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RBsumk.-Des ttudes de diffraction des rayons X et des neutrons par Li2S04 c.f.c. (entre 575 O C et 860 O C pour Li2S04 pur) montrent la grande facilite des ions sulfates a prendre toutes les orientations. Ces mouvements de rotation fortement couplte entrainent un changement permanent de la position des cations. Cela explique pourquoi tous les cations mono-et divalents ont, jusqu'a maintenant, montre une grande mobilitt dans plusieurs phases sulphates, contrairement au cas d'autres types d'tlectrolytes solides. Ces phases sulfates sont plastiques avec des proprittts qui ressemblent a celles d'autres types de mattriaux plastiques. Des Btudes de diffusion BrilIouin montrent des difftrences notables entre les phases plastiques c.f.c. et liquide Li2S04 alors que l'indice de refraction change a peine au point de fusion.

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The ionic distribution in the high-temperature rotor phase of lithium sulphate, Li2S0,. has been reinvestigated by single-crystal neutron diffraction at 923 K. Thermal vibrational tensor terms up to the fourth rank are refined 10 give a better description of the space-time-averaged Li' and SO:distributions. The picture of Li' mobility that emerges supports well the earlier proposed notion of rapidly reorienting SO:ions which effectively gate the movement of Li* ions, i.e. a 'paddle-wheel mechanism'. The mode of transport for the Li' ions would appear to be jumpsof 3. 5-3.7 A between =90% occupied, tetrahedrally distorted 8c sites (at t, t. t), into a spherical shell of radius 2.8 surrounding each of four adjacent tetrahedrally coordinated SO:ions. Each shell contains. in total, -0.2 Li' ions. These Li* ionscan then jump further intoone o! the six adjacent Sc sites. The Li+ transport thusorrursinoneofthesixdirections[110],[110].[101]etc,andgivessomesupport tothe notion (corroborated by earlier M D simulation) that the pathways are slightly curved, and hence somewhat in excess of 3.5 A. This agrees well with the value of 3.7 i 0.4 8, deduced from Raman spectroscopy. and also with diffuse neutron scattering studies which suggest Li'-Li+ pair correlation distances of 3.6-3.7 A.