Spectroscopic Study of the Reversible Chemical Reduction and Reoxidation of Substitutional Cr Ions in Sr 2 TiO 4 .

The solid-state synthesis and controllable speciation of Cr dopants in the layered perovskite Sr ₂ TiO ₄ is reported. We employed a chemical reduction procedure with NaBH ₄ at relatively mild temperatures (<450 °C) to impart sensitive control over the relative concentration of Cr ³⁺ dopants, the charge-state of oxygen-vacancy defects, and presence of Ti ³⁺ defects in highly reduced Cr-doped Sr ₂ TiO ₄ . The electron paramagnetic resonance (EPR) spectra of the reduced powder samples reveal a 12-fold increase in the Cr ³⁺ concentration within the axially compressed Ti ⁴⁺ -site of the Sr ₂ TiO ₄ host. The increase in Cr ³⁺ content is achieved through the reduction of higher-valence Cr ions that are either EPR silent or diamagnetic. The spin-Hamiltonian parameters for Cr ³⁺ substituted at the B-site of Sr ₂ TiO ₄ were refined to D = -201 × 10 ^-4 cm ^-1 , g _⊥ = 1.980, and g _∥ = 1.978. In addition, the Cr ³⁺ ion exhibits a temperature-dependent axial component to the zero-field splitting of the ⁴ A ₂ ground term that is accounted for by ligand field theory and an isotropic contraction of the Sr ₂ TiO ₄ 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 Cr ³⁺ content in the Cr-doped Sr ₂ TiO ₄ powders is discussed and contrasted to our recent study on Cr-doped SrTiO ₃ .