We have studied the volume collapse of LaMnO3 at the Jahn- Teller (JT) transition temperature TJT=750 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 Mn3+ eg 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 ΔV/V≃ 10−2. 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 behaviour is also observed under Ba doping.
@article{arxiv.cond-mat/0310700,
title = {Volume contraction at the Jahn-Teller transition of LaMnO$_3$},
author = {T. Maitra and P. Thalmeier and T. Chatterji},
journal= {arXiv preprint arXiv:cond-mat/0310700},
year = {2009}
}