Revisiting the Solid-State Synthesis of Alkali–Tantalum(V) Oxyfluorides

The solid-state synthesis of alkali–tantalum(V) oxyfluorides KTa₂O₅F and CsTa₂O₅F was revisited with the aim of streamlining their preparation as single-phase polycrystalline solids. Alkali fluorides (KF, CsF) and trifluoroacetates (KH(CF₃COO)₂, CsH(CF₃COO)₂) and tantalum oxide (Ta₂O₅) were used as precursors. Reaction temperatures were optimized by means of thermal analysis coupled with powder X-ray diffraction. Phase-pure KTa₂O₅F was obtained by heating stoichiometric mixtures of KF + Ta₂O₅ and KH(CF₃COO)₂ + Ta₂O₅ at 900–1000 °C in alumina crucibles under ambient atmosphere. Similar conditions were employed to synthesize phase-pure CsTa₂O₅F from a stoichiometric mixture of CsF + Ta₂O₅ heated to 800 °C. On the other hand, the preparation of CsTa₂O₅F from a mixture of CsH(CF₃COO)₂ + Ta₂O₅ required an excess of the trifluoroacetate precursor to obtain the targeted oxyfluoride as the sole crystalline phase. Results presented herein demonstrate that mixed-metal oxyfluorides previously thought to be synthetically challenging may be synthesized via facile solid-state reactions without the need for specialized containers and stringent conditions. In addition to streamlined synthetic routes to alkali– tantalum(V) oxyfluorides, a neutron powder diffraction study of the crystal structure of KTa₂O₅F is presented to fill a gap in crystallographic databases commonly accessed by materials and solid-state chemists.