Dielectric properties of Pb5Al3F19

P112

DIELECTRIC PROPERTIES OP Pb5A13Pl9

V.Andriamampianina, J.M.Reau, J.Ravez, S.C.Abrahams

and

P.Hagenmuller Laboratoire de Chimie du Solide du CNRS,Universite de Bordeaux I, 351 tours de la Liberation, 33405 Talence (France)

The tetragonal A5U3P19 compounds (A = Sr,Ba,Pb; M = Al,Ti,V, Cr,Pe,Gal and with the Pb5Cr3P19 -type structure undergo a first order transition without change of crystalline system at 285 ,$ Tcl< 1090 K [1,2]. The transition temperature depends strongly 2+ and R3+ cations. The atomic positions on the nature of the A of the low temperature phase satisfy the structural criteria for ferroelectricity Ill.

The present work concerns the dielec-

tric properties of Pb5A13Plg : this material has been chosen because it has the lowest value of Tc (2 285 K) and consequently relatively low dielectric losses at T Z Tc . Dielectric measurements have been performed

on

disk

either

ceramics or single crystals oriented along either Ox or Oz. The thermal dependences of s; and r;

have been determined in vacuum

or under dry helium from 100 to 600 K and in the frequency range

20 - 3.105 Hz . A jump connected with a quicker increase of c;, r; and

e ap-

pears on heating at Tc. Depending on the experimental process, this transition occurs without or with relaxation which involves thus a higher dielectric anomaly temperature when frequency increases. Another dielectric relaxation is observed at higher temperature. All these phenomena are discussed in terms of structural data and space charge electrical properties. 1 J.Ravez,V.Andriamampianina,A.Simon,J.Grannec

and S.C.Abrahams,

J.Appl. Phys. ( to appear). 2 S.C.Abrahams, J.Albertsson, C.Svensson and J.Ravex, Acta.Cryst. B 46

497

(19901 .