Cation-Anion Interactions in Solid State Oxide Fluorides

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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 dπ pπ 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, Cs2NbOF5, Rb3Na(NbOF5)2H2O, Rb3Na(MoO2F4)2H2O, Rb3Na(WO2F4)2H2O, and K3Na(WO2F4)2H2O, 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 [MOxF6-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 dπ pπ 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 CsNaNbOF5. The large, ten-coordinate cesium cations reside on inversion centers.

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  • 09/08/2018
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