Phase transitions and ferroelectric behavior in Pb(5)M(3)0 (9)F(10) (M = Mo, W) and Phase Transition in Pb(5)Ti(3)0(3)F (16).

Two new nonlinear-optic lead transition-metal oxyfluorides have been prepared and characterized. Pb(5)W(3)0(9)F(10) is tetragonal with a = 14.583(3), c = 7.365(1)angstroms and nonlinear dielectric susceptibility d(33) = 0.025(5) 10(-12)V(-1) m. The atomic arrangement permits reversal of the spontaneous polarization, with resulting maximum atomic displacements along the polar axis of about 0.4angstroms. The Curie temperature predicted on the basis of the tungsten and associated oxygen atom displacements is about 840K. The experimental phase transition temperature is 785(15)K. The dielectric permittivity reaches a maximum of 2.4x10(7) at 805(10)K as the loss tangent passes through a minimum. Pb(5)Mo(3)0(9)F(10) is isostructural with Pb(5)W(3)0(9)F(10) and exhibits similar properties. Figures of merit, for comparison with established electrooptic materials, remain to be measured. The PbF(2)-Mo0(3) system contains five distinct and newly characterized phases: cubic Pb(1-x)Mo(x) 0(3x)F(2-2x) with 0 x 0.15, orthorhombic Pb(3)mo0(3)F (6) with distorted fluorite-type structure, the closely related alpha- and beta-phases of tetragonal Pb(5)Mo(3)0(9)F(10), and tetragonal PbMo0(3)F(2). Pb(5)Ti(3)0(3)F(16) is isostructural with Pb(5)W(3)0(9)F(10), with a phase transition at 780(10)K and d(33) 10(-16)V(-1)m.