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Competing Cation–Anion Interactions and Noncentrosymmetry in Metal Oxide-Fluorides: A First-Principles Theoretical Study
journal contribution
posted on 2014-01-02, 00:00 authored by Abhishek Kumar Mishra, Michael R. Marvel, Kenneth R. Poeppelmeier, Umesh V. WaghmareAnomalous
Born dynamical charges in perovskite oxides, such as
BaTiO3 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.