Defect structure of the solid solutions ${\mathrm{Ba}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Gd}}_{\mathrm{x}}$${\mathrm{F}}_{2+\mathrm{x}}$

In this paper we present the results of a series of combined experiments on solid solutions ${\mathrm{Ba}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Gd}}_{\mathrm{x}}$ ${\mathrm{F}}_{2+\mathrm{x}}$ employing the following techniques: (a) ionic thermocurrents (ITC’s); (b) dielectric capacitance and loss measurements at 1000 Hz; and (c) EPR. In line with earlier results on solid solutions ${\mathrm{Sr}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Gd}}_{\mathrm{x}}$ ${\mathrm{F}}_{2+\mathrm{x}}$ we find a discrepancy between the numbers of dipoles as observed with the dielectric relaxation techniques [(a) and (b)] and the spectroscopic EPR method, respectively. It appears that this discrepancy is caused by the fact that in dielectric experiments the dipoles are detected through their dynamical properties, whereas the EPR results provide a static picture. With ITC we only observe those dipoles which are isolated from the remaining defect system. The most important feature of our EPR data is that trigonal ${\mathrm{Gd}}^{3+}$ ions are by far the dominant defects present in ${\mathrm{Ba}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Gd}}_{\mathrm{x}}$ ${\mathrm{F}}_{2+\mathrm{x}}$ even for 0<x<0.05. This has important consequences for the conductivity processes in heavily doped materials. The defect properties of the system ${\mathrm{Ba}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Gd}}_{\mathrm{x}}$ ${\mathrm{F}}_{2+\mathrm{x}}$ deviate from those observed for cubic solid solutions consisting of ${\mathrm{CaF}}_2$ and rare-earth fluorides, which show extensive preferential clustering.