Nucleation and growth mechanisms in solid-solid phase transitions

Abstract

We have investigated the Jahn-Teller (JT) transition accompanied by the orbital order-disorder transition in LaMnO3 by high temperature X-ray powder diffraction with synchrotron radiation and also neutron powder diffraction. The unit cell volume of LaMnO3 decreases with increasing temperature in a narrow temperature range below TJT = 750 K, and then undergoes a volume collapse at TJT. We interpret this effect as due to the more efficient packing of the MnO6 octahedra in the orbitally disordered or orbital liquid state. The orbital melting phenomenon can be qualitatively compared with the melting of ice. By constructing a model Hamiltonian involving the pseudospin of Mn eg states, the staggered JT distortion and the volume strain coordinate, we show that the anharmonic coupling between these primary and the secondary order parameters leads to the first-order J-T phase transition associated with a comparatively large reduction of the unit cell volume. We explain the temperature dependence of the JT distortions and volume strain and discuss the volume change as a function of the anharmonic coupling constant. A continuous change to a second-order transition as a function of the model parameters is obtained. This behaviour has been observed experimentally by us on doping LaMnO3 with Ba.