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.

The linear thermal expansivities of polycrystalline samples of mixtures of Li2SO4 with K2SO4 and with Na2SO4 have been measured with a “Fused-Quarz Tube” apparatus 1. Phase transitions are accompanied by changes in the expansion coefficient and also by volume changes. The observed expansion coefficient is constant over wide temperature intervals. Simple crystal structures show higher expansivities than more complex structures. The “pre-melting” phenomenon in Li2SO4 does not measurably influence the thermal expansion. A method has been devised to correct the observations for the plastic flow of the crystals close to the melting points

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The techniques of segregation during normal freezing and of zone melting have been used to establish that the melting point of 6LiNO3 is higher than that of 7LiNO3. The difference is of the order of 0.03 °C. The isotope shift of the melting point is in the opposite direction of the isotope effects found previously for phase transitions in solid lithium metal and lithium sulfate. For the latter salt a recalculation based on a more accurate value for the self-diffusion coefficient shows that the temperature of transition at about 575 °C to a fee structure is about 0.08 degr. lower for 6Li2SO4 than for 7Li2SO4.

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