NEUTRON-DIFFRACTION STUDY OF THE NONMODULATED 2212-TYPE CUPRATE BI1.5BA2.5LACU2O8.25

The structure of the nonmodulated bismuth cuprate Bi1.5Ba2.5La Cu2O8.25 has been determined from powder neutron diffraction data. This phase crystallizes in the space group I4/mmm, with a = 3.9322(l) and c = 31.240(1) angstrom. The significant features are: (i) the puckering of the [CuO2]infinity layers due to the fact that copper is located out of the basal plane of the CuO5 pyramid, (ii) the location of barium in the bismuth bilayers, whereas lanthanum occupies entirely the fluorite-type cages between the copper layers, and (iii) the complex structure of the [Bi1.5Ba0.5O2.25]infinity layers. In the latter layers barium and bismuth are located in two distinct (4e) sites, the majority of the oxygen atoms are located at the same level as bismuth, forming flat layers, and their positions correspond to a splitting with respect to the ideal one. The excess oxygen appears to be located between the double bismuth layers. The coordination of bismuth and barium is also discussed. 1994 Academic Press, Inc.