Linear birefringence behavior of incommensurate ferroelectric materials

Linear birefringence measurement is a very useful tool in studying the normal-incommensurate-commensurate phase transition sequence because this method is sensitive to variations in order parameter. We have investigated the critical exponent βof the normal-incommensurate phase transition of [N(CH₃)₄]₂ZnCl₄ and K₂ZnCl₄ crystals by the linear birefringence method. The β values evaluated from scaling analysis are consistent with the 3-d XY model. We have also studied linear birefringence behavior in the incommensurate-ferroelctric phase transition of S_0.91O_0.9C(NH₂)₂ and K₂ZnCl₄. Linear birefringence shows broad variations under the electric field, instead of a single jump or a kink. These broadenings correspond to the peak splitting behavior of the dielectric constant curve. The field-induced phase in the broadening region is attributable to the coexistence of incommensurate and ferroelectric phases.