Volume collapse in LaMnO3 at the Jahn-Teller transition temperature -: art. no. 132417

We have studied the volume collapse of LaMnO3 at the Jahn-Teller (JT) transition temperature T-JT=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+ 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 DeltaV/Vsimilar or equal to10(-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.