Applications of XRD in forensic casework
W. Heijnen, A. van de Craats, J. de Koeijer, L. Koomen, M. Warmerdam · Acta Crystallographica Section A Foundations of Crystallography · 2005
We have investigated the Jahn-Teller (JT) transition accompanied by the orbital order-disorder transition in LaMnO 3 by high temperature X-ray powder diffraction with synchrotron radiation and also neutron powder diffraction.The unit cell volume of LaMnO 3 decreases with increasing temperature in a narrow temperature range below T JT = 750 K, and then undergoes a volume collapse at T JT .We interpret this effect as due to the more efficient packing of the MnO 6 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 3+ e g 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 LaMnO 3 with Ba.