THE CRYSTAL-STRUCTURE OF THE NEW PYROELECTRIC PHASE BI4TE2O9BR2

The crystal structure of Bi4Te2O9Br2-a new phase with pyroelectric pro perties-was determined by the single crystal X-ray diffractometer tech nique (Pmm2, Z = 1, a = 5.5231(8), b = 5.5511(8), c = 9.735(1) Angstro m, R/R(W) = 0.070/0.084 for 461 reflections, MoK alpha). The structure could be described as being built up of fluorite-like triple metal-ox ygen [Bi4Te2O9] layers alternating with bromine layers along the z-axi s. Each triple [Bi4Te2O9] layer consists of two bismuth and one tellur ium cationic sublayer with oxygen atoms in tetrahedral voids among the cations; half of all octahedral voids are occupied by O3 atoms. The r elation of this crystal structure with Bi3O4Br and NdBi5O8Cl2 Sillen p hases is discussed. (C) 1995 Academic Press, Inc.