Flux crystal growth of a new BaTa2O6 polymorph, and of the novel tantalum oxyfluoride salt inclusion phase [Ba3F]Ta4O12F: Flux dependent phase formation

Abstract

A new hexagonal polymorph of BaTa2O6 and a novel salt inclusion phase, [Ba3F]Ta4O12F, were obtained via high temperature flux crystal growth using mixed alkali chloride/alkali fluoride fluxes. BaTa2O6 and [Ba3F]Ta4O12F crystallize in the hexagonal and tetragonal space groups P6/mmm and P42/mnm, respectively. The structure of BaTa2O6 consists of TaO6 octahedra that edge-share via equatorial O(2) to form Ta2O10 dimers that further connect to form a 3D structure containing channels in which the barium cations are located. The morphology of this structure is substantially different from those of the three previously reported polymorphs of BaTa2O6. The structure of the salt inclusion phase, [Ba3F]Ta4O12F, consists of slightly zig-zagging chains of trans corner-sharing (via O(3) and O(4)) TaO6 octahedra that corner-share their equatorial O(1) and O(5) oxygens with TaO5F octahedra to create the framework for the Ba3F salt inclusion. The stability of BaTa2O6 polymorphs was assessed using DFT calculations. The new hexagonal polymorph (ΔfH0 K of −2825.11 ​kJ/mol) has a higher energy than the other three known phases and is 49.17 ​kJ/mol higher than the orthorhombic polymorph.