Optical properties of ferroelectrics. digressions from phenomenological theory

Temperature dependences of birefringence were measured in (NH4) 2SO4, (NH4)2BeF4 and (CH 3NH3)Al(SO4)2. 12 H2O crystals and the photoelastic constants of the crystals were estimated. The temperature behaviour of electro-optical and dielectric characteristics was investigated. It is shown that spontaneous changes of birefringence appearing are disproportional to P2 s. The contributions of Ps and spontaneous deformation into optical constants anomalies at ferroelectric transitions are discussed