Li 2SO 4MSO 4 (M = Co, Ni) phase diagrams and considerations about α-Li 2−2x M xSO 4 solid solutions and Li 2M y (SO 4) 1+y compounds (M: divalent ions)

Abstract

Li 2SO 4CoSO 4 and Li 2SO 4NiSO 4 phase diagrams were established. In both cases, there is a limited solid solution with α-Li 2SO 4 structure (high temperature form of Li 2SO 4) and an incongruent melting compound Li 2M(SO 4) 2; with M = Co, the compound exists under two forms. Using previous data and starting from the known structure of the solid electrolyte phase α-Li 2SO 4 some remarks were made concerning the formation of the α-Li 2−2 x M x SO 4 (M divalent ion) solid solution: x values are 0.20 f Be 2+ and 0.33 for Zn 2+ which are often tetrahedrally coordinated in their compounds as Li + in α-Li 2SO 4; lower values of x oc cur 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 Li 2−2 x (4)□ x (4) M x (4) □ 1 (6)SO 4. Only a few L i 2M y (SO 4) 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 β-Li 2Co(SO 4) 2 were determined from powder pattern; tetragonal system with a = 8.104 (1), c = 14.916 (4) Å; d m = 2.77 and Z = 6; considerations were made o n its probable structure by comparison with that of Li 2Mg 2(SO 4) 3; the structure of this last compound was described and some comparisons were made with known parent compounds Li 2Fe 2(XO 4) 3 (X = Mo, W) where some vacancies still exist.