Electronically Driven Structural Distortions in Lithium Intercalates of the n = 2 Ruddlesden-Popper-Type Host Y_2Ti_2O_5S_2: Synthesis, Structure, and Properties of Li_xY_2Ti_2O_5S_2 (0 < x < 2)

摘要

Lithium intercalation into the oxide slabs of the cation-deficient n = 2 Ruddlesden-Popper oxysulfide Y_2Ti_2O_5S_2 to produce Li_xY_2Ti_2O_5S_2 (0 < x < 2) is described. Neutron powder diffraction measurements reveal that at low levels of lithium intercalation into Y_2Ti_2O_5S_2, the tetragonal symmetry of the host is retained: Li_(0.30(5))Y_2Ti_2O_5S_2, I4/mmm, a = 3.80002(2) A, c = 22.6396(2) A, 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: Li_(0.99(5))Y_2Ti_2O_5S_2, Immm, a = 3.82697(3) A, b = 3.91378(3) A, c = 22.2718(2) A, Z = 2, with ordering of Li~+ ions over two inequivalent sites. At still higher levels of lithium intercalation, tetragonal symmetry is regained: Li_(1.52(5))Y_2Ti_2O_5S_2, I4/mmm, a = 3.91443(4) A, c = 22.0669(3) A, 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.