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The crystal structure of Cs2CuP2O7 [monoclinic, Cc, a = 7.460(6), b = 12.973(10), c = 9.980(8) Åβ = 111.95(2) ° , V = 895.8(12) Å3] prepared by solid-state reactions is based upon open framework formed by corner sharing between CuO4 distorted squares and P2O7 groups. The framework is porous and has a very low framework density of 13.4 Cu+P atoms per 1 nm3. Cs+ cations reside in large framework cavities. The heteropolyhedral network in the title compound is based upon three-dimensional (3;4)-connected net that has a three-membered CuP2 ring as its elementary unit. In terms of reticular chemistry, this net should be considered as an augmented diamond (dia) net. The Cu-P net can be obtained from the latter by the replacement of its nodes by the CuP2 triangles. This replacement is strongly non-centrosymmetric, since all CuP2 triangles are oriented along the same direction, which provides a crystal chemical explanation for the absence of a symmetry centre in the structure. Cs2CuP2O7 is the first compound in the A2CuP2O7 family (A alkaline metal), which is based upon threedimensional copper pyrophosphate framework. [ABSTRACT FROM AUTHOR]