Two kinds of oxygen vacancies in lithium titaniate doped with copper as detected by EPR

Lithium titanate (Li1+xTi2-xO4) doped with Cu2+ ions was synthesized by sol-gel processing method. The structure and morphology are characterized by X-ray Diffraction (XRD), X-ray Photoemission Spectroscopy (XPS), Scanning Electron Microscopy (SEM) and Electron Paramagnetic Resonance (EPR). Spin Hamiltonian parameters describing Zeeman and hyperfine interaction for 63Cu2+ ions were obtained from EPR spectra simulations. The spectra are characteristic for (Cu-VO)0 close-associate pairs consisting of copper replacing Ti4+ ion and an adjacent doubly ionized oxygen vacancy. These oxygen vacancies serve as charge compensators for the substituting copper ions. However, there are additional narrow EPR signals at g = 2.004 in all samples, including the sample of LiTi2O4 without dopant. These signals are ascribed to single-electron trapped oxygen vacancies VO●. Superconductivity was not confirmed down to 4.2 K by magnetic measurements in all samples studied.