Crystal structure of ferroelastic Pb5Al2.96Cr0.04F19 at 300 K

Five different phases of Pb5Al3F19 ([I] to [V]) occur on cooling, The ferro elastic one [III] was stabilized at room temperature by a low rate of Cr-Al substitution. The structure was determined by X-ray diffraction analysis o n a single-domain crystal. The symmetry is triclinic, P<(1)over bar >, with cell parameters a = 10.72(3) Angstrom, b = 10.67(3) Angstrom, c = 7.24(2) Angstrom, alpha = 109.27(5)degrees, beta = 110.14(5)degrees, gamma =83.33 ( 5)degrees. The crystal network, which is closely related to those of the ot her varieties, is composed of both isolated and trans- corner-linked AIF(6) octahedra developing parallel to the c-axis, The structure is closely deri ved from that of the paraelastic phase [II], mainly by the inclination of t he previous tetragonal c-axis on the a-b plane, The loss of the mirror symm etry of the high-temperature phase leads ten a break in the Pb2+ ion alignm ent along the l-fold axis. Polydomain crystals were observed under a polari zing microscope; the ferroelastic [III]-paraelastic [II] phase transition o ccurs at 326 K on heating. (C) 2000 Academic Press.