The structural inhomogeneity of lead-cadmium fluoride systems and its implications

In this paper we present a detailed study of structural and dynamical properties of the CdF2-PbF2 systems. Particular attention is devoted to the processes involving the phase separation, a phenomenon of fundamental importance in the correct description of some dynamical properties, as introduced in our previous works. We show here, that the phase separation trend is observed in the undercooled melt, whether by cooling the liquid below its melting point, or by heating a homogeneous glass at temperatures above Tg. The degree of phase separation depends on the procedure employed to obtain the crystalline materials, and the simplest way to determine this property is by performing an isothermal treatment in the undercooled melt region. The local fluorine environments Q(n), corresponding to fluorine surrounded by nPb and 4-nCd neighbors, is used as a probe in the average local composition. We demonstrate that the increase in the structural inhomogeneity around the composition with x = 0.35 is the factor which leads to a better fluorine conducting property for the Cd0.35Pb0.65F2 solid solution, in addition to providing a correct determination of the melting temperature of the CdxPb1-xF2 compositions.