Birefringence investigations of the Sn2P2(SexS1−x)6 uniaxial ferroelectrics behaviour near the lifshitz point

The temperature dependencies of optical birefringence for nominally pure crystals Sn 2 P 2 S 6 and these crystals with the impurities of lead or selenium atoms, and solid solutions of Sn 2 P 2 (Se x S 1−x ) 6 have been investigated. The changes of the anomalies in the vicinity of the second order ferroelectric phase transition with the increase of the defectiveness of the crystalline structure and upon the approach of the phase transition to the Lifshitz point on the phase diagram have been analyzed. The experimental data agree with the known results of the renormalization group analysis of the asymptotic behavior of the dipole system near the Lifshitz point.