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.