Birefringence Induced By Lattice Imperfections In The Paraelectric Phase Of Kh2po4

Birefringence for light travelling along the c axis was observed above the Curie temperature T(c) in the strained regions characteristic of the KH2PO4 crystal. The birefringence and the area of the birefringent regions increase as the temperature approaches T(c). In the strained regions, the crystal is optically biaxial and negative, and the optical indicatrix has the same orientation as that in the ferroelectric phase. It is found that the birefringence is induced by the strain of x6 type around the lattice imperfections through a photoelastic effect.