Sodium ion transport and phase transitions in glaserite-type phosphate Na 3 Sm(PO 4 ) 2

The phosphate Na3Sm(PO4)2 with a glaserite-type structure was prepared by solid state reaction in air. At room temperature, analysis of powder X-ray diffraction data shows that the structure is orthorhombic with the Pbc21 space group and the unit cell parameters: a = 15.9365(6) Å, b = 13.94096(30) Å and c = 18.4138(5) Å. Differential thermal analysis in conjunction with dilatometry enabled the identification of two-phase transitions occurring at approximately 650 °С (α → β transition) and 986 °C (β → γ transition). As a result of the high-temperature transition to the γ-form, the lattice expands by more than 1.5%. Employing a combination of AC impedance spectroscopy and the Tubandt method, Na+ cations were unequivocally identified as the mobile charge carriers within the glaserite-type Na3Sm(PO4)2. The most pronounced increase in ionic conductivity is observed during the β → γ phase transition, which is concomitant with a substantial expansion of the Na3Sm(PO4)2.