| The early transition metal oxide fluoride molecular anions with the general formula [MOxF6--x]n- with x = 1 for M = V5+, Nb5+, Ta5+ (n = 2) and x = 2 for M = Mo6+ , W6+ (n = 2) feature distorted octahedral coordinations, that arise from electronic dpi-ppi metal-oxide orbital interactions (similar to those observed in classic ferroelectric metal oxides). The nonlinear optical response of the oxide fluoride materials would be at a maximum when the inherent "primary" out-of-center distortions in the oxide fluoride anions are crystallographically ordered and aligned. In order to attain a greater level of predictability in designing polar materials with [MOxF6-x ]n- anions, the relationship between the anionic group and the surrounding cationic network of bonds must be intimately understood.; Examination of the cation-anion interactions in KNaNbOF5, CsNaNbOF5, Rb2NbOF5, Cs2NbOF 5, Rb3Na(NbOF5)2·H2O, Rb3Na(MoO2F4)2·H 2O, Rb3Na(WO2F4)2·H 2O, and K3Na(WO2F4)2·H 2O, largely through bond valence analyses, reveals that the cationic network of bonds surrounding the [MOxF6-x ]n- anions arrange to accommodate both primary and secondary distortions, which arise due to inherent, electronic effects and chemical hardness, respectively. These observations suggest that specific cation combinations, i.e. Na/K, can result in polar ordering of the [MOx F6-x]n- anions while others do not, i.e. Na/Cs.; Polar ordering was observed in KNaNbOF5 where there was a minimum number of contacts to the nucleophilic oxide ion, thereby preserving the primary Nb-O dpi-ppi interaction. The small, eight-coordinate potassium cations distort from the centers of their coordination polyhedra toward the negatively charged trans-fluoride ions of the [NbOF5]2- anion. In contrast, an increase in the number of contacts to the oxide ion and a lengthening of the Nb-O bonds are observed in CsNaNbOF 5. The large, ten-coordinate cesium cations reside on inversion centers. |
