• ͒ vacancies were identified in disordered SrTiO 3 powders prepared by the polymeric precursor method, based on experimental measurements by x-ray absorption near edge structure spectroscopy. The paramagnetic complex states of ͓TiO 5 · V O • ͔ and ͓SrO 11 · V O • ͔ with unpaired electrons were confirmed by electron paramagnetic resonance spectroscopy. The disordered powders showed strong photoluminescence at room temperature. Structural defects of disordered powders, in terms of band diagram, density of states, and electronic charges, were interpreted using high-level quantum mechanical calculations in the density functional framework. The four periodic models used here were consistent with the experimental data and explained the presence of photoluminescence.

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In this paper, an experimental study was performed on the effect induced by different strontium precursors in the growth processes and optical properties of strontium tungstate (SrWO 4) microcrystals synthesized by the co-precipitation method. The structural behavior was analyzed by means of X-ray diffractions, Rietveld refinements, Fourier transform (FT)-Raman, and FT-infrared spectroscopies. X-ray absorption near-edge structure spectra performed at the W-L 1 and L 3 edges revealed the first coordination shell around the tungsten atoms is composed of four oxygens, i.e., existence of tetrahedral [WO 4 ] clusters inside the SrWO 4 structure. Field emission scanning electron microscopy (FE-SEM) images showed the presence of pitch and longleaf pine cone-like SrWO 4 microcrystals for most of the strontium precursors employed in the synthesis. Based on these FE-SEM images, a hypothetical crystal growth mechanism was proposed to explain the origin of these microcrystals. The optical properties were investigated by ultraviolet-visible spectroscopy and photoluminescence (PL) measurements at room temperature. The different optical band gap values found for this material, depending on the type of strontium precursor, were correlated with the existence of intermediary energy levels within the forbidden region. Finally, PL profiles were associated to the degree of distortions in tetrahedral [WO 4 ] clusters.

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