Structure-driven mixed-site borate–phosphate K5Ta8BP4O34: synthesis, structural, spectroscopic and theoretical study

Abstract

Tungstate bronze (TB) related borate–phosphate K₅Ta₈BP₄O₃₄ is afforded from boron-enriched potassium–molybdate flux. It was found to crystallize [space group P2/c (no. 13), a = 1384.36(5), b = 640.99(2), c = 2054.75(8) pm, β = 125.007(2)°, V = 1493.44(9) × 106 pm³] isotypically with the K₅M^IV₂Nb₆P₅O₃₄ (M = Ti, Zr) group of compounds. Revealed by X-ray structural analysis, occupational disorder of boron and phosphorus atoms over two crystallographically distinct positions was explained by the decrease of tetrahedral distortions. The calculated electronic structure of title compound was found to be similar to that reported for UV-Vis active niobium phosphate photocatalysts for full water decomposition. Transformation of the parental [Ta₈BP₄O₃₄] framework at incongruent melting (T = 1618 K) into M^IM^I′₂M^V₆P₃O₂₄-type phosphates was detected leading to a subtle structural relationship of phosphates bearing TB building features.