Syntheses and characterization of two alkali-metal zinc borates, α-LiZnBO3 and Li0.48Na0.52ZnBO3

Two alkali-metal zinc borates, alpha-LiZnBO3 and Li0.48Na0.52ZnBO3, have been prepared by solid-state reactions below 850 degrees C. Single-crystal XRD analyses showed that the former crystallizes in the C2/c group with a = 8.746(2) angstrom, b = 5.091 (1) angstrom, c = 6.129(1) angstrom, beta = 118.75(3)degrees, Z = 4 and the latter in the group P (1) over bar with a = 5.054(1) angstrom, b = 6.113(1) angstrom, c = 8.045(2) angstrom, alpha = 75.73(2)degrees, beta = 89.87(3)degrees, gamma = 89.86(3)degrees, Z = 4. The crystal structure of alpha-LiZnBO3 is composed of tetrahedral ZnO4 and triangular BO3 groups that are arranged into a three-dimensional (3D) network by sharing O vertices. Li0.48Na0.52ZnBO3 is also characterized by a 3D framework, but built up from corner-sharing ZnO4 tetratahedra, ZnO5 trigonal bipyramids, and BO3 triangles. Both structures afford open channels that are occupied by alkali-metal cations. The IR spectra further confirmed the presence of BO3 groups and UV-vis diffuse reflectance spectra showed band gaps of about 3.10 and 2.95 eV for the Li and Li/Na compounds, respectively. Band structure calculations indicated that both compounds are direct semiconductors with the calculated band gaps close to the observed ones. (C) 2009 Elsevier Masson SAS. All rights reserved.