CHARACTERIZATION OF 4 PHASE-TRANSITIONS IN PB5AL3F19, INCLUDING A NEWTRANSITION AT 670 K, BY IMPEDANCE AND NMR-SPECTROSCOPY

The ac (alternating current) conductivity of Pb5Al3F19 between 293 and 743 K has been analyzed within a combined complex impedance, modulus, and permittivity formalism. The antiferroelectric phase IV to ferroel astic phase LII, and the phase m to paraelastic phase II, transitions have been characterized; the latter is shown to be accompanied by high er F- ion mobility above T=368 +/-5 K. A new dielectric anomaly observ ed isochronously in epsilon'(f,T) at 670 K corresponds to an expected, but hitherto unobserved, transition from phase II to the paraelectric prototype phase I of Pb5Al3F19 A F-19 NMR investigation between 125 a nd 430 K of F- ion diffusion properties in Pb5Al3F19 shows that half t he anions have become mobile at the ferroelectric phase V to phase IV transition temperature and chat almost all the anions are mobile at th e phase TV to phase III transition temperature. These motions are show n to be short range, and no correlation is found between phase Formati on and F- ion diffusion properties in Pb5Al3F19 in the temperature ran ge below 320 K. (C) 1998 American Institute of Physics.