Na2Ca2(P2O7)F-2, the first diphosphate of the cuspidine family

The crystal structure of synthetic Na2Ca2(P2O7)F-2 was solved from single c rystal X-ray diffraction data and refined to R = 4.31% for 1234 independent reflections [I>2 sigma(I)]. The compound is monoclinic with space group sy mmetry P2(l)/n, a = 10.8730(21) Angstrom, b = 10.5516(25) Angstrom, c = 7.4 287(13) Angstrom, and beta = 109.74(2)degrees and twinned by pseudo-merohed ry with m((100)) as the plane of twinning which was accounted for in the re finement calculations. The crystal belongs to the structure family of the m ineral cuspidine and represents the first phosphate member of this group. T he structure is characterized by ribbons of four MX6-octahedra (M = Na,Ca; X = O,F) running parallel to c. Within a single ribbon the octahedra are li nked by edge-sharing, whereas two adjacent ribbons are connected via common corners. Further linkage results from the diphosphate groups, sharing each of the six equatorial oxygen atoms with adjacent octahedral ribbons. Using the modular description usually applied for the classification of cuspidin e type structures the present compound belongs to the so called type 5 and is the first representative of this class.