Phonons and structural phase transformations

This chapter discusses lattice phase transitions, soft-modes and critical scattering. The authors emphasise the complementary nature of diffuse X-ray scattering and neutron spectroscopy in the study of lattice phase transition. They present some results obtained on the ferroelectric material KNbO3 which, like BaTiO3, undergoes three phase transformations with decreasing temperature. Then they discuss certain experimental aspects of inelastic neutron scattering which are of particulare relevance for structure determinations, and for phonon spectroscopy in general. Besides the relative low cost of producing X-ray beams, their main advantage compared to neutrons is the possibility of using much smaller samples, and to be able to make photographic measurements. The main disadvantage is that the scattering cannot be analysed in energy and also results will remain in part qualitative. Then the soft mode investigated in Tb2(MoO4)3 is described. Furthermore dynamical critical phenomena are discussed by reference to investigations on ND4Br by inelastic neutron scattering. (HPOE)