A thermoluminescence study of mineral silicates extracted from herbs in the dose range 0.5–5 Gy

The presence of silicates in many personal objects suggests their potential use at low dose as fortuitous dosimeter in an accidental radiological exposure, when conventional dosimetry is not available. The goal of the present work is the dosimetric characterization of mineral silicates extracted from the plant Hibiscus Sabdariffa L, known as Jamaica flower, in the dose range 0.5–5 Gy. By studying the radiation-induced signal in time, the temperature integration region between 210 °C and 250 °C was found to be the most stable and also reduced the effects of thermal fading in the dose reconstruction process; the dose response curve was linear between 0.5 Gy and 5 Gy. By checking the change in sensitivity after repeated exposures to ionizing radiations and to high temperature heating, no variation in the glow curve shape or peak intensities were detected. To eliminate a pre-existing background signal, all the characterization measurements were performed with aliquots “annealed” by a preliminary readout of the TL.     

Kinetic vs. thermodynamic control of crystal nucleation and growth in molten silicates

A series of calcium aluminosilicate liquids have been experimentally heat-treated at high but variable states of undercooling, from just above the glass transition to the vicinity of the solidus. The mineralogy and chemistry of crystalline phases which appear in these experiments have been quantified using a combination of Transmission Electron Microscopy and Raman spectroscopy. The results show that mineral compositions are highly variable as a function of temperature, but that changes are governed by the contrasting and strongly temperature-dependent mobilities of network-modifying and network-forming cations. Whereas equilibrium crystals form near the liquidus, disordered and non-stoichiometric phases precipitate near the glass transition. Despite this apparently complex situation, the relative importance of thermodynamic and kinetic factors is found to be a single function of T − T_g (where T is temperature and T_g the glass transition temperature), regardless of the silicate composition. The existence of this mastercurve may be used to control the composition of novel composite materials such as glass ceramics.     

Lanthanum (Oxo)nitridosilicates: From Ordered to Disordered Crystal Structures

The homeotypic compounds La_16.32Ba_1.82Sr_7.86[Si₆₀N_92.32O_3.68]O₁₂ and La_13.68Sr_12.32[Si₆₀N₉₆]F_6.32O_5.68 were synthesized at high temperature (1600/1500 °C) in a radio‐frequency furnace. The crystal structures [I$\bar{4}$ m (no. 217), Z = 1, a = 13.3360(10)/13.3258(10) Å and V = 2371.8(5)/2366.4(5) ų] were solved and refined on basis of single‐crystal X‐ray diffraction data and were corroborated by lattice‐energy calculations (Madelung part of lattice energy, MAPLE) powder X‐ray diffraction data and FTIR spectroscopy. They consist of a three‐dimensional network of allside corner sharing SiN_4–xO_x tetrahedra. The framework is characterized by double dreier rings. La_16.32Ba_1.82Sr_7.86[Si₆₀N_92.32O_3.68]O₁₂ represents an oxonitridosilicate oxide and La_13.68Sr_12.32[Si₆₀N₉₆]F_6.32O_5.68 a nitridosilicate fluoride oxide, as the crystal structures contain non‐condensed (O^[0]/O,F^[0]) anions. The first compound is isotypic to Sr₃Ln₁₀Si₁₈Al₁₂O₁₈N₃₆ (Ln = Ce, Pr, Nd; Z = 2), whereas the latter describes a disordered model of the crystal structure, which is homeotypic to the mentioned SiAlONs.     

Er-doped silica–hafnia films for optical waveguides and spherical resonators

In this paper we present some result on sol–gel derived silica–hafnia systems. In particular we focus on fabrication, morphological and spectroscopic assessment of Er³⁺-activated thin films. Two examples of silica–hafnia-derived waveguiding glass ceramics, prepared by top–down and bottom–up techniques are reported, and the main optical properties are discussed. Finally, some properties of activated microspherical resonators, having a silica core, obtained by melting the end of a telecom fiber, coated with an Er³⁺-doped 70SiO₂–30HfO₂ film, are presented.     

Highly efficient broadband near-infrared luminescence in Ni-doped glass ceramics films containing cordierite nanocrystals

We report on highly efficient broadband near-infrared photoluminescence (PL) in Ni²⁺-doped glass ceramics (GCs) films fabricated by annealing Si/Ni²⁺-doped glass superlattices (SNGS). Over two orders of magnitude enhancement of PL can be achieved in comparison with that from the annealed glass film. The PL lifetime of the annealed SNGS is several milliseconds, which is much longer than those of bulk GCs. The strong PL enhancement results from the formation of high-quality cordierite nanocrystals because the Si layers act as Si source for the crystal growth. This technique can be extended to fabricate other types of high-quality GCs films.     

Atomic scale foundation of temperature-dependent bonding constraints in network glasses and liquids

The behaviour of bonding constraints with temperature is analyzed from an atomic scale approach (Molecular Dynamics, MD) combined with partial bond angle distributions (PBAD). The latter allows to have access to the second moments (standard deviations) of the distributions. Large (small) standard deviations correspond to large (small) angular excursions around a mean value, and are identified as broken (intact) bond-bending constraints. A similar procedure is used for bond-stretching constraints. Systems examined include glassy and liquid disilicate 2SiO₂-Na₂O (NS2). In the glass, MD constraint counting closely matches Maxwell enumeration of constraints using the octet binding (8-N) rule. Results show that the standard deviations of the partial bond angle distributions increase with temperature and suggest a softening of bond-bending constraints. A bimodal bonding oxygen distribution is obtained for T> Tg, and the fraction of thermally activated broken bond-bending constraints computed as a function of temperature. Overall, these results provide a microscopic rationale for extending constraint counting from chalcogenides to complex oxides, and also a numerical basis for recent functional forms of temperature-dependent constraints proposed from energy landscape approaches.     

Synthesis,Structure Characterization,and Optical Properties of the Aluminosilicate Li2Na3AlSi2O8

The aluminosilicate Li₂Na₃AlSi₂O₈ was crystallized from the Li₂CO₃–H₃BO₃ flux system. It crystallizes in the orthorhombic space group Cmca, with cell dimensions a = 14.1045 (19) Å, b = 14.7054 (19) Å, c = 7.0635 (9) Å, and Z = 8. The crystal structure consists of a two‐dimensional infinite layer, which is composed of [Al₂Si₂O₁₂] groups and [SiO₄] tetrahedra. The lithium and sodium atoms filling in the interlayer and intralayer link the layers together and balance the charge. IR spectroscopy and BVS calculations were used to verify the validity of the structure. The calculated band structures and the density of states of Li₂Na₃AlSi₂O₈ suggest that its direct gap is 4.28 eV.     

Synthesis,structures and luminescence of two new Europium(II) Silicate-Chlorides,Eu2SiO3Cl2 and Eu5SiO4Cl6

Eu₂SiO₃Cl₂ and Eu₅SiO₄Cl₆ were prepared by reaction of EuCl₂ with EuSiO₃ and Eu₂SiO₄, respectively, Sr₂SiO₃Cl₂: Eu²⁺ from mixtures of SrCO₃, Eu₂O₃, SrCl₂ · 6H₂O and SiO₂ under reducing conditions. The crystal structures of Eu₂SiO₃Cl₂ [a = 1118.7(5), c = 952.6(1) pm, tetragonal, I4/m, Z = 8, R = 3.3, R_w = 3.0%] and Eu₅SiO₄Cl₆ [a = 900.4(1), b = 1401.7(2), c = 1112.3(2) pm, β = 103.51(1)°, monoclinic, C2/c, Z = 4, R = 3.6, R_w = 2.6%] were determined from four-circle diffractometer data and compared with related compounds. The luminescence properties were investigated at 300 K and at 4.2 K; all compounds show intense bluish-green photoluminescence. Sr₂SiO₃Cl₂:Eu²⁺ shows thermoluminescence.     

On the local coordination of Fe in Fe2O3-glass and Fe2O3-glass ceramic systems containing Pb, Na and Si

The effect of the composition and casting temperature on the structural role of Fe in a series of binary (Fe₂O₃-PbO and Fe₂O₃-Na₂O) and ternary (Fe₂O₃-PbO-SiO₂) glass and glass ceramic materials is studied by means of X-ray fluorescence (XRF) mapping, X-ray absorption fine structure (XAFS) and Mössbauer spectroscopies. According to the Mössbauer results the majority of Fe exists in the Fe⁺³ state. The XRF maps reveal that Fe-rich islands evolve into the vitreous matrix of ternary samples that contain more than 40 wt% Fe₂O₃. In these samples the XAFS results disclose that 40-43 at.% of the Fe atoms belong to the Fe-rich microcrystalline islands formed by Fe_xO_y oxides. Furthermore, the structural role of Fe⁺³ in the ternary glasses is found to depend on the Fe₂O₃ content. Finally in the binary Fe₂O₃-PbO systems the majority of Fe⁺³ is octahedrally coordinated in the Fe₂O₃ and/or PbFe₁₂O₁₉ crystalline phases.