Na₇Cr₄(P₂O₇)₄PO₄

The title compound, hepta­sodium tetra­chromium(III) tetra­kis­(diphosphate) orthophosphate, was synthesized by solid-state reaction. Its structure is isotypic with that of Na₇M₄(P₂O₇)₄PO₄ (M = In, Al) compounds and is made up from a three-dimensional [(CrP₂O₇)₄PO₄]^7- framework with channels running along [001]. The three Na⁺ cations are located in the voids of the framework. One of the cations is situated on a general position, one is equally disordered around a twofold rotation axis and one is on a fourfold rotoinversion axis. The isolated PO₄ tetra­hedron of the anionic framework is also situated on the -4 axis. Structural relationships between the title compound and different diphosphates containing MP₂O₁₁ units (M = Mo, V) are discussed.