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

Motivated by the lack of facile routes to alkali-niobium(V) oxyfluorides KNb 2 O 5 F and CsNb 2 O 5 F, we investigated the reactivity of alkali trifluoroacetates KH(tfa) 2 and CsH(tfa) 2 (tfa = CF 3 COO - ) toward Nb 2 O 5 in the solid state. Tetragonal tungsten bronze KNb 2 O 5 F and pyrochlore CsNb 2 O 5 F were obtained by simply reacting the corresponding trifluoroacetate with Nb 2 O 5 at 600 °C under air, without the need for specialized containers or a controlled atmosphere. Thermolysis of KH(tfa) 2 in the presence of Nb 2 O 5 yielded single-phase polycrystalline KNb 2 O 5 F. By contrast, the reaction between CsH(tfa) 2 and Nb 2 O 5 produced a mixture of CsNb 2 O 5 F and a new oxyfluoride of formula CsNb 3 O 7 F 2 , whose crystal structure was solved using powder X-ray and electron diffraction. CsNb 3 O 7 F 2 (space group P 6/ 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 RbNb 3 O 7 F 2 was obtained upon reacting RbH(tfa) 2 with Nb 2 O 5 . Synthetic optimization enabled the preparation of RbNb 3 O 7 F 2 and CsNb 3 O 7 F 2 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.