Cr and Ni Doping of Li₄Ti₅O₁₂: Cation Distribution and Functional Properties

Abstract

Cr- and Ni-doped Li 4 Ti 5 O 12 compound has been characterized through the combined use of X-ray powder diffraction, electron paramagnetic resonance (EPR), 7 Li nuclear magnetic resonance magic-angle spinning (NMR-MAS), micro-Raman, and magnetization measurements. The doping, occurring on the octahedral site of the cubic Li 4 Ti 5 O 12 spinel lattice, strongly affects both the local and the average structural properties. The glassy character of the observed EPR signals suggests structural disorder in the stable Li 4 Ti 5 O 12 matrix and the presence of clustering phenomena or nonhomogeneous distribution of the dopant ion, as also supported by 7 Li NMR-MAS, micro-Raman, and magnetization results. The computation by numerical method of the complex EPR signal of the Cr-doped sample suggests that both Cr Ti and Cr Li substitutions occur, giving rise to two distinct EPR components, corresponding to opposite axial distortion of the relative octahedral environments. On the basis of the compositional data, defect models involving oxygen or cation vacancies are proposed to explain the conductivity of the doped material.