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

Motivated by the lack of facile routes to alkali-niobium(V) oxyfluorides KNbOF and CsNbOF, we investigated the reactivity of alkali trifluoroacetates KH(tfa) and CsH(tfa) (tfa = CFCOO) toward NbO in the solid state. Tetragonal tungsten bronze KNbOF and pyrochlore CsNbOF were obtained by simply reacting the corresponding trifluoroacetate with NbO at 600 °C under air, without the need for specialized containers or a controlled atmosphere. Thermolysis of KH(tfa) in the presence of NbO yielded single-phase polycrystalline KNbOF. By contrast, the reaction between CsH(tfa) and NbO produced a mixture of CsNbOF and a new oxyfluoride of formula CsNbOF, whose crystal structure was solved using powder X-ray and electron diffraction. CsNbOF (space group 6/) belongs to the family of hexagonal tungsten bronzes and features an open-framework structure consisting of corner-sharing Nb(O,F) octahedra with hexagonal channels occupied by Cs ions. Isomorphous RbNbOF was obtained upon reacting RbH(tfa) with NbO. Synthetic optimization enabled the preparation of RbNbOF and CsNbOF 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.