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I. INTRODUCTION

thors agree the true composition is actually grossly deficient in SrO. This gross deficiency in SrO was pointed Since the discovery of high Tc superconductivity in out by Roth12'13 along with the fact that the actual comthe series of A2B2CaM^iCu«O4+2n (A = Sr, Ba; B = Bi,Tl),1>2 a large number of papers have been published position Sr2Bi2CuO6 represented a phase which had a discussing the chemistry and properties of phases in quite different x-ray diffraction pattern. this series. Most of the effort has been concentrated on The compositions of these two phases are shown in attempts to synthesize single phase bulk materials havthe portion of the phase diagram at =900 °C illustrated ing ever higher Tc, with more and more CaCuO2 (larger in Fig. 1 (experimental details given in Ref. 3). It has value of n), and little time has been spent on the phase(s) been mentioned several times1415 that the grossly nonwith n = 1. In an effort to understand the phase equilibria stoichiometric phase exhibits superconductivity only at and crystal chemistry of the entire series, a detailed the maximum SrO content, although little evidence has study of the quaternary system SrO-CaO-Bi2O3-CuO actually been published to prove this point. The two has been undertaken. This paper discusses the current phases are easily distinguished by low angle portions status of our knowledge of the compound Sr2Bi2CuO6 of the diffraction pattern. The Raveau phase has a (n = 1); experimental details of the complete ternary peak indexable as (002) for the pseudotetragonal cell system are given elsewhere.3 Results obtained for the at =7.1-7.2° 20 (d = 12.4-12.3 A) whereas the comternary system SrO-CaO-CuO were recently published4 pound Sr2Bi2CuO6 (2:2:1) (Fig. 2) contains a peak at and data for the other two ternaries CaO-Bi2O3-CuO =7.5° 261 (d = 11.8-11.7 A). Furthermore, the Raveau and SrO-CaO-Bi2O3 will be reported in the near fusolid solution phase always has a peak at = 25.75° 20 ture.5'6 References 3-6 include new results for all of the (d — 3.457 A), representing (113) of the pseudotetragonal binary mixtures of the four-component system, the cryscell, whereas the 2:2:1 compound, Fig. 2, has no peak tallographic data for which were recently summarized.7 at all at 25.75° but contains instead a very strong peak at =-26.75° 26 (d = 3.330 A) as well as other mediumThe phase in the Sr-Bi-Cu-O system which was strong peaks between 20° and 30° not found in the Raveau first reported to show superconductivity8 was originally phase and not indexable on the tetragonal subcell. identified as having the composition Bi2Sr2Cu2O7+s, with a C-centered orthorhombic space group, a = 5.32, During heat treatments of the 2:2:1 composition b = 26.6, c = 48.8 A from electron diffraction data. (and indeed of all of the compositions for which Several papers have reported crystallographic data for Sr2Bi2Can_iCunO4+2n phases are expected) the Raveausuch a phase, generally obtained by picking a crystal type solid solution phase always forms first, along with from a m