In this paper, aggregated CaWO 4 micro-and nanocrystals were synthesized by the co-precipitation method and processed under microwave-assisted hydrothermal/solvothermal conditions (160 C for 30 min). According to the X-ray patterns, all crystals exhibited only the scheelite-type tetragonal structure. The data obtained by the Rietveld refinements revealed that the oxygen atoms occupy different positions in the [WO 4 ] clusters, suggesting the presence of lattice distortions. The crystal shapes as well as its crystallographic orientations were identified by field-emission scanning electron microscopy and high-resolution transmission electron microcopy. Electronic structures of these crystals were evaluated by the first-principles quantum mechanical calculations based on the density functional theory in the B3LYP level. A good correlation was found between the experimental and theoretical Raman and infrared-active modes. A crystal growth mechanism was proposed to explain the morphological evolution. The ultraviolet-visible absorption spectra indicated the existence of intermediary energy levels within the band gap. The highest blue photoluminescence emission, lifetime and quantum yield were observed for the nanocrystals processed in the microwave-assisted solvothermal method.

Theoretical and experimental studies were performed on the structure, optical properties, and growth of silver nanostructures in silver phosphate (Ag 3 PO 4 ). This material was synthesized by the coprecipitation method and processed in a microwave-assisted hydrothermal system at 150 °C for different times. The structural behavior was analyzed by means of X-ray diffraction, Rietveld refinement, and Raman spectroscopy. Field emission gun scanning electron microscopy as well as transmission electron microscopy revealed the presence of irregular spherical-like Ag 3 PO 4 microparticles; metallic silver nanostructures were found on their surfaces. The growth processes of Ag nanostructures when irradiated with an electron beam were explained by theoretical calculations. First-principles calculations, within a quantum theory of atoms in molecules framework, have been carried out to provide deeper insight and understanding of the observed nucleation and early evolution of Ag nanoparticles on Ag 3 PO 4 crystals, driven by an accelerated electron beam from an electronic microscope under high vacuum. The Ag nucleation and formation is a result of structural and electronic changes of the AgO 4 tetrahedral cluster as a constituent building block of Ag 3 PO 4 , consistent with Ag metallic formation. The optical properties were investigated by ultraviolet−visible spectroscopy and photoluminescence (PL) measurements at room temperature. PL properties of this phosphate were explained by the recombination phenomenon of electron−hole pairs via cluster-to-cluster charge transfer.

PbMoO 4 micro-octahedrons were prepared by the coprecipitation method at room temperature without the presence of surfactants and processed in a conventional hydrothermal at different temperatures (from 60 to 120 °C) for 10 min. These micro-octahedrons were structurally characterized by X-ray diffraction (XRD) and micro-Raman (MR) spectroscopy, and its morphology was investigated by field-emission gun scanning electron microscopy (FEG-SEM). The optical properties were analyzed by ultraviolet-visible (UV-vis) absorption spectroscopy and photoluminescence (PL) measurements. XRD patterns and MR spectra confirmed that the PbMoO 4 micro-octahedrons are characterized by a scheelite-type tetragonal structure. FEG-SEM micrographs points out that these structures present a polydisperse particle size distribution in consequence of a predominant growth mechanism via aggregation of particles. In addition, it was observed that the hydrothermal conditions favored a spontaneous formation of micro-octahedrons interconnected along a common crystallographic orientation (oriented-attachment), resulting in self-organized structures. An intense blue PL emission at room temperature was observed in these micro-octahedrons when they were excited with a 350 nm wavelength. The origin of the PL emissions as well as its intensity variations are explained by means of a model based on both distorted [MoO 4 ] and [PbO 8 ] clusters into the lattice.