Two new isotypic diphosphates, RbALiZnP(2)O(7) (RLZP) and CsLiZnP2O7 (CLZP), have been synthesized by a high-temperature solid-phase method. They both crystallize in the Pnma space group (No. 62) and have similar crystal configurations. ALiZnP(2)O(7) (A = Rb, Cs) consists of a [Li2Zn2P4O20](14- )anionic skeleton, which is composed of Li/ZnO4 tetrahedral and diphosphate groups, and the Rb or Cs atoms are located in the large hole. In this paper, we discuss the structures of the title compounds and isonomic AA'ZnP2O7 (A = alkali metal); it is found that the ionic radii of the cations and bond angles have an effect on the symmetry of the compounds. By analysis, we found that the M/P values have some effect on the dimensionality of the anionic groups in phosphates containing P2O7 groups. Thermal and spectral analyses are carried out on the title compounds. Besides that mentioned above, we also studied the relationship between the electronic structures and optical properties by theoretical calculations.