We have studied the volume collapse of ${\mathrm{LaMnO}}_3$ at the Jahn-Teller (JT) transition temperature $T_{\mathrm{JT}}=750\mathrm{}\mathrm{K}$ 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 $\Delta V/V\simeq {10}^{-2}.$ 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 behavior is also observed under Ba doping.

• Received 29 October 2003

DOI:https://doi.org/10.1103/PhysRevB.69.132417