Structural characterisation and Li conductivity of Li1/2−xSr2xLa1/2−xTiO3 (0<x<0.5) perovskites

Abstract

The crystal structure and electric properties of different compositions of the Li1/2La1/2TiO3–SrTiO3 system, which were prepared by solid-state reaction, were investigated. The limits of the Li1/2−xSr2xLa1/2−xTiO3 solid solution (0.0625≤x≤0.25) were established by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. All compositions exhibited a single cubic perovskite structure (ac≈3.87–3.90 Å; SG Pm-3m), as determined by XRD analysis. Along the solid solution, a positive deviation from Vegard's law was observed. This deviation was related to the occurrence of incipient La and Sr segregation/clustering in apparently homogeneous single phases. The room-temperature Li conductivity ranged from 7.4×10−4 to 2.8×10−5 S cm−1, and the activation energy of the samples was close to 0.33 eV, similar to that of the best Li-ion conductors. The substitution of Li by Sr decreased the number of effective vacancies (nt=[Li]+ϒ, where ϒ is the nominal A-vacancy concentration), which appreciably reduced the Li conductivity when the nt approached the 3D percolation threshold. In this series, the percolation threshold is greater than expected, which reinforces the idea that the incipient segregation is responsible for the deviation from Vegard's law.