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

摘要

The local environment of Cr~(3+) during the sol-gel synthesis of 1% and 5% Cr~(3+)-doped SrTiO3 (Cr~(3+):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 Cr~(3+) substitution into the axially-compressed Ti~(4+) site in Sr2TiO4. The best agreement between experiment and the simulated EPR spectra of Cr~(3+):Sr2TiO4 is when |D|= 0.0207 cm~(-1), g_‖ = 1.9803 and g_⊥ = 1.9793. The majority of the sample is converted to Cr~(3+):SrTiO3 with increasing annealing times at 1050 °C as detected by low-temperature emission and EPR spectroscopies, and powder X-ray diffraction.