Competing cation-anion interactions and noncentrosymmetry in metal oxide-fluorides: A first-principles theoretical study

Anomalous Born dynamical charges in perovskite oxides, such as BaTiO ₃ and KNbO₃, are known to be indicators of their tendency to turn polar through cation off-centering and measure of the interaction between d states of transition metal and p states of oxygen. Here, we use first-principles density functional theory based calculations to determine Born charges of noncentrosymmetric KNaNbOF₅ and centrosymmetric CsNaNbOF₅ with a goal to assess the cation-anion interactions relevant to the breaking of their centrosymmetry. We find that while noncentrosymmetry is favored by the primary Nb-O interaction, covalency in the competing interaction of Cs with anions suppresses it stabilizing the centrosymmetric structure and is reflected clearly in the deviation of Born effective charges (BECs) from their nominal ionic values. We identify specific features in the electronic structure that correlate with stability of the centrosymmetric structure and show that polarization of the noncentrosymmetric KNaNbOF₅ estimated using the Berry phase method is rather weak ∼0.21 μC/cm², consistent with the finding that it originates from the competition between the primary and secondary electronic distortions.