STRUCTURE OF FERROELECTRIC PB5AL3F19 AT 160 K, POLARIZATION REVERSAL AND RELATIONSHIP TO FERROELECTRIC PB5CR3F19 AT 295 K

Pb5Al3F19 at 160 K is ferroelectric, crystallizing in the tetragonal s ystem. With M(r) = 1477.9, the space group is I4cm and a = 14.07 (2), c = 7.30 (1) Angstrom, V = 1445 (3) Angstrom(3), Z = 4, D-x = 6.793 Mg m(-3). For lambda(Mo K alpha) = 0.71073 Angstrom, mu = 56.36 mm(-1). F(000) = 2480. Pb5Al3F19 undergoes a first-order phase transition on c ooling at 120 K and on heating at 270 K. The structure was determined from 656 independent F(hkl) with I > 3 sigma(I) and refined by least s quares to R = 0.0527, wR = 0.0480, S = 1.065. No atom is further than 0.83 Angstrom from the hypothetical I4/mcm atomic positions, hence the crystal is structurally ferroelectric. The mean atomic polar displace ment of the two independent Al atoms from the zero spontaneous-polariz ation location, allowing for the F atoms, is 0.207 (28) Angstrom which leads to a predicted T-c = 860 (230) K. Several additional phase tran sitions are known to form before the hypothetical phase is attained. E ach independent Al atom occupies a fluorine octahedron with average Al -F distance either 1.818 (5) or 1.824 (1) Angstrom. One Pb atom is nin e coordinated, occupying a distorted tricapped trigonal prism with ave rage Pb-F = 2.66 (22) Angstrom. The other is ten coordinated, forming a distorted bicapped cuboid, with average Pb-F = 2.61 (28) Angstrom. P yroelectric pulses are readily detectable in crystals cooled below 120 K, or subsequently on heating to 270 K. Similarly, numerous piezoelec tric resonances are observed in the same thermal ranges; the compressi on mode at similar to 1197 kHz has been identified by its characterist ic admittance circle. The mechanical quality factor Q(m) for ferroelec tric Pb5Al3F19, determined from the admittance-circle-derived equivale nt circuit, is similar to 423.