Luminescent-technique study on the structure of cation subsystem in the ionic conductor Na5RESi4O12 (RE is a rare-earth cation Tb3+, Ho3+, Er3+, Gd3+, Eu3+)

Results of experimental studies of luminescence in a group of materials with common formula Na₅RESi₄O₁₂ (RE is a rare-earth cation Tb³⁺, Ho³⁺, Er³⁺, Gd³⁺, Eu³⁺ entering the stoichiometric formula of the substance) are summarized. The luminescence spectra give information on the mobile-cation sublattice structure and dynamics. It follows from experimental results that the current opinion on the disordering of mobile sublattice is not quite correct. At relatively low temperatures (T < 100 K), a small number of typical local configurations of cations can be elucidated, which in aggregate can describe the Na⁺ cation sublattice to rather high degree of accuracy. At higher temperatures, the number of spectral lines and their positions change, which formally corresponds to changes in the symmetry of sites of the lattice, hence, is the sign of a second order phase transition. This phenomenon is given an explanation based on a suggestion on the local dynamic averaging of the mobile cation electrical fields.