Crystal structures of new layered perovskite-type oxyfluorides, CsANb2O6F (A = Sr and Ca) and comparison with pyrochlore-type CsNb2O5F

New layered perovskite-type oxyfluorides, CsSrNb2O6F and CsCaNb2O6F, were prepared via solid state reactions and their crystal/electronic structures were compared to those of a compositional analogue, CsNb2O5F which has a 3-dimensional pyrochlore-type structure. The CsSrNb2O6F and CsCaNb2O6F phases are derived from the Dion-Jacobson family of a general formula A[A'n-1BnX3n+1] with n = 2. Rietveld refinements based on synchrotron X-ray diffraction data revealed that CsSrNb2O6F belongs to the tetragonal system with a = 3.86716(1) angstrom, c = 11.45257(2) angstrom (P4/mmm, Z = 1), while CsCaNb2O6F adopts a 2a(p) x 2a(p) x 2c(p) superstructure with orthorhombic unit cell parameters, a = 7.64813(1) angstrom, b = 7.67516(1) angstrom, c = 22.43663(3) angstrom (Bmmb, Z= 8). The anion distributions in the lattices were confirmed by applying Bond Valence Sum (BVS) method; the F ions occupy the central sites of the corner-sharing double perovskite block in the layered structures, but no ordering of anions was found in the pyrochlore structure. Through UV-Vis diffuse reflectance measurement, the band gap energy was estimated to 3.43 eV for CsSrNb2O6F, 3.65 eV for CsCaNb2O6F and 4.00 eV for CsNb2O5F. The band gap energy showed a good correlation with the Nb-O(F)-Nb bond angle(theta) in the oxyfluorides. As the Nb-O(F)-Nb bond angle deviates from 180 degrees, the value of E-g increase. The good linearity observed in the E-g vs. cos(180 - theta) plot suggests that E-g can be finely controlled by adjusting of the Nb-O(F)-Nb bond angle in the lattice.