New solid solutions of Li_xNa_1–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 Li_xNa_1–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 Li_xNa_1–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.