Stable polar oxynitrides through epitaxial strain

We investigate energetically favorable structures of ABO(2)N oxynitrides as functions of pressure and strain via swarm-intelligence-based structure prediction methods, density functional theory (DFT) lattice dynamics and first-principles molecular dynamics. We predict several thermodynamically stable polar oxynitride perovskites under high pressures. In addition, we find that ferroelectric polar phases of perovskite-structured oxynitrides can be thermodynamically stable and synthesized at high pressure on appropriate substrates. The dynamical stability of the ferroelectric oxynitrides under epitaxial strain at ambient pressure also implies the possibility to synthesize them using pulsed laser deposition or other atomic layer deposition methods. Our results have broad implications for further exploration of other oxynitride materials as well. We performed first-principles molecular dynamics and find that the polar perovskite of YSiO2N (I4cm) is metastable up to at least 600 K under compressive epitaxial strain before converting to the stable wollastonitelike structures (I4/mcm). We predict that YGeO2N, LaSiO2N, and LaGeO2N are metastable as ferroelectric perovskites (P4mm) at zero pressure even without epitaxial strain.