Anharmonicity and disorder in simple and complex perovskites:: a high energy synchrotron and hot neutron diffraction study

We report a study of simple ABO(3) type perovskites BaTiO3, PbTiO3, KNbO3, SrTiO3 and the relaxer perovskites PbSc1/2Nb1/2O3 (PSN) in their cubic phase using hard synchrotron radiation and hot neutrons. Gram-Charlier expansions of the thermal parameters have been performed and have revealed interesting features about the probability density function and the one-particle potential of the different atoms. This description is compared with other descriptions in terms of the split atom model and in terms of the rotator model. Structural trends regarding the order-disorder versus displacive character of the phase transitions have been obtained. It is concluded that SrTiO3 and BaTiO3 are quasi-harmonic systems whereas KNbO3 shows weak anharmonicity and PbTiO3 and PSN display strong anharmonic features.