CRYSTAL-CHEMISTRY OF NONCENTROSYMMETRIC ALKALI-METAL NB AND TA OXIDE PYROBORATES

The compounds CsNbOB2O5 and CsTaOB2O5 have been synthesized and structurally characterized by single-crystal X-ray diffraction methods. Each crystallizes with two formula units in the orthorhombic system and space group Pmn2(1). Cell parameters for the Nb compond are a = 7.527(2), b = 3.988(1), c = 9.717(2) Angstrom, and V = 291.7(2) Angstrom(3); cell parameters for the Ta compound are a = 1.5479(9), b = 3.9064(4), c = 9.7713(9) Angstrom, and V = 288.11(9) Angstrom(3). The structures of these materials are compared to those of RbNbOB2O5 and KNbOB2O5, and results are correlated to the linear and nonlinear optical properties of the crystals. X-ray data on the solid-solution series Rb1-xKxNbOB2O5 and Rb1-xCsxNbOB2O5 are also presented. (C) 1995 Academic Press,Inc.