Volume contraction at the Jahn-Teller transition of LaMnO$_3$

TL;DR

This study models the volume collapse in LaMnO$_3$ at its Jahn-Teller transition, revealing a first-order phase change with significant volume reduction driven by anharmonic coupling, and explores how doping influences this transition.

Contribution

The paper introduces a model Hamiltonian that explains the volume contraction and transition order in LaMnO$_3$ at the Jahn-Teller transition, including effects of anharmonic coupling and doping.

Findings

First-order transition with ~1% volume reduction.

Temperature dependence of Jahn-Teller distortions explained.

Transition order can change to second order with parameter variation.

Abstract

We have studied the volume collapse of LaMnO$_3$ 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 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 behaviour is also observed under Ba doping.