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Two new congruent-melting alkali metal diphosphates,Rb2CdP2O7 and Cs2CdP2O7,were synthesized by conventional solid-state reactions.Single-crystal X-ray structural analyses showed that M2CdP2O7(M = Rb,Cs)feature two-dimensional [CdP2O7]2– layers that are composed of CdO5 pyramids and P2O7 dimers.The Rb+and Cs+cations ions fill the interlayers.The flexible P2O7 dimers could be bent and distorted if the Cs+ion was substituted by the Rb+ion,which led to crystallization of the Rb2CdP2O7 compound in the lower symmetry monoclinic space group P21/c(No.14)and crystallization of the Cs2CdP2O7 compound in the high-symmetry orthorhombic space group Pnma(No.53).Thermal analyses showed that the two compounds melt congruently and that the melting point of Rb2CdP2O7 compound is lower than that of the Cs2CdP2O7 compound owing to distortion of the P2O7 dimers.IR spectroscopy and UV/Vis–near-IR diffuse reflectance spectroscopy were performed on the reported compounds.