Аннотация:Neodymium substitution in rare earth elements (REE) was studied in the systems Nd5−xLnxMo3O16+y, where Ln = La, Ce, Pr with y∼0.5 by means of X-ray powder diffraction (including the Rietveld algorithm), scanning electron microscopy (SEM) and by measurements of the electrical conductivity. At x values of 0.5, 0.3, and 5.0 the mixtures of REE oxides with MoO3 transform to single-phase solid solutions with fluorite-related structure and doubled fluorite unit cell parameters as a result of solid state reactions occurring at 1050 °C. In the systems with La and Pr the unit cell parameter of solid solutions increases with x. While in the system with Ce this parameters decrease. Conductivity of ceramic solid electrolytes increases up to 17 times compared with the unmodified Nd5Mo3O16+y.