J. MATER. CHEM.. 1994, 4( 12), 1871-1873 Phase Diagram of the Bi-Sr-Cu-0 System Elena Yu. Vstavskaya," Vladimir A. Cherepanov? Andrei Yu. Zuev,aSimon D. Suttonb and J. Stuart Abell*b a Department of Chemistry, Ural State University, 5 7 Lenin Av, Yekaterinburg, Russia School of Metallurgy and Materials, The University of Birmingham, Birmingham, UK B 75 2TT ~ ~ ~ ~~~ The phases present in the Bi-Sr-Cu-0 system in air, at 865 "C, have been investigated using X-ray diffraction techniques, with particular emphasis on the homogeneity ranges of the ternary oxide phases. Only three such phases were found; a tetragonal form and an orthorhombic form of Bi,Sr,CuO, (both exhibiting homogeneity ranges), and Bi,Sr,Cu,O,. Eight binary oxides were also found. No evidence was found at this temperature for the previously reported phases, Bi,Sr,Cu,O, and Bi,,Sr,,Cu,O,.Since the discovery of superconductivity in the Bi,Sr,Ca, -,CU,O, + 2n system, much scientific effort has been expended in attempting to produce single-phase samples of the higher T, members of the series (higher values of n), with rather less effort being spent on the much lower T,, but nevertheless important base member, Bi2Sr,Cu06 (n=1). In this work we have re-investigated the phase relationships within the Bi-Sr-Cu-0 phase diagram in an attempt to clarify some of the discrepancies in the literature. Bi-Cu-0 System The only phase in this system that has been previously described is B~,CUO,,'-~ which can be fabricated at tempera- tures below its melting point of 840rC.1.5 Two eutectic temperatures were found; 770 'C at 15 mol% CuO and 835 "C at 54 mol% C~0.l.~ Sr-Cu-0 System A number of phases have been reported in this system.Sr,CuO, fprmed in a$ at 930'C has a tetragonal structure (a=3.901 A, c =7.488 A). During annealing at 980 "C a betra- gonalaorthorhombic p$ase transition occurs (a = 12.699 A, b = 3.908 A and c =3.497 A). This transition has been reported to be irreversible.6 Detailed studies of Sr,Cu03 above the phase transition have also been described.' SrCuO, also forms in airs,9 whilst SrCu,02 forms in a reduced oxygen atmosphere." One more phase, with a CuO content of 60-64% has been found, but different authors ascribe different formulae; Sr,Cu,O, (62.5% Cu0),",l2 Sr,Cu,O, (60% CuO)', and Sr4Cu7012 (63.64% CuO).14 Recently, Siegrist et ~11.'~ described it as Sr14Cu,40,, (63.5% oCuO) with an vrthorhom- bic unit cell (a= 11.45 A, b =13.35 A and c= 3.98 A).Sr-Bi-0 System The phase diagram of the Sr-Bi-0 system has been studied by Roth et a1.16 and Guillermo et and a number of phases identified; Bi,SrO,, Bi,Sr,O, and Bi,Sr,06. Ikeda et found pne more pFase BiSr30z (ophorhombic cell: a= 17.147 A. b= 16.758 A and c = 16.998A). Several groups have studied the rhombohedra1 P-pha~e"-'~ and the tetragonal y-phase.21.'2 The P-phase exists between 11and 30 mol% Sr and the ?-phase between 43 and 46 mol% Sr. A detailed study of the Bi,03-Sr0 phase diagram has been made in a previous paper.23 Bi-Sr-Cu-0 System A number of single phases and solid solutions have been reported in the Bi-Sr-Cu-0 sy~tem.'*,~~-~~Th ese include Bi,Sr,CuO, (the 2201 superconductor).24 Bi,,Srl,Cu,O,,'R Bi,Sr,Cu20,,'8 B~,S~,CU,O,'~ and the solid solutions Bi, + 3r2-.CuO, (the so-called Raveau- or R-phase)26 and Bi2+xSr2-~C~l+yOz(.x=0.1-0.6 and y=0-~,/2).~~ Experimental About 230 samples of different composition were prepared by the usual ceramic techniques, using Bi203, SrCO, and CuO all of a purity of greater than 99.5%.Annealing temperatures were in the range 780-865°C and annealing times were 70-250 h, with intermediate re-grinding under alcohol in an agate mortar. Some samples in the Sr-Cu-0 system were fired at 930 'C followed by annealing at 865 "C.All samples were quenched from 865 'C. Phase identification was per-formed by powder X-ray diffraction (XRD) using a DRON-3 diffractometer with Cu-Kr radiation. Data were collccted at 0.04" increments over the range 20" <20 <60", and lattice parameters were calculated using a least-squares method. Results and Discussion Sr-Cu-0 System Three compounds have been found in this system ir air at 865 "C: Sr,Cu03, SrCuO, and Sr,,Cu,,O,, . Compounds with the nominal compositions SrCu,O,, Sr,Cu,O,". and S~,CU,O,~~were not obtained. The sample with the former composition was found to consist of Sr,,Cu,,O,, and CuO, and the last two compositions consisted of varying amounts of Srl,Cu,,O,l and SrCuO,. Sr,Cu,O,, was found to be identical with Sr,,Cu,,O,, within experimental error.Sr-Bi-0 System The following phases have been found in this sysiem in air at 865°C: a 8-phase with a variable composition (0.11dtSr<0.30), Bi,SrO,, a 7-phase with the general formula Bi, +*Sr1 -*O, (0.08<x<0.14), Bi,Sr,O,, Bi2Sr30, and BiSr,O,. Those samples with composition tsr<0.11 either coexist with the liquid phase or are completely molten. The homogeneity range of the P-phase is in good agreement with the literature, that of the ?-phase is broader than previously published. The phase diagram of the Bi,O,-SrO system is described in detail in a previous paper.,, Bi-Sr-Cu-0 System A compound with the nominal composition Bi,Sr !Cu06 (2201), having a tetragonal structure (A) (aM 5.4 A, c c:24 A) has been obtained at a temperature of less than 790°C after firing for 24 h. Increasing the temperature to 865 "C leads to the transformation of A to an orthorhombic structure (B).It was found that phases A and B have considerable homogeneity ranges and that not all compositions of phase A have under- gone this transformation at 865°C. At 790°C the tetragonal phase A has a homogeneity range that includes the sample with the nominal composition Bi,,Sr,,Cu,O, (described earlier as single phase18). This homogeneity range decreases with increasing temperature, as demonstrated by the series Bi,+xSr2-xCu0,. Samples with the following compositions were prepared: x=O, 0.05, 0.1, 0.125, 0.2, 0.3, 0.4, 0.5 and 0.6.After firing at 790°C, analysis by XRD showed all samples to be single phase and tetragonal with the exception of x= 0.4, 0.5 and 0.6, which also contained some excess Bi,SrO,. After firing at 830 "C, the samples with x=0,0.05 and 0.1 had transformed into the orthorhombic phase B. Raising the firing temperature to 865°C showed no change in the position of this boundary. Increasing the Bi component of phase A decreases the melting point and results in a liquid phase at this temperature. In order to confirm the boundaries of the homogeneity ranges of the two phases, lattice parameters were measured and unit-cell volumes calculated for several samples lying on each of lines 1,2 and 3 in Fig. 1. These are presented in Tables 1-3. These results show that neither phase conforms to Vegard's Law, but this is not unusual and many 'non-ideal' solid solutions are known.27 However, outside the single phase regions, where the limiting compositions of the solid solutions remain, the lattice parameters remain constant, and thus define the extent of the phase fields.The homogeneity range of phase A is in good agreement with, but slightly wider than, that obtained by Ikeda et a1." However, Bi,,Sr,,Cu,O,, described by Ikeda et al. as single phase, was found to lie within the B range. Samples in the field, of 2-3 mol% width, between the homogeneity ranges of phases A and B contained both these phases. Because of its relative narrowness it is difficult to conclude whether this is a field of equilibrium coexistence of Bi(%) \ 50 '\ \ \ 60x', 'j70 / 20 BiSr30, Bi2Sr306 Bi2Sr205 Bi2Sr04 Sr(%) Fig.1 Selected area within the Bi-Sr-0-0 phase diagram at 865 "C in air, showing the major phases and phase fields J. MATER. CHEM., 1994, VOL. 4 Table 1 Parameters and unit-cell volumes of phase t3 (compositions on line 1) at 865 "C in air mole fraction of metal component N(i)= n(i)/(nBi +nSr +nCu) 0.360 0.400 0.240 5.418(5) 23.333(7) 24.430( 1) 3088.8 0.375 0.400 0.225 5.418(5) 23.333(7) 24.430( 1) 3088.8 0.400 0.400 0.200 5.422(5) 23.332( 7) 24.436(8) 3091.8 0.410 0.400 0.190 5.424(5) 23.322(6) 24.438( 7) 3091.8 0.425 0.400 0.175 5.427(0) 23.303(2) 24.436(4) 3090.2 0.440 0.400 0.160 5.426(6) 23.341 (7) 24.453(5) 3097.2 0.450 0.400 0.150 5.426(6) 23.341(7) 24.453(5) 3097.2 Table 2 Parameters and unit-cell volumes of phase B (compositions on line 2) at 865 "C in air ~ mole fraction of metal component N(i)= n(i)/(nBi +nSr +nCu) 0.375 0.375 0.250 5.418(5) 23.317(4) 24.416(6) 3084.9 0.380 0.380 0.240 5.418(5) 23.317(4) 24.316( 6) 3084.9 0.390 0.390 0.220 5.420( 1) 23.301(0) 24.330(8) 3085.5 0.400 0.400 0.200 5.422( 5) 23.332( 7) 24.436(8) 3091.8 0.410 0.410 0.180 5.423( 1) 23.316(8) 24.442(0) 3090.7 0.420 0.420 0.160 5.423( 1) 23.316(8) 24.422(0) 3090.7 Table 3 Parameters and unit-cell volumes of phase A (compositions on line 3) at 865 "C in air mole fraction of metal component N(i)=n(i)/(nBi+nSr+nCu) 0.410 0.310 0.280 5.380(8) 24.621(0) 712.8 0.425 0.325 0.250 5.380(8) 24.62 1 (0) 712.8 0.440 0.340 0.220 5.391(7) 24.617(6) 715.6 0.450 0.350 0.200 5.394(4) 24.615(4) 716.3 0.460 0.360 0.180 5.395(5) 24.631(7) 717.1 0.470 0.370 0.160 5.395(5) 24.631(7) 717.1 the two phases, or a metastable region in the vicinity of a transition boundary between them.A similar situation exists near the boundaries of these phase fields and excess CuO. Special attention was paid to the area in the vicinity of the compositions Bi2Sr3Cu20z (C) and Bi,Sr,Cu,O, (D), both described as single phase by Tkeda et a[." Samples with the exact compositions corresponding to the phases C and D, and others varying from them by ra. 1-2 mol% in all direc- tions were prepared. All samples, including those exactly corresponding to phases C and D, were composed of combi-nations of three of the following phases: phase B, phase D, Bi,Sr,O,, SrCuO, and Sr,,Cu,,O,, .Increasing the firing time at 865°C to 150-170 h did not result in the formation of single-phase material. Two series of samples (located on lines 4 and 5 on Fig. 1) were further fired for a total of 200-250 h at 865 "C. XRD showed that the intensities of the reflections from phase B and from the Sr-Cu-0 phases decreased and the refleFtions from B~,Sr,Cu,O, (D),oorthorhombic with II =34.035 A, b = 24.050 A and c =5.389 A, increased. None of the samples were single phase. At all stages of the investigation the sample with J. MATER. CHEM., 1994, VOL. 4 h u)c.-C ?em v u)c 3 0 * I ,,I, ,,,,I , 1 20 25 30 35 40 2Wdegrees Fig.2 XRD pattern from a sample with nominal composition Bi,Sr,Cu,O,. 9.Bi,Sr,CuO, peaks; X, Bi,Sr,Cu,O, peaks nominal composition C contained three phases: B, D and Sr14Cu24041(Fig. 2). Owing to the Sr14Cu24041peaks both being very weak and being situated on the shoulders of peaks of the majority phases, they have not been marked on the figure. Conclusions Equilibrium reactions within this phase diagram are clearly sluggish and can easily prevent the required phase formation unless extreme care is taken during the sintering process. Consequently, the comparison of results from different groups must consider the precise reaction temperatures and times. We have found that at 865°C in air the following binary oxides exist within the Bi-Sr-Cu-0 phase diagram; Sr,CuO,, SrCuO,, Sr14Cu24041,Bi,SrO,, Bil +$r1 -xO, (0.08<x d 0.14), Bi2Sr,0s, Bi,Sr,O, and BiSr,O,.Only two ternary oxides were observed; Bi,Sr,CuO, (existing as both a tetra-gonal and an orthorhombic phase, both with significant homo-geneity ranges) and Bi,Sr,Cu,O,. No evidence was seen for either Bi,Sr,Cu20, (C) or Bi,,Sr,,Cu,O,, which falls within the boundary of the phase B homogeneity range. Over nearly one half of the diagram, in the Bi-rich area, all compositions were liquid at this temperature. The authors are grateful to Dr. Yu. M. Yarmoshenko for useful discussions and help with the structural work.V.A.Ch. is grateful to the Royal Society for the award of a Fellowship. References 1 J-S. Boivin, D. Thomas and G. Iridot, C.R. Seances Acad. Sci., Ser. C, 1973,276, 1105. 2 R. S. Roth, J. R. Dennis and H. F. 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