Spectroscopic Study of the Reversible Chemical Reduction and Reoxidation of Substitutional Cr Ions in Sr2TiO4.

Abstract

The solid-state synthesis and controllable speciation of Cr dopants in the layered perovskite Sr2TiO4 is reported. We employed a chemical reduction procedure with NaBH4 at relatively mild temperatures (<450 °C) to impart sensitive control over the relative concentration of Cr3+ dopants, the charge-state of oxygen-vacancy defects, and presence of Ti3+ defects in highly reduced Cr-doped Sr2TiO4. The electron paramagnetic resonance (EPR) spectra of the reduced powder samples reveal a 12-fold increase in the Cr3+ concentration within the axially compressed Ti4+-site of the Sr2TiO4 host. The increase in Cr3+ content is achieved through the reduction of higher-valence Cr ions that are either EPR silent or diamagnetic. The spin-Hamiltonian parameters for Cr3+ substituted at the B-site of Sr2TiO4 were refined to D = -201 × 10-4 cm-1, g⊥ = 1.980, and g∥ = 1.978. In addition, the Cr3+ ion exhibits a temperature-dependent axial component to the zero-field splitting of the 4A2 ground term that is accounted for by ligand field theory and an isotropic contraction of the Sr2TiO4 lattice with decreasing temperature. The observed changes to the electronic structure upon reduction are quantitatively reversible upon reoxidation of the sample under aerobic annealing at the same temperature and duration as the reduction conditions. This temperature dependence of the Cr3+ content in the Cr-doped Sr2TiO4 powders is discussed and contrasted to our recent study on Cr-doped SrTiO3.