Short-Range\nOrder and Local Distortions in Entropy\nStabilized Oxides

An idealized high entropy oxide is characterized by perfect\nchemical\ndisorder and perfect positional order. In this work, we investigate\nthe extent to which short-range order (SRO) and local structural distortions\nimpede that idealized scenario. Working in the entropy stabilized\nα-PbO<sub>2</sub> structure, we compare a two-component system,\n(Ti,Zr)O<sub>2</sub>, with a four-component system, (Ti,Zr,Hf,Sn)O<sub>2</sub>, using a combination of experimental and computational approaches.\nSpecial quasi-random structures are used in conjunction with density\nfunctional theory calculations to investigate the local distortions\naround specific elements revealing significant local distortions that\nare relatively insensitive to the number of chemical constituents.\nUsing finite temperature Monte Carlo simulations, we are able to reproduce\nthe previously experimentally observed SRO and transition temperature\nfor the two-component system. However, the ideal configurational entropy\nis never reached, so some amount of SRO is expected even at synthesis\ntemperatures. On the other hand, the order–disorder transition\ntemperature is dramatically lower and experimentally inaccessible\nfor the four-component system, while the configurational entropy is\ncloser to ideal at synthesis conditions and less sensitive to temperature.\nTotal scattering measurements and pair distribution function analysis\nof slow-cooled and quenched samples support this view. In summary,\nwe demonstrate that SRO effects in high entropy materials are less\nprevalent as more components are added in, provided the pairwise interaction\nstrengths remain comparable, while local distortions are less affected\nby the number of components.