temperature factors. It remains to be seen whether or not a sample with composition R6T6M 5 forms an identical structure but without T-T waist contacts. (1)% V= 252.6 (1) A 3, Z= 2, Dx= 3.732 (1) Mg m -3, F(000) = 272, T= 295 K. X-ray powder diffraction data (Cr Ka l, 2 = 2.28975 A, R = 0.083 for 35 reflections), X-ray single-crystal data (MoKa, 2=0.7107A, /z=5.86mm -l, R=0.040 for 672 reflections), and neutron powder diffraction (2= 1.8820/k, R t = 0.051 for 50 reflections). Isomorphism with thortveitite, 8c2Si207, is confirmed, but the refinements, using both a linear and a split-atom model, indicate a disordered bridging oxygen in the diphosphate anion with a non-linear P-O-P bond of 165.9 (1) °. The metal-oxygen distances conform with those of similar compounds. The JCPDS Diffraction File No. is 35-1497 for Mn2P207.Introduetlon. Among the condensed diphosphates there are often groups of isomorphous compounds, e.g. many divalent-metal M2P20 7 diphosphates crystallize with the thortveitite-type structure. The crystal structure of thortveitite, (Sc,Y)2Si207, was originally determined by 0108-2701/84/121995-05501.50 Zachariasen (1930). The space-group symmetry was reported as C2/m, which implies a linear Si-O-Si bond in the disilicate anion. The metal ions are octahedrally coordinated. Since then many studies of thortveitite-type M2X207 compounds (X = Si, P, V or As) have been published, which have in common the peculiarities that the bridging oxygen has a very large thermal motion, and the XO4 tetrahedra are more regular than they should be, considering that they share corners with each other (cf Cruickshank, 1961).
