Syntheses and characterization of two alkali-metal zinc borates, alpha-LiZnBO_3 and Li_(0.48)Na_(0.52)ZnBO_3

摘要

Two alkali-metal zinc borates, alpha-LiZnBO_3 and Li_(0.48)Na_(0.52)ZnBO_3, have been prepared by solid-state reactions below 850 deg C. Single-crystal XRD analyses showed that the former crystallizes in the C2/c group with a = 8.746(2) A, b = 5.091 (1) A, c = 6.129(1) A, beta = 118.75(3) deg, Z = 4 and the latter in the group P1-bar with a = 5.054(1) A b = 6.113(1) A, c = 8.045(2) A, alpha = 75.73(2) deg, beta = 89.87(3) deg, gamma = 89.86(3) deg, Z = 4. The crystal structure of alpha-LiZnBO_3 is composed of tetrahedral ZnO_4 and triangular BO_3 groups that are arranged into a three-dimensional (3D) network by sharing O vertices. Li_(0.48)Na_(0.52)ZnBO_3 is also characterized by a 3D framework, but built up from corner-sharing ZnO_4 tetratahedra, ZnO_5 trigonal bipyramids, and BO_3 triangles. Both structures afford open channels that are occupied by alkali-metal cations. The IR spectra further confirmed the presence of BO_3 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.