Speciation of Cr(iii) in intermediate phases during the sol–gel processing of Cr-doped SrTiO3 powders

Abstract

The local environment of Cr³⁺ during the sol–gel synthesis of 1% and 5% Cr³⁺-doped SrTiO₃ (Cr³⁺:SrTiO₃) bulk powders was systematically studied by structural characterization techniques and dopant-specific spectroscopies. After calcination at 800 °C, the precursors were annealed between 850 °C and 1050 °C for up to 6 h. We observe the formation of numerous phases in addition to the final product of SrTiO₃. One of these is a metastable Ruddelsden–Popper phase (Sr₂TiO₄) that forms when the precursor is annealed at 1050 °C for less than 2 h. Electron paramagnetic resonance (EPR) spectroscopy reveals a new signal that correlates with appearance of the Sr₂TiO₄ phase and is consistent with Cr³⁺ substitution into the axially-compressed Ti⁴⁺ site in Sr₂TiO₄. The best agreement between experiment and the simulated EPR spectra of Cr³⁺:Sr₂TiO₄ is when |D| = 0.0207 cm⁻¹, g_∥ = 1.9803 and g_⊥ = 1.9793. The majority of the sample is converted to Cr³⁺:SrTiO₃ with increasing annealing times at 1050 °C as detected by low-temperature emission and EPR spectroscopies, and powder X-ray diffraction.