Anion order in oxysulfide perovskites: origins and implications

Heteroanionic oxysulfide perovskite compounds represent an emerging class of new materials allowing for a wide range of tunability in the electronic structure that could lead to a diverse spectrum of novel and improved functionalities. Unlike cation ordered double perovskites-where the origins and design rules of various experimentally observed cation orderings are well known and understood-anion ordering in heteroanionic perovskites remains a largely uncharted territory. In this contribution, we present and discuss insights that have emerged from our first-principles-based electronic structure analysis of a prototypical anion-ordered SrHf(O0.5S0.5)(3) oxysulfide chemistry, studied in all possible anion configurations allowed within a finite size supercell. We demonstrate that the preferred anion ordering is always an all-cis arrangement of anions around an HfO3S3 octahedron. As a general finding beyond the specific chemistry, the origins of this ordering tendency are traced back to a combined stabilization effect stemming from electronic, elastic, and electrostatic contributions. These qualitative notions are also quantified using state-of-the-art machine learning models. We further study the relative stability of the identified ordering as a function of A (Ca, Sr, Ba) and B (Ti, Zr, Hf) site chemistries and probe chemistry-dependent trends in the electronic structure and functionality of the material. Most remarkably, we find that the identified ground-state anion ordering breaks the inversion symmetry to create a family of oxysulfide ferroelectrics with a macroscopic polarization >30 mu C/cm(2), exhibiting a significant promise for electronic materials applications.