Stable polar oxynitrides through epitaxial strain

We investigate energetically favorable structures of $AB{\mathrm{O}}_2\mathrm{N}$ 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 ${\mathrm{YSiO}}_2\mathrm{N}$ ($I4cm$) is metastable up to at least 600 K under compressive epitaxial strain before converting to the stable wollastonitelike structures ($I4/mcm$). We predict that ${\mathrm{YGeO}}_2\mathrm{N}, {\mathrm{LaSiO}}_2\mathrm{N}$, and ${\mathrm{LaGeO}}_2\mathrm{N}$ are metastable as ferroelectric perovskites ($P4mm$) at zero pressure even without epitaxial strain.

Figure 1

Crystal structures of oxynitride compounds in (a) the $P{6}_{1}22$ structure, (b) the $P4mm$ structure, (c) the $I4/mcm$ structure, (d) the $I4cm$ structure, (e) the $P{3}_{1}21$ structure, (f) the $P{3}_2$ structure, and (g) the $Cmc{2}_1$ structure. Cyan, blue, red, and light grey spheres represent Y/La/Bi, Si/Ge/Hf, O, and N atoms, respectively.

Figure 2

Calculated enthalpies per formula unit (f.u.) of various predicted structures as functions of pressure with respect to the $P{6}_{1}22$ structure of the oxynitrides.

Figure 3

(a) The calculated enthalpies of the $P4mm$ structure with respect to the most stable structure of the various oxynitrides at 30 GPa. (b) The calculated enthalpies of the $P4mm$ structure of ${\mathrm{LaSiO}}_2\mathrm{N}$ as a function of epitaxial strain with respect to the $P{3}_{1}21$ structure at 30 GPa. Positive strains here and elsewhere in this paper are extensional.

Figure 4

Soft phonon modes for the $P4mm$ structures of the oxynitrides as functions of the epitaxial strain at ambient pressure.