The effect of the addition of 11 mol % R2O3 (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 HfO2, solid solutions with a cubic structure such as fluorite are formed. The HfO2-Sc2O3 sample is an Hf7Sc2O17 phase having a fluorite-type lattice with rhombohedral distortions that undergoes reversible changes in the cubic structure at a temperature of ~ 760 ° C. It has been established that the nature of the dopant practically does not affect the microstructure of the HfO2-R2O3 ceramics; all samples are coarse-grained with a grain size of up to 10 μm. It was shown that the conductivity of HfO2-R2O3 samples is determined by the volume of grains. The most promising materials for use as a solid oxide electrolyte are HfO2-Tm2O3 and HfO2-Yb2O3, in which high conductivity is combined with structural stability.