  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.
