature {T2) where a different assemblage of phases was observed; 3) returned to Ti to demonstrate reversal of the reaction(s) between Ti and Ti. All experimental details are given in Tables la and lb. Phase identification was made by x-ray powder diffraction using a high angle diffractometer with the specimen packed into a cavity 0.127 or 0.254 mm deep in a glass slide. The diffractometer, equipped with a theta compensator slit and a graphite diffracted beam monochromator, was run at l/4°20/min with CuKa radiation at 40 KV and 35 MA. The radiation was detected by a scintillation counter and solid state amplifier and recorded on a chart with l°/2 0 = 1 in. For purposes of illustration and publication, the diffraction patterns of selected specimens were collected on a computercontrolled, step scanning goniometer and the results plotted in the form presented. Equilibrium in this system has proven to be so difficult to obtain that a few specimens were prepared by utilizing lactic acid in an organic precursor route to obtain more intimate mixing at low temperatures [9]. This procedure yielded an essentially single phase amorphous precursor for the composition that contains 66.7 mol % BizOs. At higher Bi contents, pure Bi metal was formed by carbothermic reduction under even the lowest temperature drying procedures in air.
Specimens for solidus and liquidus determinations in the CaO-CuO system were prepared by dissolving mixtures of cupric nitrate and calcium nitrate in distilled water and then drying. The specimens were calcined two or three times between 500 and 700 °C with intermittent grinding. Samples of Cai-xCu02 were heated in a horizontal tube furnace for 36 to 120 h in air or in oxygen. In determining the exact stoichiometry of the compound previously reported as "CaCu02" [7], however, a citrate synthesis route was used [15]. Dried anhydrous calcium carbonate and basic cupric carbonate (Cu(OH)2:CuC03) were dissolved in dilute nitric acid and complexed with excess citric acid monohydrate. After drying, the resulting friable, low-density material was calcined at 700 °C either in air or in a flowing oxygen atmosphere until x-ray diffraction revealed the presence of fewer than three phases. It took 18 to 84 h for these synthesis reactions to reach completion.            [20]. fcc"-metastablephase with larger rhombohedral distortion of cubic symmetiy, with superstructure equal to 42 and faint incommensurate superstructure pterpendicular to the hexagonal [hOl] plane. bcc=body centered cubic solid solution; symmetiy often distorted and generally with superstructure.

C5B14 = Ca5Bii4026
CB2=CaBi204 C2B3 = Ca4Bi60i3 CB = Ca2Bi205(triclimic) C-mon=metastable C-centered monochnic phase near Ca6Bi70i6j. ' Although Ca4Bi60i3 has formed during first 700 °C heat treatment, further heating and grinding resulted in formation of Ca2Bi305, which increased with the third heat treatment, indicating that the 2:3 phase was formed metastably but the 1:1 compound is the stable phase. 'Amount of 2:3 decreasing and amount of Cai_jCu02 may be increasing very slightly.

Experimental Results and Discussion
Most of the experiments performed on the binary and ternary mixtures of CaO-Bi203-CuO are reported in Table la. Additional experiments specifically designed in an attempt to obtain crystals large enough for x-ray single crystal studies are detailed in Table lb. Crystallographic data for various phases are reported in Table 2.

The System BiiOj-CuO
A phase diagram for this system was already published [16], and was redrawn as Fig. 6392 in Phase Diagrams for Ceramists (PDFC) [17]. It apparently contains only one compound Bi2Cu04, (BaCu). No attempt was made to reinvestigate the melting relations of this system because it does not have any great effect on the phase equilibria of the ternary system with CaO.

The System CaO-CuO
Although a revised phase diagram for this system was previously reported [7], further experimental evidence (Table la) was accumulated in this study and the diagram was revised again [18] as shown in Fig. 1. The CaCu203 compound, which was reported to be stable only above 950 °C [19], was found to be stable between 985 and 1018 °C. Previously determined temperatures, 1020 and 1013 °C [20,7] for the decomposition of CaCu203(CCu2) and for eutectic melting, respectively, are within experimental error of the new values, 1018 ± 2 °C and 1012 ± 2 "C.
3.2.1 Ca2Cu03 The Ca2Cu03(C2Cu) compound decomposes into CaO plus liquid above 1034 ± 2 °C, which is slightly above the previous estimate of 1030 °C [20,7]. The composition of the eutectic reaction is 20CaO-80CuO±5%, as determined from the presence or absence of the Ca2Cu03 phase in samples of varying compositions that were quenched from 1020 °C.
3.2.2 Cai-jtCuOi Samples prepared with an original Ca:Cu ratio of 45.33:54.67 contained no detectable CaO or CuO after heating in oxygen at 700 °C, as demonstrated by x-ray diffraction (Fig. 2 and Table 3). Compositions with original Ca:Cu ratios of 45.20:54.80 and 45.45:54.54 ('^'S-.G) yielded x-ray patterns which indicated the presence of excess CuO and excess CaO, respectively. Therefore, the Ca:Cu ratio for this compound is 0.453:0.547 or Cai_iCu02 with the composition Cao.828Cu02 (.«=0,172) at 700 °C in oxygen. The single phase region for this phase probably varies with temperature and partial pressure of oxygen. The composition and structural analyses of this phase have been recently reported [15]. The x-ray powder diffraction pattern for Cai_xCu02 is shown in Fig. 2 and the indexed data is given in Table 3. This compound decomposes into Ca2Cu03 plus CuO above 755 °C in air and 835 °C in oxygen. In Fig. 1, the experiments conducted in air and those conducted in an oxygen atmosphere are indicated by the dashed line and the crosses, respectively. At 675 °C, Cai-iCu02 can be synthesized from CaC03 plus CuO but the run product never fully equilibrates to a single-or two-phase assemblage. Rather, the metastable three-phase assemblage Cai-xCu02+CaO + CuO persists: after five cycles of heating with intermittent grinding the relative proportions of phases were Cai-:txCu02>CaO> CuO and they remained that way for an additional overnight heat treatments. Because of its greattional 31 overnight heat treatments. Because of its great persistence, Cai-jCu02 is interpreted as being an equilibrium phase, but it should be noted that reversal of its decomposition (synthesis from CuO + Ca2Cu03) was not successfully demonstrated.    (7), b =6.321 (2), and c = 10.573 (2) A. " Superstructure probably not accounted for by S-vectors.

CuiO in the Binary
System CU2O, which is known to be stable in air only above 1026 °C, was found in this system above 1012 °C. Therefore, Cu* and Cu^"*" must have coexisted in the samples that were quenched in air from temperatures between 1012 and 1026 °C. The CU2O observed in samples that were quenched from below 1026 °C is probably formed during solidification of the liquid phase; i.e., an oxygen deficiency in the liquid may result in the solidification of CU2O as well as CuO.

The System CaO-BizOs
The phase equilibria diagram for the system CaO-BizOa was reported in [21] and redrawn as Fig. 6380 in PDFC [17]. It is reproduced here as Fig. 3 with the scale changed to l/2Bi203-CaO instead of Bi203-CaO, to maintain consistency with the other phase diagrams in this report. An interpretation of the experimental results recorded in Table 1 was published in [19] and it is shown in Fig. 4 (cf. Fig. 3). The major differences between our new diagram and the one presented in [21] are: 1) the composition of "Ca7Biio022" [21,22] is revised to Ca4Bi60i3 (2:3) and its crystal structure is reported in [23]; 2) the composition of "CavBieOie" [21,22] is now reported as Ca2Bi205, and its crystal structure is given in [24]; 3) a metastable phase ~Ca6Bi70i6js was formed at about 925 "C on the CaO-rich side of Ca2Bi205, but at about 885 °C on the CaO-poor side; 4) melting relations have been determined in the region of 20-50 mol % CaO.

3J.1. Rhombohedral Solid Solution (Sillen Phase-Rhomb)
The rhombohedral solid solution was first reported by Sillen [25]. Phase relations in the CaO-rich region of the Sillen phase field were previously [20] represented as exhibiting a congruent transition to the fee solid solution, and the present experiments indicate such a point at (~ 22 mol % CaO, -835 °C). Conflant et al. [21] reported a phase transition from one rhombohedral phase to another at about 735-740 °C. Differential thermal analysis of a 1:6 ratio CaO:l/2Bi203 specimen confirms the presence of a reversible transition at about 735 "C. Samples quenched from -750 °C are clearly rhombohedral as previously reported [21,22], but x-ray patterns (Figs. 5a, 5b; Tables 4, 5, 6) from samples that were quenched from ^735°C exhibit peak splitting and faint superstructure reflections (Fig. 5b). The diffraction patterns for both the high and low temperature forms are much sharper if the specimens are not ground after quenching. Apparently, it is easy to induce mechanical deformation in these samples by grinding. The peak splitting can be indexed with an orthorhombic cell a =6.8188(3), b =3.9531 (2), and c =27.830(1) A, which is most easily observed in the rhombohedral (0,2,13) and (3,0,9) reflections corresponding to (2,2,13) + (4,0,13) and (3,3,9) + (6,0,9), respectively, in the orthorhombic indexing (Figs. 5a, 5b, and Tables 5, 6). Dimensionally the unit cell is orthorhombic, but the symmetry cannot be higher than monoclinic because it is the derivative of a rhombohedral (rather than hexagonal) high symmetry phase. Single crystals prepared at 700 °C with a salt eutectic flux (Table lb) (2), and c =27.830(1) A. ^ Apparently due to an unidentified structure.   [26] demonstrated that the solidus temperature of fee BiaOa (ai in [21]) increases with additions of CaO. Conflant et al. [21] depicted its homogeneity range as extending to temperatures above the rhombohedral Sillen phase, and they did not include a congruent melting point. The present work and [18], however, indicate that there is a congruent melting point between 20 and 23 mol % CaO at about 885 °C. The phase diagram in [21] includes a dashed line which defines a small a/ region in the CaO-rich, low temperature portion of the fee field. Present results are essentially in agreement with this finding; i.e., all x-ray diffraction patterns from quenched "fee" samples that contain at least 20 mol % CaO exhibit the superstructure peaks described in [21] plus a very slight splitting of cubic diffraction maxima that was not described in [21] (Fig. 6, Table 7). The observed splitting of substructure peaks of ai' fits rhombohedral symmetry with flH = 7.7427(9), CH = 9.465(1) A, c/fl= 1.2224. The complete field, extending to about 30 mol % CaO, is labeled "fee" because neither the data presented here nor that in [20] provides a sound basis for drawing definitive phase boundaries. The minimum shown in Fig. 4 at -773 °C for the CaO-rich end of this solid solution is in relatively good agreement with the value of 785 "C which can be interpreted from [21] (Fig. 3). When a single-phase specimen of composition near this minimum (5:14-3:8, CaO:l/2Bi203) is quenched after 10 min annealing at -760 °C (-13 °C below the equilibrium minimum), the rhombohedral splitting of cubic maxima was greatly enhanced; this is the a" phase of [21] (Fig. 6; Table 8). As with the rhombohedral Sillen-type phases, these rhombohedrally distorted fee phases are highly susceptible to mechanical damage during routine grinding, therefore the line splitting of ai' can only be seen if the quenched specimen is not ground. X-ray analysis of this sample yielded flH = 7.616, CH = 9.6477, C/A =1.2668, whereas hexagonal indexing of a truly cubic pattern would give c/a= 1.2247; [1,1,1]C = [0,0,0,3]H and [2,2,0]c = [2,2,4 4,0]H. Thus, the rhombohedrally distorted phase that was quenched from the stable "fee" region (ai') had a da ratio that was slightly smaller than the cubic value, but the metastable lower-temperature phase (ai") that was quenched from below the "fee" region had a da ratio that was considerably larger than the cubic value. Single crystal x-ray precession patterns from the a" phase ( Fig. 7) can be indexed with either a monoclinic or a rhombohedral cell with a =4asub as shown in Table 8.

The "Body-Centered-Cubic" Solid Solution ("bcc")
The phase referred to as body-centeredcubic ("bcc") solid solution was reported as a high temperature phase in [21]. In the present study this phase was found to extend from about 35 to 45 mol % CaO. The exact boundaries of the two-phase "fcc-bcc" region were not determined because the compositions of coexisting phases were not consistently reproduced. Just as with the "fee" phase the "bcc" phase also exhibits line splitting and superstructure. Distortions from cubic symmetry (Fig. 8, Table 9), seem to be greatest in samples that are quenched from the region near the decomposition point of the 2:3 phase, (Fig. 9, Table 10). Single crystal x-ray diffraction precession data (Fig. 10) confirm the distortion recorded in Fig. 9 and Table  10 and indicate the nature of the superstructure.  CaO-rich phase boundaries of the "bcc" field have not been precisely determined in part because of complications arising from the presence in many experiments of a metastable phase (see "C-mon" below). This bcc-type phase was found to be stable down to a minimum temperature of 825±5°C (Fig. 4) which is in good agreement with the value of 819 °C interpreted from [20] (see Fig. 3).         I  I  I  I  I  I  I  1_   20  30  40  50  60  70  80  90  100 110 120 130 140 Degrees Two Theta  Table 9. X-ray powder diffraction data for the body centered cubic phase (Ca0:l/2Bi203 mol ratio 9:10, 1000 °C quench)

"
CasBiuOu" (C5Bi4-5:14) A compound with the composition Ca5Bii4026 was previously reported [21,22] as stable up to at least 650 °C, We have no contrary evidence and indeed an apparently single phase x-ray diffraction pattern can be obtained for the 5:14 ratio (26.32% CaO; Fig. 11, Table 11) by annealing a quenched liquid of this composition overnight at 650 °C. The exact composition should be regarded as provisional, however, pending a crystal structure determination. The xray pattern in Table 11 corresponds well with that published in [22] except for a small but consistent shift in observed d amounting to ~ 1/4° 26 for CuKa radiation. Apparently the earlier work had an unrecognized deviation in calibration of the diffraction data. The diffraction pattern has not yet been indexed even with the aid of some single crystal data (Fig. 12). The complexity of the pattern and consideration of the single crystal data suggests triclinic symmetry. Degrees Two Theta   Degrees Two Theta

40
At 732±7°C the 5:14 phase decomposes to a mixture of the rhombohedral phase plus CaBi204 (1:2). This equilibrium was demonstrated by both the breakdown of single phase material after heating above this range, and by nucleation of 5:14 in a two phase mixture of rhombohedral + 1:2 below it. This is considerably lower than the value of 772 °C which may be interpreted from [21] (Fig. 3).
3.3.5. CaBi204 (CB^-l:!) The compound CaBi204 was synthesized at 650 °C [22] and reported as stable up to about 800 "C [21] where it was shown (Fig. 3) to decompose to fee plus 2:3. Apparently inconsistent data in our own work required us to determine the decomposition temperature by simultaneous quenching of single phase 1:2, originally prepared by annealing at 650 °C, and reheating a sample of quenched liquid from which fee plus 2:3 was synthesized. These experiments suggest that the 1:2 phase is not stable above 778 ±5 °C. This may be compared with the value of 799 °C which can be interpreted from [21] (Fig. 3).
The 1:2 phase often occurs along with other phases in samples that are air quenched from temperatures greater than about 800 °C. The x-ray powder diffraction pattern of the 1:2 phase Fig. 13, Table  12, corresponds well with that reported in [22] except for the observed shift in 2 6 mentioned in section 3.3.4. Several attempts were made to synthesize single crystals of the 1:2 phase (see Table lb), but the only procedure that succeeded was to anneal single phase 1:2 4-a 50/50 NaCl/KCl flux (50/50 flux/charge) at 775 "C and then cool at 1 °C/h to 645 °C. The single crystal x-ray diffraction precession data are shown in Fig. 14. The x-ray powder diffraction pattern was indexed on the Ccentered monoclinic cell C2/c obtained from the single-crystal precession data. The lattice parameters refined by least-squares analysis with the aid of calculated structure factors and the calculated powder pattern based on single crystal structure determination are a =16.6295 (8)           3.3.6. Ca4Bi«Oi3 (€283-2:3) The compound "Ca7Biio022", (41.176 mol % CaO) was reported in [22] and [21], and the phase diagram shown in [21] can be interpreted as indicating that it decomposes at about 848 °C. (Fig. 3 in [20]). Experiments performed in the present work (Table 1) indicate that the composition of this phase is really 2:3 (40 mol % CaO) rather than 7:10, but the decomposition temperature (Table 1 and Fig. 4) of 855 ±5 °C is in good agreement with [21]. The x-ray powder diffraction pattern of this phase is shown in Fig. 15 and recorded in Table 13. These results agree well with those in [22] (except for the shift in 2 ^previously mentioned). Single crystals of Ca4Bi60i3 were grown both by utilizing a 50/50 NaCl/KCl flux and by reannealing a quenched liquid.
The compound is orthorhombic a =17.3795(5), b =5.9419(2), c =7.2306(2) A, with a C-centered space group, as determined from single crystal x-ray precession photographs Fig. 16) and x-ray diffraction datarefined by least squares. A complete crystal structure determination [23] including single crystal x-ray analysis, neutron diffraction Rietveld analyses, and measurements of second harmonic generation, proved that the true space group is the non-centrosymetric C2mm. The crystal structure was reported in [23] from data collected on crystals prepared in this study. A complete discussion of the indexing of this phase with comparison to the calculated powder pattern is given in [27]. The crystal structure determination [23] reveals that Bi*^ occurs in two coordination types with 2/3 of the Bi*^ ions five-coordinate and 1/3 of the Bi"^^ ions only three-coordinate, by oxygen. Determinations of the crystal structures of more of these phases will perhaps result in a better understanding of the role played by Bi^* coordination in 3-and 4-component superconductors.
3.3.7. Ca2Bi20s (C2Bj-l:l) The compound "CaTBisOie", (53.846 mol % CaO) was reported in [22] and [21], and the phase diagram in [21] (redrawn as Fig. 3) can be interpreted as indicating that it decomposes at about 929 °C. Experiments performed in the present work (Table 1) combined with a structure determination performed on crystals prepared in this study [24] indicate that the composition of this phase is really 1:1 (50 mol % CaO) rather than 7:6. The x-ray powder diffraction pattern of the phase shown in Fig. 17 and Table 14 agrees well with that reported in [22] (except for the shift in 2 0 noted above). Single crystal x-ray diffraction precession photographs (Fig. 18) indicate that the 1:1 compound is triclinic, and powder x-ray diffraction data [27] yield least squared values of a =10.1222(7), b =10.146(6), c = 10.4833(7) A, a = 116.912(5), /3 = 107.135(6), y = 92.939(6)°. The indexing of this pattern out to high angles in 2 0 could only be accomplished with the aid of calculated structure factors and the calculated powder pattern based on the single crystal structure determination reported in [24]. The structure determination reveals a unique Bi*^ coordination of U-shaped BisOn groups with one five-fold coordinated Bi*^ bridging two four-fold "saw-horse" shaped polyhedra [24]. Degrees Two Theta  Degrees Two Theta   Table 14. X-ray powder diffraction data for the compound Table 14. X-ray powder diffraction data for the compound Ca2Bi205 CaiBizOj--Continued 3J.8 "C-mon" Metastable Phase ~Ca«+xSr«_xBii4033 (j:-^6) When the 1:1 phase is heated between 885 and 925 "C for 20 min to 3 h a metastable C-centered monoclinic phase is formed which may be nearly single phase [a = 21.295(4), b =4.3863(8), c =12.671(2) A, and ^ = 102.74(1)°]. After overnight heat treatments, however, this phase decomposes to a "bcc" plus CaO assemblage. Comparison of the X-ray powder diffraction patterns (Fig. 19, Table 15) for this phase and for Ca6+iSr6-jBli4033 (A;-4.8) indicates that it is the metastable end member extension of the stable ternary solid solution.

The System CaO-Bi203-CuO
Ternary phase relations of the system CaO-l/2Bi203-CuO have been studied at temperatures between 700 and 900 °C. No ternary compounds were discovered, but new data on the CaO-l/2Bi203 and CaO-CuO binaries have been incorporated. The ternary phase relations at 700-750 and 750-800 "C are shown in Figs. 20 and 21 respectively. There remains some uncertainty about the equilibrium phase relations involving Cai-xCu02.
To verify that the three-phase equilibria inferred from synthesis runs (products of a synthesis from CaCOs, Bi203, and CuO) reflected equilibrium phase assemblages, various three phase mbctures of pre-made binary compounds were reacted isothermally. For example, such experiments demonstrate that a mechanical mixture of CaiBieOn + 7Ca2Cu03 + 3Ca4.533Cu5.4670io (bulk composition 51.80: 9.84: 38.36) is metastable with respect to a mixture of Ca2Bi205 + Ca2Cu03+ Ca4.533Cu5.4670io at 700 "C. Because the nucleation (or increase in volume fraction) of Cai-iXCu02 from binary compounds was never demonstrated at 700 "C (see Sec. 3.2.2) the possibilities of three phase equilibria including Ca2Cu03 (and/or Cai-xCu02) plus Bi6Ca40i3 can not be ruled out. For example, the mechanical mixture 5Ca2Cu03 + Ca4Bi60i3 which has a bulk composition of 56:24:20 shows no convincing evidence of Cai-xCu02 even after six heating/grinding treatments at 700 °C. Degrees Two Theta Fig. 19. X-ray powder diffraction pattern comparing the "C-mon" metastable phase -Cafi+jBrft-^BinOja x->6 to the ternary x-^O Table 15. X-ray powder diffraction data for the "C-mon"