Studies on Li 2 SO 4 ‐MgSO 4 and Li 2 SO 4 ‐Li 4 SiO 4 systems

The phase equilibria as well as the properties and crystal structures of the compounds formed in both Li 2 SO 4 ‐MgSO 4 and Li 2 SO 4 ‐Li 4 SiO 4 systems have been studied by means of x‐ray diffraction technique (at high and room temperatures) as well as by the thermal analyses (DTA, DSC, TGA, etc. ). In Li 2 SO 4 ‐MgSO 4 system there exists a compound Mg 4 Li 2 (SO 4 ) 5 formed by peritectic reaction at 840°C and decomposed at 105°C into the Li 2 SO 4 ‐base solid solution and MgSO 4 · Mg 4 Li 2 (SO 4 ) 5 and Li 2 SO 4 ‐base solid solution conduct an eutectic reaction at 663°C with the composition of eutectic point lying in 22 mol% MgSO 4 . The solubility of MgSO 4 in Li 2 SO 4 is a little smaller than 10 mol% while at the same time the Li 2 SO 4 phase transition temperature decreases from 574 to 560°C On the other hand, no noticeable solid solubility of Li 2 SO 4 in MgSO 4 has been observed. The reaction\documentclass{article}\pagestyle{empty}\usepackage{amssymb,amsmath}\begin{document}$$ 4{{\rm MgSO}}_4 + {{\rm Li}}_2 {{\rm SO}}_4 \underrightarrow {105^\circ {{\rm C}}}{{\rm Mg}}_4 {{\rm Li}}_2 ({{\rm SO}}_4)_5 $$\end{document}is an endothermal one and its heat of formation is 2.57 kJ/mol. The activation energy of the reaction calculated by thermal peak displacement method at various heating rates is 173.5 kJ/mol (1.80 ev). The crystal Mg 4 Li 2 (SO 4 ) 5 belongs to orthorhombic system with lattice parameters at 180°C: a = 8.577, b =8.741, c = 11.918 Å. The space group seems to be either P 222 or P mmm. Assuming that there are two formula units in a unit cell, the density calculated is then 2.20 g/cm 3 very close to that of Li 2 SO 4 or MgSO 4 . Meanwhile, in Li 2 SO 4 ‐Li 4 SiO 4 system a new phase Li 8‐2 x (SiO 4 ) 8‐ x (SO 4 ) x is formed by peritectic reaction at 953°C with a range of composition x =0.96 −0.58. The crystal belongs to ortho‐rhombic system with lattice parameters at x =0.8: a = 5.002, b = 6.173 and c =10.608Å. The density observed is 2.31 g/cm 3 and there are 2 formula units in an unit cell. It is shown from the measurements of piezoelectric and laser SHG coefficients of the crystal that the crystal posseses a symmetrical center with the space group belonging to P mmn. The lattice parameter c has a maximum at x =0.8. In the air Li 8‐2 x (SiO 4 ) 2‐ x (SO 4 ) x can absorb 7.6 wt% water vapour and other gases which can only be desorbed by heating it at a temperature above 350°C. Neither absorption nor desorbtion can change its crystal structure, a characteristic similar to that of zeolite molecular sieve. The dewater activation energy of Li 8‐2 x (SiO 4 ) 2‐ x (SO 4 ) x is 171.5 kJ/mol. Li 8‐2 x (SiO 4 ) 2‐ x (SO 4 ) x and Li 4 SO 4 bring about an eutectic reaction at 823°C with its eutectic composition being 12 mol% Li 4 SiO 4 . No observable solubility of Li 4 SiO 4 in Li 3 SO 4 has been noticed. The solubility of Li 2 SO 4 in Li 4 SiO 4 is approximately equal to 5 mol%. With Li 2 SO 4 being dissolved in, the phase transition temperature of Li 4 SiO 4 is decreased. After being fused, the specimens Li 3 SO 4 ‐MgSO 4 and Li 2 SO 4 ‐Li 4 SiO 4 are cooled at a rate of 10°C/min, their metastable eutectic systems are resulted respectively.