LI2SO4-MSO4 (M = CO, NI) PHASE-DIAGRAMS AND CONSIDERATIONS ABOUT ALPHA-LI2-2XMXSO4 SOLID-SOLUTIONS AND LI2MY(SO4)1+Y COMPOUNDS (M-DIVALENT IONS)

Li2SO4-CoSO4 and Li2SO4-NiSO4 phase diagrams were established. In both cases, there is a limited solid solution with alpha-Li2SO4 structure (high temperature form of Li2SO4) and an incongruent melting compound Li2M (SO4)2; with M = Co, the compound exists under two forms. Using previous data and starting from the known structure of the solid electrolyte phase alpha-Li2SO4 some remarks were made concerning the formation of the alpha-Li2-2xMxSO4 (M divalent ion) solid solution: x values are 0.20 for Be2+ and 0.33 for Zn2+ which are often tetrahedrally coordinated in their compounds as Li+ in alpha-Li2SO4; lower values of x occur for other divalent cations and x < 0.04 for Ca, Sr, Ba cations which are never tetrahedral coordinated in their compounds; so, the probable structural formula of the solid solution is Li2-2x(4) square x(4)Mx(4) square 1(6)SO4. Only a few Li2My(SO4)1+y compounds exist; y = 0.5 with M = Zn, y = 1 with M = Co, Ni and Zn and y = 2 with M = Mg and Mn. Cell parameters of beta-Li2Co(SO4)2 were determined from powder pattern; tetragonal system with a = 8.104 (1), c = 14.916 (4) angstrom; d(m) = 2.77 and Z = 6; considerations were made on its probable structure by comparison with that of Li2Mg2(SO4)3; the structure of this last compound was described and some comparisons were made with known parent compounds Li2Fe2(XO4)3 (X = Mo, W) where some vacancies still exist.