A multinuclear NMR study of sodium-borovanadophosphate glasses

xNaVO₃ · yNaPO₃ · (1 − y)NaBO₂ glasses were prepared where x = 0.0, 0.05, 0.5 and 0 ? y ? 1. These glasses have been investigated with multinuclear MAS NMR. ⁵¹V NMR spectra show that two vanadate configurations are possible, assigned as four and five coordinated vanadium. The concentration of the latter group decreases upon addition of sodium-borate. The presence of four and three coordinated boron sites can be deduced from the ¹¹B NMR spectra. The latter boron sites appear only when borate groups are connected to each other. ³¹P NMR spectra of borophosphates and borovanadophosphates show that the ratio of pyro- and metaphosphates is greater in the glasses containing vanadate than in simple borophosphates. These results indicate phosphate to be the most acidic agent and therefore the best chain-terminating group in these glasses.     

The effect of fictive temperature on the structure of E-glass: A high resolution,multinuclear NMR study

The variation of the structure of E-glass with fictive temperature has been examined with high resolution, multinuclear (¹¹B, ²⁷Al, ²⁹Si, ¹⁷O, and ¹⁹F) MAS NMR. The previously observed decrease in 4-coordinated boron with increasing fictive temperature is confirmed and, for the first time, is directly correlated with an increase in non-bridging oxygens. An increase in 5-coordinated Al is also observed, and is linked to an increase in association of F with Al at higher fictive temperature.     

Structural organisation in oxide glasses from molecular dynamics modelling

Classical molecular dynamics modelling has been used to obtain new models of 50CaO·50P₂O₅ and 50MgO·50SiO₂ glasses and, together with previously published models of 63CaO·37Al₂O₃, and 50CaO·50SiO₂ glasses, these have been inspected to evaluate structural features. For the first time, models of glasses near the eutectic in three systems, aluminate, silicate, and phosphate, with the same modifier, Ca, have been compared. All have short range order which is similar to that in crystals of the same composition, 5CaO·3Al₂O₃, CaSiO₃ and Ca(PO₃)₂. There is a clear trend in bonding of bridging oxygen to Ca, which is dominant in aluminate glass, common in silicate glass, and absent in phosphate glass. Preliminary results for 50MgO·50SiO₂ glass show unusual behaviour because ~ 5% of oxygen is present as “non-network” oxygen, i.e. bonded only to Mg. The models show broader Qⁿ distributions than seen in NMR experiments, and this remains an area for improvement of MD modelling of glasses. The distributions of Ca in the models have been studied using the pair distribution function T_CaCa(r) which is found to be similar in the three glasses, and also similar to the previous experimental measurement for 50CaO·50SiO₂ glass. The distributions of Ca are markedly different in the glasses compared to the crystals, being isotropic in the former and anisotropic in the latter, which should be a factor in glass forming ability.     

Structure of fluorochlorozirconate glasses using molecular dynamics

The structure of fluorochlorozirconate glass with composition Zr_0.55Ba_0.20La_0.05Na_0.20F_2.95−xCl_x with 0 ? x ? 0.9, i.e., up to ∼30% chlorine anions, has been simulated using molecular dynamics methods applied to a unit cell comprising 790 ions. The structure comprises a network of (mainly) corner and edge-linked ZrF_8−pCl_p, ZrF_7−qCl_q and LaF_7−rCl_r polyhedra with sodium, barium and additional chlorine ions occupying irregular positions in space between the polyhedra. For chlorine concentrations above about 15%, the number of chlorine ions occupying corner bridging positions increases markedly which is expected to reduce the glass temperature. For chlorine concentrations greater than about 20%, the spatial distribution of ions is not uniform and clustering into zirconium rich and sodium, barium and chlorine rich regions occurs. The possibility that this phenomenon may be a precursor to the observed precipitation of barium chloride crystals just above the glass temperature is discussed.     

Structure of organic-inorganic hybrid low-melting glasses from Si NMR and ab initio molecular orbital calculations

The structure of organic-inorganic hybrid glass precursor Me₂Si(OPO(OH)₂)₂ and low-melting glasses SnO-Me₂SiO-P₂O₅ has been examined by ²⁹Si static and MAS NMR, respectively. The ²⁹Si MAS NMR spectra of SnO-Me₂SiO-P₂O₅ were deconvoluted into three Gaussian peaks, whose chemical shifts were located at −3.1, −10.2 and −18.1 ppm. Ab initio molecular orbital calculations have also been carried out for the clusters modeling the structure of precursor and glasses. From the calculations of ²⁹Si NMR shielding constants, the experimental chemical shifts at −10.2, −3.1 and −18.1 ppm were assigned to ²⁹Si atoms in P-O-Si(Me)₂-O-P network linkage, in P-O-Si(Me)₂-OH terminating structure and in P-O-Si(Me)₂-O-Si network linkage, respectively. Based on the area ratio of the assigned three peaks, the network structures around Si constructing SnO-Me₂SiO-P₂O₅ low-melting glasses were discussed in detail.     

The nature of phase separation in binary oxide melts and glasses. I. Silicate systems

An exhaustive review of compositional and thermal extents of miscibility gaps in 41 binary silicate systems permits identification of three groups of cations exhibiting different immiscibility behaviours. The first group comprises network-modifier cations with an ionic radius larger than about 87.2 pm. They have coordination numbers equal to, or higher than, 5 and their miscibility gap size increases linearly with increasing ionic potential. The second group involves cations with an ionic radius larger than 26 pm and smaller than about 87.2 pm (in octahedral coordination). They have at least two coordination numbers: the first one is always 4 and the other 5 (or more). For this reason they are called amphoteric. Their miscibility gap sizes do not increase linearly with an increase of the ionic potential, but follow curves. The third group includes cations with variable crystal field stabilization energies. They are characterized by larger miscibility gap sizes than expected when they are compared with cations with similar ionic radii despite the fact that some of them (e.g. Cr³⁺) may behave as an amphoteric element because their ionic radii in octahedral coordination are smaller than about 87.2 pm. The origin of phase separation in binary silicate systems is due to coulombic repulsions between poorly screened cations bounded by bridging oxygen strongly polarized towards the silicon, and by non-bridging oxygen.     

Glassy states: Concentration glasses and temperature glasses compared

The behavior of glass-forming systems in the equilibrium state above the glass temperature is still a heavily investigated field. Surprisingly, the behavior of the glass itself is less widely investigated. Even less investigated is the behavior of glass-forming materials in which composition is changed. Here we look at the behavior of glasses after temperature-jumps and compare that behavior with that of glasses subjected to concentration-jumps. Moisture and carbon dioxide are used as the plasticizing environments. Surprisingly, the glass created by jumping (down) to a given final condition via a change in concentration is more stable than that formed by a change in temperature – this in spite of the external condition of temperature and chemical activity (RH or carbon dioxide pressure) being the same. Furthermore, the concentration glass under such conditions has a higher excess volume than the temperature glass and its response does not ‘merge’ with that of the temperature glass, hence, the concentration glass is not the same as a temperature hyperquenched glass.     

Estimating accuracy of 17O NMR measurements in oxide glasses: Constraints and evidence from crystalline and glassy calcium and barium silicates

The use of ¹⁷O nuclear magnetic resonance (NMR) spectroscopy to measure site populations in silicate and aluminosilicate glasses has provided insights and challenges to conventional models of glass structure. In order to better understand the level of accuracy and precision achievable, we have synthesized crystalline barium metasilicate (BaSiO₃), barium orthosilicate (Ba₂SiO₄), tricalcium silicate (Ca₃SiO₅), a barium silicate glass ((BaO)_0.45(SiO₂)_0.55), and a calcium silicate glass ((CaO)_0.56(SiO₂)_0.44), and report ¹⁷O NMR spectra for all of these. After correcting the observed intensities for quadrupolar effects, we measure an NBO content of 66.7% ± 0.6% for the BaSiO₃, compared to the known value of 66.7%. Applying the same techniques for the glasses gives an NBO content of 58.8% ± 0.8% (vs. the expected 55.5% ± 1.4% from stoichiometry) for the barium silicate and 76.9% ± 1.2% (vs. 78.6% ± 1.4%) for the calcium silicate. Within our uncertainties, we find no evidence for deviation from conventional models of glass structure for the glasses studied here. We also see no NMR signal (detection limit of about 0.5%) at the expected position for “free” oxide ions (bonded only to Ca^2 +), as newly constrained by our data for crystalline Ca₃SiO₅, which contains this species.     

The nature of phase separation in binary oxide melts and glasses. II. Selective solution mechanism

A comprehensive review of structural data in binary silicate systems indicates that the tetrahedral critical radius (87.2 pm) of binary silicate melts (or glasses) is associated with the silicon tetrahedral network that defines the structure of the melt. In a binary system, most of the cages present in the melt are made of six and five-membered rings of silicon tetrahedra. Cages bounded by six or more-membered rings can host cations of all sizes. However, cations that enter in cages made of five-membered rings are discriminated by their ionic radius. Cations with ionic radii larger than about 87.2 pm (network modifiers) cannot enter in pentagonal apertures; cations with radii smaller than 87.2 pm (amphoteric cations) can. Cages bounded by pentagonal rings play a key role in phase separation by selecting which cations can fit in them, adopt a four-fold coordination, and reduce the size of miscibility gaps, i.e. the cages permit explaining why some cations are amphoteric. This result is important because it shows that a structural control is exerted by the solvent (here SiO₂) upon immiscibility which creates a selective solution mechanism that affects small (<87.2 pm) cations in binary silica-rich melts.     

The nature of phase separation in binary oxide melts and glasses. III. Borate and germanate systems

A review of immiscibility data in binary borate and germanate systems was performed in order to compare miscibility gap consolute temperatures with ionic potentials and radii of their associated cations. The trends obtained demonstrate that a selective solution mechanism similar to the one identified for the binary silicate systems is present in the borate and germanate binaries. More importantly, the borate and germanate immiscibility data permitted the identification of a new group of cations depicting an immiscibility behaviour different from the ones identified in binary silicate systems. The new group involves highly polarizable cations possessing a lone pair of electrons. This lone pair of electrons together with oxygen bonded by strong covalent bonds to modifier cations provides efficient shielding to the cations' nuclei which considerably reduces the coulombic repulsions and produces miscibility gaps with very low consolute temperatures. A new group of cations having an homogenizing effect on melts (i.e. a capacity to make immiscible melts single phase) is thus reported. Experimental and spectroscopic data suggest that miscibility gaps associated with cations having a lone pair of electrons exist in binary silicate systems such as TlO_1/2-SiO₂, PbO-SiO₂, SnO-SiO₂ and Bi₂O₃-SiO₂. The consolute temperature of their miscibility gaps is expected to be relatively low and metastable.     

On the electrochemical behaviour of undercooled melts and glasses

The electrochemical behaviour of undercooled melts and glasses is considered in detail. First the thermodynamic properties of the corresponding phases are calculated and then the temperature course of the emf and of other thermodynamic functions of a galvanic cell in which the net reaction is: undercooled melt (glass)→crystal, are determined. Some additional problems concerning the electrochemistry of amorphous electrodes are analyzed, e.g. hydrogen overvoltage upon hydrogen evolution on crystalline or amorphous electrodes of a given metal. Several variants are discussed for the possible experimental realization of a galvanic cell in which the emf is determined by the difference in the chemical potentials of the undercooled melt (or the glass) and the crystal, i.e. of a galvanic cell with a vitreous and a crystalline electrode made of one and the same electronic conductor or of a bipolar cell with an amorphous and a crystalline electrolyte, etc. An attempt is made to estimate the interal electric (Galvani) potential at the melt/crystal interface by using data for the difference of the Peltier coefficient between the respective phases.The analysis of the electrochemical behaviour of vitreous, liquid and crystalline electrodes is performed on the basis of formal thermodynamics and in terms of classical model approaches. It is shown how to determine by electrochemical measurements the main thermodynamic parameters of glasses which are considered as frozen-in non-equilibrium systems.     

In situ high temperature 27Al NMR study of structure and dynamics in a calcium aluminosilicate glass and melt

The temperature dependence (25–1400 °C) of ²⁷Al NMR spectra and spin–lattice relaxation time constants T₁ have been studied for a calcium aluminosilicate (43.1CaO–12.5Al₂O₃–44.4SiO₂) glass and melt using an in situ high temperature probe, and the glass has been characterized by ambient temperature, high field MAS NMR. The peak positions and the line widths show a consistent behavior as motional averaging of the quadrupolar satellites increases with increasing temperature. The rate of decrease with temperature of T₁ drastically increases near the glass transition temperature T_g, which suggests a change in NMR relaxation process from vibrational to translational motions. Above the T₁ minimum (≈1200 °C), NMR correlation times obtained from T₁ are in good agreement with shear relaxation times estimated from viscosity, suggesting that microscopic nuclear spin relaxation is controlled by the same dynamics as macroscopic structural relaxation, and thus that atomic-scale motion is closely related to macroscopic viscous flow.     

A MAS NMR structural study of cadmium phosphate glasses

Cadmium phosphate glasses, of general formula xCdO(1−x)P₂O₅ (0.25?x?0.6), have been prepared by melting in alumina crucibles, with resulting dissolution of up to 6.4 mol% Al₂O₃. The local structure in these glasses has been studied using ³¹P, ²⁷Al and ¹¹³Cd nuclear magnetic resonance. The distribution of [PO₄]Qⁿ species as a function of composition has been shown to follow the simple binary model. The rate of change of the chemical shift of the ³¹P species in the Q² environment depends on the bond order, which in turn reflects the extent of double bonding between phosphorus and oxygen.     

Atomic structure of beryllium boroaluminate glasses: A multi-nuclear NMR spectroscopic study

The short-range structure and network speciation have been studied in a series of beryllium boroaluminate glasses using ¹¹B, ²⁷Al and ⁹Be NMR spectroscopy. All glasses are characterized by a coexistence of BeO₄, BO₃, BO₄, AlO₄, AlO₅ and AlO₆ species. The concentrations of BO₃, AlO₅ and AlO₆ species in these glasses are significantly higher and the geometry of the B-O and Al-O coordination polyhedra are unusually disordered compared to those in other alkali and alkaline-earth boroaluminate glasses reported previously in the literature. These results indicate that Be atoms may not play the typical role of a network-modifying cation in these glasses. This structural scenario is consistent with the highest field strength and electronegativity of Be among all alkali and alkaline-earth metals.     

Structure and lattice dynamics of binary lead silicate glasses investigated by infrared spectroscopy

The polar lattices dynamics of seven binary lead silicate glasses have been studied by infrared spectroscopy. The analysis of the reflectivity spectra with a dielectric function model, based on a modified Gaussian profile, allows a quantitative evaluation of the presence of lead cations within different structural sites. From the role of the lead cations versus the degree of polymerization of the silicate network and the comparison with literature results, we may to give a scenario for explaining the observed structural evolution of the glass matrix and more particularly the drastic change occurring around 45% of lead content. Below this threshold, lead cations act only as modifiers of the silicate network. Above, the glass structure is deeply modified; a lead network involving around 10% of the lead content appears in glasses whose composition is just above the threshold and progressively grows at the expense of the silicate network with the increase of lead content. For high lead content, lead cations can act as modifiers of the silicate network or as network formers. Results also show that the analysis of far infrared measurements combined with the knowledge of the UV edge optical response is very promising to characterize the local disorder around cations in glasses.     

Structure and Raman spectra of glasses containing several glass-forming oxides and no glass-modifying oxide

Binary glasses containing no modifying oxides, such as SiO₂---GeO₂, SiO₂---B₂O₃, SiO₂---P₂O₅, GeO₂---B₂O₃, Al₂O₃---P₂O₅ and ternary glasses SiO₂---GeO₂---P₂O₅, Al₂O₃---B₂O₃---P₂O₅, B₂O₃---SiO₂---P₂O₅, Al₂O₃---ZrO₂---P₂O₅ have been prepared by melting and CVD methods. The Raman spectra have also been measured. Structural characteristics of SiO₂, GeO₂, B₂O₃, P₂O₅ in different glass systems are analysed. There exist coordination number changes in B₂O₃- and GeO₂-containing glasses and linkage changes between tetrahedra (SiO₄) and (PO₄) in SiO₂ and P₂O₅ containing glasses. The structure of Al₂O₃ containing glasses is homogeneous and the structure of B₂O₃ containing glasses is inhomogeneous. These experimental results are in coincidence with the X-ray small angle scattering analysis.     

Structure of lithium borate and strontium borate glasses with high modifier contents

The structure of lithium borate and strontium borate glasses with high Li₂O and SrO contents was examined using the experimental results of ESR and the optical absorption of Cu²⁺ ions, and the Mössbauer spectra of Fe³⁺ ions and Raman spectra of these glasses. The results indicate the existence of non-bridging oxygens belonging to orthoborate and pyroborate groups, structural groups of diborate and high pressure forms of metaborate above 30 mol% of the modifier contents in these glasses. Among these four structural groups, the diborate group rapidly decreased with an increase of the modifier contents, while the other three groups increased with an increase of the modifier contents.