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Single crystals of barium strontium diphosphate, Ba(1.01)Sr(0.99)P(2)O(7), were prepared by a solid-state reaction. The compound is isostructural with α-Ba(2)P(2)O(7), α-Sr(2)P(2)O(7) and BaPbP(2)O(7). The structure has only one diphosphate group of the dichromate type that is repeated by symmetry to form sheets. These sheets present mirror planes perpendicular to the a axis, situated at x = 1/4 and 3/4. All the cations and three of the five independent O atoms are located on these mirror planes. The Ba(2+) cations are nine-coordinated by O atoms. The Ba(2+) and Sr(2+) cations are distributed on three different sites, in contrast with the isostructural compounds where only two sites are fully occupied. The refined site occupancies sum to a Ba:Sr ratio of 1.0133 (18):0.9867 (13), which leads to the reported deviation from the BaSrP(2)O(7) stoichiometry. The Raman spectrum of the compound is also reported and discussed.