A Novel Discovery of Negative Thermal Expansion in Rare-earth Pyrochlore through Anion Order-Disorder Transition

TL;DR

This paper reports the first observation of negative thermal expansion in rare-earth pyrochlores caused by oxygen anion migration, revealing a new mechanism for NTE in complex oxides and its pressure sensitivity.

Contribution

It uncovers the origin of NTE in rare-earth pyrochlores through anion order-disorder transition, advancing understanding of structural behaviors in complex oxides.

Findings

NTE originates from oxygen anion migration between lattice sites.

Anion migration causes lattice contraction and polyhedral distortion.

External pressure delays the anion migration transition.

Abstract

In this study, we report for the first time the occurrence and investigation of the negative thermal expansion (NTE) effect in rare-earth pyrochlores. It is found that the NTE originates from the migration of oxygen anions from 48f sites to 8b sites, where one-twelfth of the original anions gradually occupy half of the available oxygen vacancies. This initial rapid transition leads to the distortion and rotation of polyhedral units, effectively contracting the lattice and manifesting as macroscopic NTE. The transition is sensitive to external isotropic pressure, where increasing pressure delays the onset of anion migration. This study deepens our understanding of NTE in complex oxides and demonstrates the utility of deep learning potentials for exploring intricate structural behaviors.