LiCoTeO: synthesis, single-crystal structure and physical properties of a new tellurate compound with Co/Co mixed valence and orthogonally oriented Li-ion channels.

A tellurate compound with Co/Co mixed valence states and lithium ions within orthogonally oriented channels was realized in LiCoTeO. 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 CoO and TeO octahedra was not observed. LiCoTeO 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, R = 0.0174, wR = 0.0462, 608 F 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 LiCoTeO were found in two orthogonal orientations. The Co/Co 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 Co and Co, the title compound, however, showed no magnetic ordering down to low temperatures. The ionic conductivity of LiCoTeO was determined via alternating current (AC) electrochemical impedance spectroscopy and was found to be in the range of 1.6 × 10 S cm at 573 K.