Phase transition at 555K in nonlinear optic Pb(5)Cr(3)F(19).

A new nonlinear-optic crystal has been grown and characterized. The a-lattice constant of tetragonal Pb(5)Cr(3)F(19), 14.384 (5)angstrom at 295K, expands sharply by 0.6% on heating through the Curie temperature (T(c) = 555(5)K) as the c-lattice constant, 7.408(2)angstrom at 295K, abruptly contracts by 0.6% and the uniaxial indicatrix of the crystal reverses sign from negative to positive. The phase transition is first order. The a-lattice constant below T(c) fits a second order polynomial of the form a(i)(T) = a(i)(295) [1 + alpha(i)deltaT + beta(i)(deltaT)(2)] with alpha(1) = 1.2x10(-6)K(-1) and beta(1) = 9.8x10(-8)K(- 2); the c-lattice constant thermal variation is linear with alpha(3) = 10.3x10(-6)K(-1). Above T(c), the thermal expansion of both axes becomes linear with alpha(1) = 2.5 and alpha(3) = 4.9x10(-6)K(-1) for a(555) = 14.568(8) and c(555) = 7.388 (5)angstrom. The birefringence at 295K is 310 x 10(-4) and at 640K is -35 x 10(-4), for lambda = 543nm. The entropy change at T(c) is 13(3) J mol(-1)K(-1).