Short-range order and local distortions in entropy stabilized oxides

TL;DR

This study investigates how short-range order and local distortions affect the idealized disorder in entropy stabilized oxides, revealing that adding more components reduces SRO effects and local distortions are relatively unaffected by composition.

Contribution

It combines experimental and computational methods to analyze local distortions and SRO in multi-component oxides, providing new insights into their structural behavior.

Findings

Significant local distortions are insensitive to the number of elements.

SRO is present at synthesis temperatures and less prevalent with more components.

Local distortions are less affected by the number of components.

Abstract

An idealized high entropy oxide is characterized by perfect chemical disorder and perfect positional order. In this work, we investigate the extent to which short-range order (SRO) and local structural distortions impede that idealized scenario. Working in the entropy stabilized $\alpha$-PbO$_2$ structure, we compare a two-component system, (Ti,Zr)O$_2$, with a four-component system, (Ti,Zr,Hf,Sn)O$_2$, using a combination of experimental and computational approaches. Special quasi-random structures are used in conjunction with density functional theory calculations to investigate the local distortions around specific elements revealing significant local distortions that are relatively insensitive to the number of chemical constituents. Using finite temperature Monte Carlo simulations, we are able to reproduce the previously experimentally observed SRO and transition temperature for the two-component system. However, the ideal configurational entropy is never reached, so SRO is expected even at synthesis temperatures. On the other hand, the order-disorder transition temperature is dramatically lower and experimentally inaccessible for the four-component system, while the configurational entropy is closer to ideal and less sensitive to temperature. Total scattering measurements and pair distribution function analysis of slow-cooled and quenched samples support this view. In general, we demonstrate that SRO effects in high entropy materials are less prevalent as more components are added in, provided the pairwise interaction strengths remain comparable, while local distortions are less affected by the number of components.