Lithium intercalation into ␤-VOPO 4 has been performed, both chemically and electrochemically, with X-ray diffraction characterization of the resulting phases at various levels of intercalation-deintercalation. It is shown that intercalation occurs through a first-order transition at 3.98 V vs. Li ϩ /Li 0 , which leads to a ␤-LiVOPO 4 -like structure but with lithium ordering which doubles the a and b unit cell parameters. Starting from chemically intercalated ␤-Li 0.92 VOPO 4 , the reversible process involves 0.55 electron per transition metal, i.e., 90 mAh/g, at a C/50 rate which owing to the 3.95 V potential plateau, gives an attractive specific energy of 355 Wh/kg. Fundamentally, it is shown herein that the redox energy of V 5ϩ /V 4ϩ couple in an octahedral coordination involving (V ϭ O) mϩ unit is included between those observed in oxides and in M 2 (PO 4 ) 3 compounds with (PO 4 ) 3Ϫ oxo-anions.

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structure structure (solids and liquids) D 2000 -008Phase Transition and Crystal Structures of LiSn 2 (PO 4 ) 3 .-Compound (IV) shows a reversible phase transition from rhombohedral (space group Rr3rc, Z = 6, 450 K) to triclinic (Pr1r, Z = 2, 250 K) monitored by powder X-ray and neutron diffraction, DSC and 31 P and 7 Li MAS NMR. The high temperature phase has a NASICON-type structure which is responsible for the relationship between conductivity and activation energy. The low temperature phase is a structural distortion of the rhombohedral phase with cages occupied by two Li atoms. The 4-fold coordination of Li appears to be the driving force for the phase transition. -(MORIN, E.; ANGENAULT, J.; COUTURIER, J. C.; QUARTON, M.; HE, H.; KLINOWSKI, J.; Eur. J. Solid State Inorg. Chem. 34 (1997) 9, 947-958; Lab. Cristallochim. Solide, CNRS, Univ. P. et M. Curie, F-75252 Paris, Fr.; EN)

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