Phase stability of entropy stabilized oxides with the $\alpha$-PbO$_2$ structure

TL;DR

This study combines experimental and computational methods to analyze the phase stability of entropy stabilized oxides with the $ ext{α}$-PbO$_2$ structure, focusing on tetravalent cations and explaining their stability.

Contribution

It introduces a pairwise approach to predict phase stability of high entropy oxides and confirms the stability of the $ ext{α}$-PbO$_2$ structure for specific tetravalent cation combinations.

Findings

$ ext{α}$-PbO$_2$ is the lowest energy structure among candidates.

No other five-component tetravalent cation compounds are expected to form.

Experimental verification supports the predicted phase stability.

Abstract

The prediction of new high entropy oxides (HEOs) remains a profound challenge due to their inherent chemical complexity. In this work, we combine experimental and computational methods to search for new HEOs in the tetravalent $A$O$_2$ family, using exclusively $d^0$ and $d^{10}$ cations, and to explain the observed phase stability of the $\alpha$-PbO$_2$ structure, as found for the medium entropy oxide (Ti, Zr, Hf, Sn)O$_2$. Using a pairwise approach to approximate the mixing enthalpy, we confirm that $\alpha$-PbO$_2$ is the expected lowest energy structure for this material above other candidates including rutile, baddeleyite, and fluorite structures. We also show that no other five-component compound composed of the tetravalent cations considered here is expected to form under solid state synthesis conditions, which we verify experimentally. Ultimately, we conclude that the flexible geometry of the $\alpha$-PbO$_2$ structure can be used to understand its stability among tetravalent HEOs.