Study of the crystal structures of sodium magnesium and sodium nickel diphosphates

Single-crystal X-ray crystallography studies have shown that diphosphates N a3.64Mg2.18(P2O7)(2) and Na3.64Ni2.18(P2O7)(2) crystallize with the same st ructural type and the same space group P (1) over bar. Their triclinic latt ice parameters are equal to a = 10.901 (2), b = 9.765 (2), c = 6.382 (1) An gstrom, alpha = 112.43 (1)degrees, beta = 99.64 (1)degrees, gamma = 107.53 (1)degrees, Z = 2 and a = 10.889 (5), b = 9.705 (4), 6.358 (4) Angstrom, al pha = 112.46 (4)degrees, beta = 99.92 (4)degrees, gamma = 107.54 (4)degrees , Z = 2, respectively. The structure could be regarded as a packing of diph osphate groups [P2O7](4-) and [MO6] octahedra (M = Mg, Ni) delimiting cavit ies and tunnels which host sodium cations, The tunnels are running along [0 01]. The structure is characterized by mixed (Na, M) sites with an occupati on factor ratio equal to ca. 0.82/0.18. Sodium cations are located in five different sites: two cavities tone penta- and the other octa-coordinated) t otally occupied and three octahedral interstices, which are partially fille d by Na(3), Na(4), and Na(5) according to the respective occupation factors of 0.15, 0.42, and 0.25 for Na3.64Mg2.18(P2O7)(2) and 0.13, 0.40, and 0.29 in the case of Na3.64Ni2.18(P2O7)(2). (C) 2000 Academic Press.