Changes of infrared and Raman spectra induced by structural phase transitions. II. Examples

For pt.I see ibid., vol.9, p.1571 (1976). The general group-theoretical method of discussing the frequency and strength changes in infrared and Raman spectra due to Landau-type structural phase transitions derived previously, is applied to 18 types of the best known structural phase transitions in dielectric crystals: SrTiO 3 , LaAlO 3 , SiO 2 , NH 4 Cl, NH 4 Br, NaNO 3 , BaMnF 4 , (CH 3 NH 3 ) 2 MnCl 4 , DyVO 4 , NaN 3 , KH 2 PO 4 , BaTiO 3 , LiTaO 3 , SbSI, NaNO 2 , (NH 4 ) 2 SO 4 , Gd 2 (MoO 4 ) 3 , and RbH 3 (SeO 3 ) 2 . The frequency splitting is in the lowest order in all cases proportional to eta s or eta s 2 , where eta s is the spontaneous value of the order parameter. The infrared or Raman strength of modes which become active in the distorted phase is in the lowest order proportional to delta s 2 , or delta s 4 . The available experimental data are discussed and in most cases a good agreement with theory is found.