Anion order and spontaneous polarization in LaTiO\textsubscript{2}N oxynitride thin films
The perovskite oxynitride LaTiO\textsubscript{2}N is a promising material for photocatalytic water splitting under visible light. One of the obstacles towards higher efficiencies of this and similar materials stems from charge-carrier recombination, which could be suppressed by the built-in electric field in polar materials. In this study, we investigate the spontaneous polarization in epitaxially strained LaTiO\textsubscript{2}N thin films via density functional theory calculations. The effect of epitaxial strain on the anion order, resulting out-of-plane polarization, energy barriers for polarization reversal and the corresponding coercive fields are studied. We find that for compressive strains larger than 4\% the thermodynamically stable anion order is polar along the out-of-plane direction and has a coercive field comparable to other switchable ferroelectrics. Our results show that strained LaTiO\textsubscript{2}N could indeed suppress carrier recombination and lead to enhanced photocatalytic activities.