Li6P6O18: X-ray powder structure determination of lithium cyclohexaphosphate

Preparation, characterization by X-ray diffraction, IR absorption, DTATG analysis, and ab-initio crystal structure determination are reported for anhydrous lithium cyclohexaphosphate Li₆P₆O₁₈. It crystallizes in a monoclinic cell (space group P2₁/n N^o14, Z=2) with a = 7.9911(3)Å, b = 17.0319(6)Å, c = 5.3208(2)Å and β = 99.433(1)°. X-ray powder diffraction data were fitted by Rietveld method leading to R_Bragg = 0.034. The crystal structure of Li₆P₆O₁₈ is built up from P₆O₁₈ ring anions, with 1 internal symmetry. Six lithium coordination polyhedra (two pseudo square pyramids and four tetrahedra) share common edges and corners as to form Li₆O₁₆ groups. These cationic groups build up a layer in the (1 0 1¯) plane showing large distorted hexagonal tunnels and interconnect the P₆O₁₈ rings in a three dimensional way.