Synthesis, structure and optical properties of two new Perovskites: Ba2Bi2/3TeO6 and Ba3Bi2TeO9

Two new ordered Perovskite compounds, Ba2Bi2/3TeO6 and Ba3Bi2TeO9, have bee n synthesized and structurally characterized from neutron powder diffractio n data. Ba2Bi2/3TeO6 is cubic (space group=Fm (3) over bar m; a=8.4536(2) A ngstrom), with a 1:1 rock salt ordering of Te6+ and Bi3+ on the octahedral sites. To maintain charge balance the bismuth site is only 2/3 occupied. Ba 3Bi2TeO9 is trigonal (space group= P (3) over bar c1: a =6.18313(6) Angstro m, c= 14.8645(2) Angstrom), with a 2.1 ordering of Bi3+ and Te6+, and out-o f phase tilting of the octahedra about all three pseudocubic axes (Glazer t ilt system a(-)a(-)a(-)). The two structures also differ from each other in the local coordination of Bi3+ This ion is shifted toward an octahedral fa ce in Ba3Bi2TeO9, whereas ideal octahedral coordination is observed in Ba2B i2/3TeO6. This shift is driven by the valency requirements of oxygen and ai ded by the stereoactive lone electron pair on Bi3+. These structural change s lead to a distinct change in the optical band gap, from similar to 2.8 eV in Ba2Bi2/3TeO6 to similar to 2.3 eV in Ba3Bi2TeO9. Substitutional doping studies were carried out in the hope of increasing conductivity. Although t he dopants appear to be homogeneously distributed, the resulting compositio ns were found in each case to be electrically insulating. Extended Huckel b and structure calculations were performed in order to obtain a qualitative understanding of the optical and electrical properties. (C) 2000 Elsevier S cience Ltd. All rights reserved.