PHASE-TRANSITION AND CRYSTAL-STRUCTURES OF LISN2(PO4)(3)

LiSn2(PO4)(3) undergoes a reversible phase transition which we have ch aracterized by powder X-ray diffraction, DSC and P-31 and Li-7 NMR. Th e crystal structures of the two phases have been determined by powder neutron diffraction at 250 and 450 K using the Rietveld method. The HT phase has a NASICON type-structure, rhombohedral R (3) over bar c wit h a = 8.6334(7)Angstrom and c = 21.533(3) Angstrom. The LT phase, tric linic P (1) over bar, with a = 8.4029(1) Angstrom, b = 8.4408(2) Angst rom, c = 8.8897(1) Angstrom, alpha = 118.116(1)degrees, beta = 90.180( 1)degrees, gamma = 119.681(1)degrees, is a structural distortion of th e HT form with cages occupied by two lithium atoms. The preference of lithium for 4-fold coordination appears to be the driving force for th e transition.