Microscopic origin of the structural phase transitions at the Cr2O3 (0001) surface

TL;DR

This study uses first-principles calculations and Monte Carlo simulations to uncover the microscopic mechanisms behind the unusual structural phase transitions observed on the Cr2O3 (0001) surface, highlighting the roles of surface site competition, strain, and magnetic order.

Contribution

It provides a detailed microscopic understanding of the surface phase transitions in Cr2O3 (0001) through a combined computational approach, including a configurational Hamiltonian and thermodynamic analysis.

Findings

Surface undergoes an ordering transition at 165 K.

Epitaxial strain and magnetic order influence surface configurational frustration.

Mechanism for disordering transition involves strain-induced frustration.

Abstract

The surface of a Cr2O3 (0001) film epitaxially grown on Cr undergoes an unusual reentrant sequence of structural phase transitions. In order to understand the underlying microscopic mechanisms, the structural and magnetic properties of the Cr2O3 (0001) surface are here studied using first-principles electronic structure calculations. Two competing surface Cr sites are identified. The energetics of the surface is described by a configurational Hamiltonian with parameters determined using total energy calculations for several surface supercells. Effects of epitaxial strain and magnetic ordering on configurational interaction are also included. The thermodynamics of the system is studied using Monte Carlo simulations. At zero strain the surface undergoes an ordering phase transition at 165 K. Tensile epitaxial strain together with antiferromagnetic ordering drive the system toward strong configurational frustration, suggesting the mechanism for the disordering phase transition at lower temperatures.