THERMAL-DEPENDENCE OF THE PB5AL3F19 LATTICE-PARAMETERS IN PHASES I-IV

Pb5Al3F19 undergoes three first-order phase transitions; on cooling fr om phase I (4/m) to phase II (2/m) at 365 K, from phase II to phase II I (4/m) at 305 K, and from phase III to phase IV (4mm) at 120 K. The l engths of the a- and c-axes undergo continuous change at the highest t emperature phase transition as the beta-angle increases from 90-degree s below 365 K; no difference is detectable between the lengths of the a- and b-axes in monoclinic phase II, within an accuracy of 0.007 angs trom. The beta-angle temperature dependence follows a power law with e xponent that does not differ significantly from 1/2. At the 305 K firs t-order phase transition, the beta-angle abruptly changes from its max imum value of 90.41-degrees in phase II to 90-degrees in tetragonal ph ase III; the a- and c-axes, however, maintain their continuous change in length through this transition. The 120 K first-order phase transit ion is characterized by an abrupt 0.77% decrease in the length of the normalized a-axis, and an equally abrupt 1.89% increase in the c-axis length, on cooling from phase III to phase IV. The a- and c-axes in al l four phases are characterized by linear thermal coefficients, with m agnitudes presented below. The normalized a-axis lengths in phases I, II and III extrapolated to 0 K differ by less than 0.021 angstrom from 14.174 angstrom, the corresponding c-axis lengths at 0 K differ by le ss than 0.004 angstrom from 7.194 angstrom. The normalized unit cell v olume thermal dependence of phases I, II and III is linear and continu ous between 400 and 120 K but undergoes a discontinuity of 0.40% at th e transition from phase III to phase IV.