~) Solid State Co,nmunications, Vol. 79, No. 8, pp. 635-640, 1991. Printed m Great Britain. 0038-1098/9153.00+.00 Pergamon Press pie DETERMINATION OF CHARGE CARRIERS PARAMETERS OF THE PbI_xBIxOx/2F 2 SOLID SOLUTION. N.F.UVAROV*, E.F.BAIRETDINOV*, J.M.REAU** and P.HAGENMULLER** * Institute for Solid State Chemistry,Derzhavina, 18 , Novosibirsk 91 , USSR . **Laboratoire de Chimle du Solide du CNRS,Universit~ de Bordeaux I, 351 cours de la LibEration, 33405 Talence Cedex, France. Received II april 1991 by P.Burlet The bulk impedance parameters of polycrystalllne samples of the Pbl_xBixOx/2F 2 solid solution with fluorlte-type structure have been determined from an analysis of a.c. conductlvlty data measured in a wide temperature range. Hopplng rates of mobile F- ions are extracted from frequency dependences of conductivi- tles at different temperatures. The concentration of charge carriers in the investlgated samples (0.05 ~ x ~ 0.40) has been estimated : it increases wlth x and its value for x = 0.40 , equal to about 5-6~ , is of the same order of magnltude as the maximum percentage of interstitial anions of F"-type in the Pbl_xBlxF2+ x solid solution in which they play an essentlal ro- le in the long-range motlons. The conductlvity enhancement observed with increaslng x in Pbl_xBXxOx/2F 2 is mainly a mobllity effect. i. INTRODUCTION. MF 2 solld electrolytes with fluorlte-type structure are known to undergo a dlffuse transition to a disordered phase at a tem- perature T c well below the meltlng tempe- rature Tm: thls Faraday transltion is as- sociated with the development of dlsorder in the anlon sublattlce responslble for the high ion conductivity and is accompa- nled by a specific heat anomaly [1,2]. Varlous investigations of MF 2 crystals do- ped with trivalent rare-earth ions have shown that T C is conslderably reduced by doping [3]. On the other hand, the study of electrl- cal properties of a large number of solid solutions 2+ 2+a F Ml-xM'x 2+~x of fluorite-type structure has polnted out that the partial replacement of M 2+ host cations by M 3+ or M 4+ guest lons improves drastically the conductivlty [4-6]. The conductivlty of an ionlc conductor is determined by both concentration of the mo- bile ions and mobility which is dependent on the ion hopping rate. Varlous experimen- tal methods, such as Hall effect [7,8], do- ping of ionic crystals with aliovalent im- puritles [9], inelastic neutron scattering [i0,ii] or NMR techniques [12], have been used to separate those two terms: but it is generally quite difficult. Recently it has 635