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Heteroanionic materials have recently positioned in the center of multiple investigations. The latter is motivated by the physical, optical, and chemical properties that can emerge when properly combining different, but compatible anions, within the same compound. Oxynitride perovskite compounds have shown to offer a wide range of different tunable properties depending on the O/N rate, for instance, the absorption of light in the visible regime makes them ideal candidates for photocatalysis applications where polar compounds offer a notable advantage [1].
In this study, we theoretically explored the origin of the polar and ferroelectric responses experimentally observed in perovskite oxynitrides SrNbO$_2$N and SrTaO$_2$N [2,3]. With this aim, first-principles calculations of the structural and vibrational properties were performed. We show that such ferroelectric behavior is extremely sensitive to the anionic ordering in both Sr(Nb,Ta)O$_2$N systems, where cis- and trans- orderings are identified as the most probable orderings in the crystals. Furthermore, We found that both cis- and trans- type can hold ferroelectricity, although the mechanisms in how such a stable phase emerges in each ordering are quite different. Aditionally, we show that spontaneous polarization characterizing such ferroelectric phases can be considerably enhanced by applying in-plane epitaxial strain, achieved by using strontium titanate as substrate.
[1] Fuertes, A. (2015). “Metal oxynitrides as emerging materials with photocatalytic and electronic properties”. Materials Horizons, 2(5), 453-461.
[2] Kim, Y. I., Woodward, P. M., Baba-Kishi, K. Z., \& Tai, C. W. (2004). “Characterization of the structural, optical, and dielectric properties of oxynitride perovskites AMO$_2$N (A= Ba, Sr, Ca; M= Ta, Nb)”. Chemistry of materials, 16(7), 1267-1276.
[3] Kikkawa, S., Sun, S., Masubuchi, Y., Nagamine, Y., \& Shibahara, T. (2016). “Ferroelectric response induced in cis-type anion ordered SrTaO2N oxynitride perovskite”. Chemistry of Materials, 28(5), 1312-1317.
