Phase transitions and ferroelectric behavior in Pb5M3O9F10(M=Mo, W) and phase transition in Pb5Ti3O3F16

Abstract

Two new nonlinear optic lead transition-metal oxyfluorides have been prepared and characterized. Pb5W3O9F10 is tetragonal with a=14.583(3), c=7.365(1) Å and nonlinear dielectric susceptibility d33=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 0.4 Å. The Curie temperature predicted on the basis of the tungsten and associated oxygen atom displacements is about 840 K. The experimental phase transition temperature is 785(15) K. The dielectric permittivity reaches a maximum of 2.4×107 at 805(10) K as the loss tangent passes through a minimum. Pb5Mo3O9F10 is isostructural with Pb5W3O9F10 and exhibits similar properties. The PbF2–MoO3 system contains five distinct and newly characterized phases: cubic Pb1−xMoxO3xF2−2x with 0≤x≤0.15, orthorhombic Pb3MoO3F6 with distorted fluorite-type structure, the closely related α and β phases of tetragonal Pb5Mo3O9F10, and tetragonal PbMoO3F2. Pb5Ti3O3F16 is isostructural with Pb5W3O9F10, with a phase transition at 780(10) K and d33<10−16 V−1 m.

Multiferroic magnetoelectric fluorides: why are there so many magnetic ferroelectrics?

“…Ravez et al (1975Ravez et al ( , 1987 report ten members of this ferroelectric family, eight of which are magnetic multiferroics, such as Sr 5 Co 3 F 19 and Sr 5 Cr 3 F 19 . Multiferroic oxyfluorides of this family can be fabricated by substituting +4 metal ions for +3 ions and an equal number of oxygen ions for fluorine, such as Pb 5 Ti 3 O 3 F 16 (Arquiscanouet et al 1986, Arquis et al 1986. The ferroelastic properties of this family were analyzed by Ravez in 1993, and switching below 555 K shown by Abrahams et al (1990).…”