Electronically driven structural distortions in lithium intercalates of the n=2 Ruddlesden-popper-type host Y2Ti2O5S2:: Synthesis, structure, and properties of LixY2Ti2O5S2 (0 <x< 2)

Lithium intercalation into the oxide slabs of the cation-deficient n = 2 Ruddlesden-Popper oxysulficle Y2Ti2O5S2 to produce LixY2Ti2O5S2 (0 < x < 2) is described. Neutron powder diffraction measurements reveal that at low levels of lithium intercalation into Y2Ti2O5S2, the tetragonal symmetry of the host is retained: Li0.30(5)Y2Ti2O5S2, 14/mmm, a = 3.80002(2) Angstrom, c = 22.6396(2) Angstrom, Z = 2. The lithium ion occupies a site coordinated by four oxide ions in an approximately square planar geometry in the perovskite-like oxide slabs of the structure. At higher levels of lithium intercalation, the symmetry of the cell is lowered to orthorhombic: Li0.99(5)Y2Ti2O5S2, Immm, a = 3.82697(3) Angstrom, b = 3.91378(3) Angstrom, c = 22.2718(2) Angstrom, Z = 2, with ordering of Li+ ions over two inequivalent sites. At still higher levels of lithium intercalation, tetragonal symmetry is regained: Li1.52(5)Y2Ti2O5S2, 14/mmm, a = 3.91443(4) Angstrom, c = 22.0669(3) Angstrom, Z = 2. A phase gap exists close to the transition from the tetragonal to orthorhombic structures (0.6 < x < 0.8). The changes in symmetry of the system with electron count may be considered analogous to a cooperative electronically driven Jahn-Teller type distortion. Magnetic susceptibility and resistivity measurements are consistent with metallic properties for x > 1, and the two-phase region is identified as coincident with an insulator to metal transition.