Compositional and structural variations in the ternary system Li-Al-Si

The Li-Al-Si ternary system has been investigated and four ternary phases characterized using both powder and single crystal X-ray diffraction data. LiAlSi and Li7Al3Si4 crystallize in the F (4) over bar 3m, cubic system, Li18Al2Si6 in tetragonal I4(1)/amd and Li15Al3Si6 in hexagonal P6(3)/m space group. The structure of LiAlSi (a = 5.94 Angstrom), previously reported from powder data, has been confirmed. The new compound Li7Al3Si4 has been identified and, due to a higher lithium content, its cell parameter is slightly larger (a = 6.115 Angstrom). The tetragonal cell (a = 6.179(1) Angstrom, c = 12.199(4) Angstrom) of Li18Al2Si6 may be considered as a (1 x 1 x 2) supercell of the cubic one. Structural variations are strongly correlated to compositional fluctuations and atomic substitutions, in every case the Si-Al network displays some covalent character. In the Li15Al3Si6 hexagonal cell, bonding is more covalent leading to a heterographite-like Si-Al network.