27Al NMR Study of the Structure of Lanthanum- and Yttrium-Based Aluminosilicate Glasses and Melts

Gamma, neutron, and heavy charged ion shielding properties of Er3+-doped and Sm3+-doped zinc borate glasses

This study aimed to investigate the nuclear radiation shielding properties of erbium (Er)-reinforced and samarium (Sm)-reinforced borate glasses. In the 0.015–15 MeV photon energy range, attenuation coefficients, as well as half-value layer tenth-value layers, and the mean-free path have been calculated. Additionally, effective, and equivalent atomic numbers, effective atomic weight, electron density, and exposure and energy absorption build-up factors were also calculated. To evaluate the overall nuclear radiation attenuation competencies of Er-rich and Sm-rich glasses, effective removal cross-section values for fast neutrons and projected range/mass stopping power values for alpha and proton particles were also determined. The glass sample BZBEr2.0 had the highest linear and mass attenuation coefficients (µ and µ m), effective conductivity (C eff), the effective number of electrons (N eff), and effective atomic number (Z eff) values as well as the lowest half-value layer (T 1/2), tenth value layers (T 1/10), mean free path (λ), exposure build-up factor, and energy absorption build-up factor values. µ m values were reported as 2.337, 2.556, 2.770, 2.976, 2.108, 2.266, 2.421, 2.569, and 2.714 for BZBEr0.5, BZBEr1.0, BZBEr1.5, BZBEr2.0, BZBSm0.0, BZBSm0.5, BZBSm1.0, BZBSm1.5, and BZBSm2.0 glass samples at 0.06 MeV, respectively. The results showed that Er has a greater effect than Sm regarding the gamma-ray shielding properties of borate glasses. The results of this investigation could be used in further investigations and added to older investigations with the same aim, to aid the scientific community in determining the most appropriate rare-earth additive, to provide adequate shielding properties based on the requirement.

Liquid structure under extreme conditions: high-pressure x-ray diffraction studies

Under extreme conditions of high pressure and temperature, liquids can undergo substantial structural transformations as their atoms rearrange to minimise energy within a more confined volume. Understanding the structural response of liquids under extreme conditions is important across a variety of disciplines, from fundamental physics and exotic chemistry to materials and planetary science.In situexperiments and atomistic simulations can provide crucial insight into the nature of liquid-liquid phase transitions and the complex phase diagrams and melting relations of high-pressure materials. Structural changes in natural magmas at the high-pressures experienced in deep planetary interiors can have a profound impact on their physical properties, knowledge of which is important to inform geochemical models of magmatic processes. Generating the extreme conditions required to melt samples at high-pressure, whilst simultaneously measuring their liquid structure, is a considerable challenge. The measurement, analysis, and interpretation of structural data is further complicated by the inherent disordered nature of liquids at the atomic-scale. However, recent advances in high-pressure technology mean that liquid diffraction measurements are becoming more routinely feasible at synchrotron facilities around the world. This topical review examines methods for high pressure synchrotron x-ray diffraction of liquids and the wide variety of systems which have been studied by them, from simple liquid metals and their remarkable complex behaviour at high-pressure, to molecular-polymeric liquid-liquid transitions in pnicogen and chalcogen liquids, and density-driven structural transformations in water and silicate melts.

Structure and dynamics of high-temperature strontium aluminosilicate melts

We report the study of high-temperature melts (1600-2300 °C) and related glasses in the SrO-Al2O3-SiO2 phase diagram considering three series: (i) depolymerized ([SrO]/[Al2O3] = 3); (ii) fully polymerized ([SrO]/[Al2O3] = 1); and (iii) per-aluminous ([SrO]/[Al2O3] < 1). By considering the results from high-temperature 27Al NMR and high-temperature neutron diffraction, we demonstrate that the structure of the polymerized melts is controlled by a close-to-random distribution of Al and Si in the tetrahedral sites, while the depolymerized melts show smaller rings with a possible loss of non-bridging oxygens on AlO4 units during cooling for high-silica compositions. A few five-fold coordinated VAl sites are present in all compositions, except per-aluminous ones where high amounts of high-coordinated aluminium are found in glasses and melts with complex temperature dependence. In high-temperature melts, strontium has a coordination number of 8 or less, i.e. less than in the corresponding glasses. The dynamics of high-temperature melts were studied from 27Al NMR relaxation and compared to macroscopic shear viscosity data. These methods provide correlation times in close agreement. At very high temperatures, the NMR correlation times can be related to the oxygen self-diffusion coefficient, and we show a decrease of the latter with increasing Si/(Al + Si) ratios for polymerized melts with no compositional dependence for depolymerized ones. The dominant parameter controlling the temperature dependence of the aluminum environment of all melts is the distribution of Al-(OSi)p(OAl)(4-p) units.

Structure and Crystallization of Alkaline-Earth Aluminosilicate Glasses: Prevention of the Alumina-Avoidance Principle

Aluminosilicate glasses are considered to follow the Al-avoidance principle, which states that Al-O-Al linkages are energetically less favorable, such that, if there is a possibility for Si-O-Al linkages to occur in a glass composition, Al-O-Al linkages are not formed. The current paper shows that breaching of the Al-avoidance principle is essential for understanding the distribution of network-forming AlO₄ and SiO₄ structural units in alkaline-earth aluminosilicate glasses. The present study proposes a new modified random network (NMRN) model, which accepts Al-O-Al linkages for aluminosilicate glasses. The NMRN model consists of two regions, a network structure region (NS-Region) composed of well-separated homonuclear and heteronuclear framework species and a channel region (C-Region) of nonbridging oxygens (NBOs) and nonframework cations. The NMRN model accounts for the structural changes and devitrification behavior of aluminosilicate glasses. A parent Ca- and Al-rich melilite-based CaO-MgO-Al₂O₃-SiO₂ (CMAS) glass composition was modified by substituting MgO for CaO and SiO₂ for Al₂O₃ to understand variations in the distribution of network-forming structural units in the NS-region and devitrification behavior upon heat treating. The structural features of the glass and glass-ceramics (GCs) were meticulously assessed by advanced characterization techniques including neutron diffraction (ND), powder X-ray diffraction (XRD), ²⁹Si and ²⁷Al magic angle spinning (MAS)-nuclear magnetic resonance (NMR), and in situ Raman spectroscopy. ND revealed the formation of SiO₄ and AlO₄ tetrahedral units in all the glass compositions. Simulations of chemical glass compositions based on deconvolution of ²⁹Si MAS NMR spectral analysis indicate the preferred formation of Si-O-Al over Si-O-Si and Al-O-Al linkages and the presence of a high concentration of nonbridging oxygens leading to the formation of a separate NS-region containing both SiO₄ and AlO₄ tetrahedra (Si/Al) (heteronuclear) in addition to the presence of Al_[4]-O-Al_[4] bonds; this region coexists with a predominantly SiO₄-containing (homonuclear) NS-region. In GCs, obtained after heat treatment at 850 °C for 250 h, the formation of crystalline phases, as revealed from Rietveld refinement of XRD data, may be understood on the basis of the distribution of SiO₄ and AlO₄ structural units in the NS-region. The in situ Raman spectra of the GCs confirmed the formation of a Si/Al structural region, as well as indicating interaction between the Al/Si region and SiO₄-rich region at higher temperatures, leading to the formation of additional crystalline phases.

NMR investigations unveil phase composition-property correlations in Sr0.55Na0.45SiO2.775 fast ion conductor

This paper reports results of ²³Na and ²⁹Si solid-state NMR investigations carried out on sodium strontium silicate ion conductor, Sr_0.55Na_0.45SiO_2.775 and presents the first experimental evidence to show that different synthesis conditions induce multiple devitrified phases. Along with 1-dimensional NMR, ²³Na MQMAS spectra have been used to identify the phases corresponding to polymorphs of Na₂Si₂O₅, in addition to the crystalline SrSiO₃ and the glass/amorphous Na₂Si₂O₅ phases. The surprising observation of about an order of magnitude higher ionic conductivity achieved in devitrified samples is attributed to the growth of the crystalline δ-Na₂Si₂O₅ phase within the amorphous Na₂Si₂O₅ phase domains, identified using NMR. Together with XRD and conductivity measurement data, the study leads to the identification of the chemical phase composition and an understanding of the composition-property-structure correlation in this material. Present findings, while do not show any evidence of Na doping in the SrSiO₃ phase confirming earlier reports, explain the large discrepancy in the conductivity reported in the literature.

Steady photodarkening of thulium alumino-silicate fibers pumped at 1.07 μm: quantitative effect of lanthanum, cerium, and thulium

By pumping thulium-doped silica-based fibers at 1.07 μm, rapid generation of absorbing centers leads to photoinduced attenuation (PIA). This detrimental effect prevents exploiting laser emissions in the visible and near infrared. We report on the characterization of the PIA versus the fiber core composition, particularly the concentration of thulium (Tm), lanthanum (La), and cerium (Ce) ions. We show that UV emission induced by Tm-Tm energy transfers is the source of photodarkening and that lanthanum and cerium are efficient hardeners against PIA.

Direct 17O NMR experimental evidence for Al–NBO bonds in Si-rich and highly polymerized aluminosilicate glasses

Double-resonance ¹⁷O{²⁷Al} NMR unambiguously evidences Al–NBO bonds in rare-earth aluminosilicate glasses.

MAS NMR spectra of quadrupolar nuclei in disordered solids: the Czjzek model

Structural disorder at the scale of two to three atomic positions around the probe nucleus results in variations of the EFG and thus in a distribution of the quadrupolar interaction. This distribution is at the origin of the lineshape tailing toward high fields which is often observed in the MAS NMR spectra of quadrupolar nuclei in disordered solids. The Czjzek model provides an analytical expression for the joint distribution of the NMR quadrupolar parameters upsilon(Q) and eta from which a lineshape can be predicted. This model is derived from the Central Limit Theorem and the statistical isotropy inherent to disorder. It is thus applicable to a wide range of materials as we have illustrated for 27Al spectra on selected examples of glasses (slag), spinels (alumina), and hydrates (cement aluminum hydrates). In particular, when relevant, the use of the Czjzek model allows a quantitative decomposition of the spectra and an accurate extraction of the second moment of the quadrupolar product. In this respect, it is important to realize that only rotational invariants such as the quadrupolar product can make sense to describe the quadrupolar interaction in disordered solids.