Crystal structure and thermal properties of the LixNa1–xKZnP2O7 solid solutions and its relation to the MM′ZnP2O7 diphosphate family

Abstract

New solid solutions of LixNa1–xKZnP2O7, belonging to the MM’ZnP2O7 structural family (M = Li+, Na+, Zn2+; M’ = Na+, K+), were prepared and phase equilibria along the LiKZnP2O7–NaKZnP2O7 section were reported. These solid solutions were investigated single-crystal X-ray diffraction, high-temperature X-ray powder diffraction (HTXRD), differential scanning calorimetry, and impedance spectroscopy. A miscibility gap was found within the range of 0.8 < x < 1.0, and a binodal curve was constructed using HTXRD data. The similarity of the thermal and compositional deformation of LixNa1–xKZnP2O7 solid solutions is revealed: increasing the radius of the metal by 0.01 Å leads to the same deformations of the crystal structure as increasing the temperature by 31 °C. The Li/Na↔Zn cation exchange lead to an increase in conductivity above 300 °C. The alkali mixed effect in activation energy occurs for LixNa1–xKZnP2O7 solid solutions near x = 0.5. The phase with the 2a×2b×c superstructure ordering was observed as a result of the solid solution melting.