Phase transitions and potential ferroelectricity in noncentrosymmetric KNaNbOF5

Abstract

Increasing the number of known ferroelectrics requires expanding the chemical design space and mechanisms producing ferroelectricity. To that end, we examine the displacive, noncentrosymmetric-to-centrosymmetric phase transition in the oxyfluoride ${\mathrm{KNaNbOF}}_{5}$ using ab initio calculations and Landau theory. We predict an intermediate $Pnma$ phase occurs in the transition from the known low- and high-temperature phases, $Pna{2}_{1}$ and $Cmcm$, respectively, which we subsequently confirm using high-resolution and in situ $^{19}\mathrm{F}$ magic-angle spinning nuclear magnetic resonance. Using the intermediate structure, we then calculate the monodomain polarization switching barrier in ${\mathrm{KNaNbOF}}_{5}$ to be $\ensuremath{\sim}93$ meV per formula unit, comparable to existing ferroelectrics. The reversal of the polarization is due to uncompensated antipolar displacements of oxygen and fluorine and does not require oxygen and fluorine site exchange as in other nonswitchable heteroanionic materials, which makes ${\mathrm{KNaNbOF}}_{5}$ a viable oxyfluoride for a switchable electric polarization.