Li3Co1.06(1)TeO6: synthesis, single-crystal structure and physical properties of a new tellurate compound with CoII/CoIII mixed valence and orthogonally oriented Li-ion channels.

Abstract

A tellurate compound with CoII/CoIII mixed valence states and lithium ions within orthogonally oriented channels was realized in Li3Co1.06(1)TeO6. The single-crystal structure determination revealed two independent and interpenetrating Li/O and (Co,Te)/O substructures with octahedral oxygen coordination of the metal atoms. In contrast to other mixed oxides, a honeycomb-like ordering of CoO6 and TeO6 octahedra was not observed. Li3Co1.06(1)TeO6 crystallizes orthorhombically with the following unit cell parameters and refinement results: Fddd, a = 588.6(2), b = 856.7(2), c = 1781.5(4) pm, R1 = 0.0174, wR2 = 0.0462, 608 F2 values, and 33 variables. Additional electron density in tetrahedral voids in combination with neighboring face-linked and under-occupied octahedral lithium sites offers an excellent possible diffusion pathway for lithium ions. According to the symmetry of the crystal structure the diffusion pathways in Li3Co1.06(1)TeO6 were found in two orthogonal orientations. The CoII/CoIII mixed valence was investigated via X-ray photoelectron spectroscopy (XPS), revealing a composition comparable to that derived from single-crystal X-ray diffractometry. Magnetic susceptibility measurements underlined the coexistence of CoII and CoIII, the title compound, however, showed no magnetic ordering down to low temperatures. The ionic conductivity of Li3Co1.06(1)TeO6 was determined via alternating current (AC) electrochemical impedance spectroscopy and was found to be in the range of 1.6 × 10-6 S cm-1 at 573 K.