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

Two new ordered Perovskite compounds, Ba2Bi2/3TeO6 and Ba3Bi2TeO9, have been synthesized and structurally characterized from neutron powder diffraction data. Ba2Bi2/3TeO6 is cubic (space group=Fm (3) over bar m; a=8.4536(2) Angstrom), 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. Ba3Bi2TeO9 is trigonal (space group= P (3) over bar c1: a =6.18313(6) Angstrom, c= 14.8645(2) Angstrom), with a 2.1 ordering of Bi3+ and Te6+, and out-of phase tilting of the octahedra about all three pseudocubic axes (Glazer tilt 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 face in Ba3Bi2TeO9, whereas ideal octahedral coordination is observed in Ba2Bi2/3TeO6. This shift is driven by the valency requirements of oxygen and aided by the stereoactive lone electron pair on Bi3+. These structural changes 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 the dopants appear to be homogeneously distributed, the resulting compositions were found in each case to be electrically insulating. Extended Huckel band structure calculations were performed in order to obtain a qualitative understanding of the optical and electrical properties. (C) 2000 Elsevier Science Ltd. All rights reserved.