Paraelectric, piezoelectric and pyroelectric crystals: II Phase transitions

For pt. I, see ibid., vol. 6, 121 (1973). The response of a crystal in the neighbourhood of a structural phase transition is discussed. It is shown that the behaviour of the scattering cross section is very dependent upon the symmetry of the two phases. The weakly anharmonic crystal model is used within the framework of the Landau theory of phase transitions to discuss nonpiezoelectric/ ferroelectric, piezoelectric/ferroelectric and transitions involving an increase in the size of the unit cell both of the improper ferroelectric kind and otherwise. In every case it is shown that the theory gives rise to the possibility of quasi-elastic as well as inelastic scattering below T c , but above T c quasi-elastic scattering is expected in the piezoelectric/ferroelectric and improper ferroelectric phase transitions.