Exploiting the Reactivity of Metal Trifluoroacetates to Access Alkali-Niobium(V) Oxyfluorides

Motivated by the lack of facile routes to alkali-niobium(V) oxyfluorides KNb₂O₅F and CsNb₂O₅F, we investigated the reactivity of alkali trifluoroacetates KH(tfa)₂ and CsH(tfa)₂ (tfa = CF₃COO^-) toward Nb₂O₅ in the solid state. Tetragonal tungsten bronze KNb₂O₅F and pyrochlore CsNb₂O₅F were obtained by simply reacting the corresponding trifluoroacetate with Nb₂O₅ at 600 °C under air, without the need for specialized containers or a controlled atmosphere. Thermolysis of KH(tfa)₂ in the presence of Nb₂O₅ yielded single-phase polycrystalline KNb₂O₅F. By contrast, the reaction between CsH(tfa)₂ and Nb₂O₅ produced a mixture of CsNb₂O₅F and a new oxyfluoride of formula CsNb₃O₇F₂, whose crystal structure was solved using powder X-ray and electron diffraction. CsNb₃O₇F₂ (space group P6/mmm) belongs to the family of hexagonal tungsten bronzes and features an open-framework structure consisting of corner-sharing Nb(O,F)₆ octahedra with hexagonal channels occupied by Cs⁺ ions. Isomorphous RbNb₃O₇F₂ was obtained upon reacting RbH(tfa)₂ with Nb₂O₅. Synthetic optimization enabled the preparation of RbNb₃O₇F₂ and CsNb₃O₇F₂ as single-phase polycrystalline solids at 500 °C under flowing synthetic air. Both oxyfluorides were found to be semiconductors with a band gap of ≈3.5 eV. The discovery of these two oxyfluorides highlights the importance of probing the reactivity of solids whose full potential as fluorinated precursors is yet to be realized.