Kinetic Study on the Formation of the Aragonite-Like Phase Ba(CuO x ) 1-Y (CO 3 ) y

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.C. Matacotta°, P. Nozar°°, P.Scardi** and

*C.M.B.M., Materials Dep, 38050 Trento, Italy, [email protected] "**Dept. of Materials Engineering, University of Trento, 38050 Trento, Italy °CNR-ISM, Area Ricerca Bologna, 40129 Bologna, Italy "°°ICTP HTSLAB, p.o.box 586, 34100 Trieste, Italy °°°CNR-LAMEL Area Della Ricerca, 40129 Bologna, Italy ABSTRACT A systematic study on phase formation and stabilisation in the Ba-Cu-C-O system in the temperature range 20-500'C, under various atmospheres, by traditional thermal analysis techniques, high temperature X-ray diffraction and high resolution electron microscopy, has permitted to identify and characterise the formation kinetics of a new copper containing phase isomorphic to y-BaCO 3. INTRODUCTION Since 1989 [1,2] a number of new layered perovskites having alternate carbonate and copper-oxygen layers have been reported. In 1992 superconductivity has been found in (Sr,Ba)2Cul+x(CO3)x02+y [3] and soon after a large number or related superconducting compounds have been discovered, some of them exhibiting Tc above 100K [4,5]. All these compounds which, from a chemical point of view, can be classified as oxycarbonates, are characterised by perovskite-like structures that are typical of oxides so that they are sometimes indicates as carbocuprates. This definition is particularly appropriate in barium-rich compounds with large basal plane dimensions (Cu-O-Cu distances in excess of 4 A) where the structural differentiation in copper- and carbon-containing layers is lost. Both Cu and C can sit in a more or less ordered way within the same layer as cuprate groups CuOx are partially substituted by carbonate groups CO 3 [6,7]. A similar situation has been recently found in nonperovskitic compounds like BaCuO 2 , where the non stoichiometric behaviour, reported by several authors in the past [8], has been recently attributed [9] to the presence of carbonate groups in the cuprate structure. Very recently we have found the existence of a new phase in the Ba-Cu-C-O system with a crystal structure strictly derived from the aragonite-like y-BaCO 3 that can be expressed as Ba(CuOx)y(CO 3)liy [10]. The first clues on the existence of such phase were given by optical methods (IR and Raman modes in the spectral region typical of the Cu-O 495 Mat. Res. Soc. Symp. Proc. Vol. 398 ©1996 Materials Research Society

vibrations) and electron microscopy techniques. EDX analysis on extremely small portions of grains exhibiting the crystal features of 7BaCO 3 showed a Cu/Ba ratio ranging from 0.1 to 0.5. Figure 1 shows a typical HREM image of a thin transparent section of Ba(CuOx)y(CO 3)i.y oriented along the [001] direction. The domain structure of the grain is evident. The brightness pattern of some domains is exactly simulated by the structure of y-BaCO 3 . Other domains present an alteration of bright and dark rows that is not possible to simulate unless different atom species are allowed to sit in the octahedral cavities of the barium atoms hexagonal packing. Selected area electron diffraction patt