Phase composition, microstructure, and electroconductivity of R$_{2}$O$_{3}$ solid electrolytes (R = Sc, Y, Ho, Er, Tm, Yb, Lu)

The effect of the addition of 11 mol.% R$_{2}$O$_{3}$ (R = Sc, Y, Ho, Er, Tm, Yb, Lu) on the phase and elemental composition, microstructure, and electrical conductivity of hafnium oxide was studied. When all additives, with the exception of scandium, are introduced into HfO$_2$, solid solutions with a cubic structure such as fluorite are formed. The HfO$_2$–Sc$_{2}$O$_{3}$ sample is an Hf$_{7}$Sc$_{2}$O$_{17}$ phase having a fluorite-type lattice with rhombohedral distortions that undergoes reversible changes in the cubic structure at a temperature of $\sim$ 760$^{\circ}$C. It has been established that the nature of the dopant practically does not affect the microstructure of the HfO$_2$–R$_{2}$O$_{3}$ ceramics; all samples are coarse-grained with a grain size of up to 10 $\mu$m. It was shown that the conductivity of HfO$_2$–R$_{2}$O$_{3}$ samples is determined by the volume of grains. The most promising materials for use as a solid oxide electrolyte are HfO$_2$–Tm$_{2}$O$_{3}$ and HfO$_2$–Yb$_{2}$O$_{3}$, in which high conductivity is combined with structural stability.