Valmor R. Mastelaro

Structural conditions that leads to photoluminescence emission in SrTiO3: An experimental and theoretical approach

Complex cluster [TiO5⋅VOz] and [SrO11⋅VOz] (where VOz=VOX, VO•, VO••) vacancies were identified in disordered SrTiO3 powders prepared by the polymeric precursor method, based on experimental measurements by x-ray absorption near edge structure spectroscopy. The paramagnetic complex states of [TiO5⋅VO•] and [SrO11⋅VO•] 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.

Cluster Coordination and Photoluminescence Properties of alpha-Ag2WO4 Microcrystals

In this paper, we report our initial research to obtain hexagonal rod-like elongated silver tungstate (α-Ag(2)WO(4)) microcrystals by different methods [sonochemistry (SC), coprecipitation (CP), and conventional hydrothermal (CH)] and to study their cluster coordination and optical properties. These microcrystals were structurally characterized by X-ray diffraction (XRD), Rietveld refinements, Fourier transform infrared (FT-IR), X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopies. The shape and average size of these α-Ag(2)WO(4) microcrystals were observed by field-emission scanning electron microscopy (FE-SEM). The optical properties of these microcrystals were investigated by ultraviolet-visible (UV-vis) spectroscopy and photoluminescence (PL) measurements. XRD patterns and Rietveld refinement data confirmed that α-Ag(2)WO(4) microcrystals have an orthorhombic structure. FT-IR spectra exhibited four IR-active modes in a range from 250 to 1000 cm(-1). XANES spectra at the W L(3)-edge showed distorted octahedral [WO(6)] clusters in the lattice, while EXAFS analyses confirmed that W atoms are coordinated by six O atoms. FE-SEM images suggest that the α-Ag(2)WO(4) microcrystals grow by aggregation and the Ostwald ripening process. PL properties of α-Ag(2)WO(4) microcrystals decrease with an increase in the optical band-gap values (3.19-3.23 eV). Finally, we observed that large hexagonal rod-like α-Ag(2)WO(4) microcrystals prepared by the SC method exhibited a major PL emission intensity relative to α-Ag(2)WO(4) microcrystals prepared by the CP and CH methods.

29Si MAS–NMR studies of Qn structural units in metasilicate glasses and their nucleating ability

The purpose of this work is to verify the possible existence of a relationship between the similarity of the local structure of the network-forming cation Si4+ (Qn units and chemical shifts) in glasses and isochemical crystals and the nucleating ability of these glasses. Four metasilicate glasses with widely different volume nucleation rates: Na2Ca2Si3O9 and Na4CaSi3O9 (very large), CaSiO3 (intermediate) and CaMgSi2O6 (undetectably small) were chosen. We present magic angle spinning nuclear magnetic resonance spectroscopy (MAS–NMR) data for Na2Ca2Si3O9 and Na4CaSi3O9 glasses and for their respective isochemical crystalline phases for the first time. Additionally, we repeat NMR measurements of glasses and crystals previously studied by other authors (CaSiO3 and CaMgSi2O6) to test the consistency of our experimental techniques and method of analysis. Different central chemical shifts of Q2 resonances in parent glasses and their isochemical crystals were measured, indicating structural differences. The relative amount of Qn groups in each glass was obtained from the deconvolution of the 29Si MAS–NMR spectra. The shape of the Qn distribution for each system was considered as a measure of the similarity of the connectivities of SiO4 tetrahedra in each glass with respect to its isochemical crystal (which has only Q2 groups). A correlation was found between the shape of the Qn distribution and the nucleation tendency of these glasses, indicating that similarities between the tetrahedra connectivities in glass and isochemical crystal has a role in determining the internal nucleation tendency of the metasilicate glasses studied.

Relation between photoluminescence emission and local order-disorder in the CaTiO3 lattice modifier

In this letter, the authors propose that photoluminescence emission in CaTiO3 is affected not only by disorder in the lattice former but also by structural disorder in the lattice modifier. Structural disorder was evaluated by Ti, Ca K-edge x-ray absorption near-edge structure experiments and by photoluminescence emission. The preedge feature of the Ca K edge was related to the intensity of photoluminescence emission. The results of the preedge feature of the Ca K-edge x-ray absorption near-edge structure confirm the presence of different Ca coordination numbers, namely, Ca–O11 and Ca–O12.

Presence of excited electronic state in CaWO4 crystals provoked by a tetrahedral distortion: An experimental and theoretical investigation

By combining experimental techniques such as x-ray diffraction, Fourier transform Raman, ultraviolet-visible, x-ray absorption near edge structure, extended x-ray absorption fine structure spectroscopy, and theoretical models, a general approach to understand the relationship among photoluminescence (PL) emissions and excited electronic states in CaWO4 crystals is presented. First-principles calculations of model systems point out that the presence of stable electronic excited states (singlet) allow us to propose one specific way in which PL behavior can be achieved. In light of this result, we reexamine prior experiments on PL emissions of CaWO4.

Structural studies in lead germanate glasses: EXAFS and vibrational spectroscopy

The Raman, IR absorption and EXAFS spectra at the Ge K-edge and Pb LIII-edge of eight lead germanate glasses, with general formula xPbO(1−x)GeO2 with x = 0.20, 0.25, 0.33, 0.40, 0.50, 0.53, 0.56 and 0.60, have been measured. The occurrence of [GeO6] units besides [GeO4] could not be deduced unambiguously from the data. The vibrational and EXAFS data agree with a progressive depolymerization of the network. Starting from all Ge atoms linked to four bridging oxygens in GeO2 (x = 0), the number of tetrahedral units with one or two non-bridging oxygens increases with x. At low content, Pb2+ ions act as modifiers in the germanate structure, but to a lesser extent than an equivalent number of alkaline ions.

Relationship between short-range order and ease of nucleation in Na2Ca2Si3O9, CaSiO3 and PbSiO3 glasses

Abstract A relationship between the short-range order around the modifier cations and the crystal nucleation tendency in silicate glasses is demonstrated. New extended X-ray absorption fine spectroscopy (EXAFS) results on the local structure around calcium and lead atoms were obtained and analyzed for both vitreous and crystalline samples. Three different silicate systems were studied: wollastonite (CaSiO3) and soda-lime–silica (Na2Ca2Si3O9), for which volume nucleation is easily observed and lead metasilicate (PbSiO3) for which nucleation occurs only on the sample surfaces in typical laboratory conditions. In the glasses that have a high nucleation tendency (Na2Ca2Si3O9 and CaSiO3), the local structures of these modifier cations are similar to their short-range order in the isochemical crystalline phases, whereas the local structure in the glass that presents a low nucleation tendency (PbSiO3) is quite different from that of its isochemical crystal phase.

Amorphous lead titanate: a new wide-band gap semiconductor with photoluminescence at room temperature

This paper describes a new amorphous wide-band gap semiconductor with photoluminescence (PL) at room temperature. The amorphous PbTiO3 was prepared by a sol–gel-like process in powder and thin film form. The optical property and the PL behaviour showed a direct relation to the amorphous structure. The PL peak energy can be controlled by the change of the exciting surge energy. Copyright

A novel ozone gas sensor based on one-dimensional (1D) α-Ag2WO4 nanostructures

This paper reports on a new ozone gas sensor based on α-Ag₂WO₄ nanorod-like structures. Electrical resistance measurements proved the efficiency of α-Ag₂WO₄ nanorods, which rendered good sensitivity even for a low ozone concentration (80 ppb), a fast response and a short recovery time at 300 °C, demonstrating great potential for a variety of applications.

Residual stresses in a soda-lime-silica glass-ceramic

Abstract The present study was undertaken to test the validity of existing models for: (i) the residual internal stresses which arise due to thermal and elastic mismatch in duplex systems; and (ii) the critical particle diameter for spontaneous cracking. The specimens studied were bulk nucleated, partially crystallized 17.20Na2O32.09CaO48.12SiO22.59P2O5 (mol%) glasses. The residual stress was determined by an X-ray diffraction technique for a particular set of hkl planes. The crystal diameters and volume fractions crystallized were measured by an image analyzer system. The experimental residual stress was in agreement with the calculated value within the measurement errors. On the other hand, the theoretical value of the critical particle diameter, calculated by an energy balance approach, was more than ten times smaller than the experimental value, suggesting that the semi-spherical crack assumed in the model does not hold for our system. A preliminary fitting with a fracture mechanics model was also carried out.