Volume collapse inLaMnO3at the Jahn-Teller transition temperature

Abstract

We have studied the volume collapse of ${\mathrm{LaMnO}}_{3}$ at the Jahn-Teller (JT) transition temperature ${T}_{\mathrm{JT}}=750\mathrm{K}$ which has recently been found in high-temperature powder x-ray and neutron diffraction experiments. We construct a model Hamiltonian involving the pseudospin of ${\mathrm{Mn}}^{3+}$ ${e}_{g}$ states, the staggered JT distortion and the volume strain coordinate. We show that the anharmonic coupling between these primary and secondary order parameters leads to the first-order JT phase transition associated with a comparatively large reduction of the unit cell volume of $\ensuremath{\Delta}V/V\ensuremath{\simeq}{10}^{\ensuremath{-}2}.$ We explain the temperature dependence of JT distortions and volume strain and discuss the volume change as function of the anharmonic coupling constant. A continuous change to a second-order transition as function of model parameters is obtained. This behavior is also observed under Ba doping.