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 phas e using hard synchrotron radiation and hot neutrons. Gram-Charlier expansio ns of the thermal parameters have been performed and have revealed interest ing features about the probability density function and the one-particle po tential of the different atoms. This description is compared with other des criptions in terms of the split atom model and in terms of the rotator mode l. Structural trends regarding the order-disorder versus displacive charact er of the phase transitions have been obtained. It is concluded that SrTiO3 and BaTiO3 are quasi-harmonic systems whereas KNbO3 shows weak anharmonici ty and PbTiO3 and PSN display strong anharmonic features.