Number Of Bridging Oxygen Atoms Research Articles

The mixed network former effect in glasses: solid state NMR and XPS structural studies of the glass system (Na2O)x(BPO4)1−x

The structural organization of sodium borophosphate glasses with composition (Na(2)O)(x)(BPO(4))(1-x) (0.25 ≤x≤ 0.55) has been investigated by differential scanning calorimetry, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), as well as single- and double resonance (11)B and (31)P magic-angle spinning (MAS) NMR. (11)B MAS-NMR data indicate the dominance of anionic four-coordinated boron units, and (31)P MAS NMR reveals the successive transformation of neutral P(3) into singly charged P(2) units and their further transformation into doubly charged P(1) units at high Na(2)O contents. The quantification of these units provides detailed insight into the competition of the network formers borate and phosphate for the network modifier oxide. At low modifier content (x < 0.35), the anionic species are almost exclusively borate (B(4)) units, whereas at higher sodium concentrations, large numbers of anionic phosphate (P(2) and P(1)) species are formed. O-1s XPS data provide a quantitative distinction between B-O-B, B-O-P, and P-O-P linkages as well as non-bridging oxygen atoms, and comparable numbers can be extracted from (11)B and (31)P MAS-NMR experiments. Both XPS as well as (31)P{(11)B} and (11)B{(31)P} rotational echo double resonance (REDOR) NMR results reveal strong interactions between the two network formers boron oxide and phosphorus oxide, resulting in a preferred formation of B-O-P linkages. For higher Na(2)O contents, however, the successive network modification diminishes this preference, resulting in close-to-statistical network connectivities. Compositional trends of T(g) in the Na(2)O-B(2)O(3)-P(2)O(5) glass forming system can be correlated with the overall network connectedness, expressed by the total number of bridging oxygen atoms per network former species. However, separate linear correlations are observed for different compositional lines, indicating also the relevance of the type of network former linkages present.

Thermodynamic simulation of the behavior of network-modifying cations in multicomponent silicate melts

1128 The structure and physicochemical properties of silicate and magmatic melts are currently described mostly within the scope of a model representing the structure of melts as an ensemble of silicon–oxygen structural units Q n (where n is the number of bridging oxygen atoms). The concentrations of these structural units depend on the composition and temperature and can be experimentally determined from Raman spec� tra or calculated by means of thermodynamic simula� tions [1–3].

Spectroscopic, mechanical and magnetic characterization of some bismuth borate glasses containing gadolinium ions

The ultrasonic parameters, the optical parameters along with the IR spectroscopy and magnetic susceptibility studies have been employed to explore the role of Gd 2O 3 in the structure of the glasses xGd 2O 3–60B 2O 3–10MoO 3–(30- x)Bi 2O 3, with 0 ≤ x ≤ 7 mol %. IR analysis indicates that Gd 2O 3 is preferentially incorporated into the borate network-forming BO 4 units. It is assumed that Bi 2O 3 and MoO 3 enter the structure as modifiers in the form of BiO 6 and MoO 6 only. The compositional dependence of the mechanical and the optical parameters are interpreted in terms of the transformation of the structural units BO 3 into BO 4, the increase in the number of bridging oxygen atoms, and the substitution of high bond strength Gd–O, in the place of low bond strength Bi–O bond. The results of the magnetic susceptibility reveal the paramagnetic behavior as described by the Curie-Weiss law and indicating the presence of weak antiferromagnetic exchange interactions between Gd 3+ ions. The magnetic entropy change of the glasses was determined according to the temperature and magnetic field dependence of magnetization.

Structure−Property Relations in Mixed-Network Glasses: Multinuclear Solid State NMR Investigations of the System xAl2O3:(30 − x)P2O5:70SiO2

New mixed-network glasses along the composition line xAl2O3−(30 − x)P2O5−70SiO2 have been prepared and characterized in terms of their density, thermal expansion coefficient, refractive index, and characteristic temperatures. The compositional changes in these macroscopic properties have been correlated with structural information, obtained via Raman spectroscopy and state-of-the-art solid state NMR techniques, including 27Al, 29Si, and 31P magic-angle spinning (MAS) NMR, 27Al triple quantum MAS NMR, as well as static 31P spin echo decay spectroscopy. In addition, the extent of P−O−Al connectivity has been quantified on the basis of 27Al{31P} rotational echo double resonance (REDOR) and 31P{27Al} rotational echo adiabatic passage double resonance (REAPDOR) measurements. Both the macroscopic and the structural properties show nonlinear dependences on x, including abrupt changes at a nominal Al/P ratio of 1 (x = 0.15), where no glasses can be formed by melt quenching under the conditions used in this study. The structure of phosphorus-rich glasses (Al/P < 1) is characterized by four-, five- and six-coordinated Al species, whose second coordination sphere is dominated by phosphorus. 31P static and MAS NMR spectra suggest the presence of at least three distinct phosphorus environments, corresponding to silicon-bonded P(3) units, anionic metaphosphate P(2) species interacting with octahedral aluminum, and tetrahedral PO4/2 groups (P(4) units) bonded similarly as in AlPO4. (In this P(n) nomenclature, the superscript denotes the number of bridging oxygen atoms attached to a P atom.) The latter species is also the dominant phosphorus environment in the Al-rich glasses (Al/P > 1), where the alumina component is involved in Al−O−P, Al−O−Si, and possibly also Al−O−Al linkages. All of these results indicate that the structure of these glasses is dominated by the strong mutual affinity of the phosphorus oxide and alumina components. To quantify this affinity, the experimental REDOR and REAPDOR results have been compared with a cluster model assuming that both components react completely under formation of aluminum phosphate-like domains, thereby maximizing the number of Al−O−P linkages. Both the REDOR and the REAPDOR results show, however, clear deviations from such a structural scenario, supporting a more homogeneous glass structure with a certain degree of connectivity randomization.

Group connectivity in binary silicate glasses: A quasi-chemical approach and molecular dynamics simulation

The medium-range order of molecular dynamics (MD) simulated binary alkali silicate glass with 5, 10, 15, 20, 25, and 33.3mol% of Na2O is studied in terms of the connectivity groups Qn,a1,…,an, where n stands for the number of bridging oxygen atoms, and a1,…,an indicate the type of the connected Qn units. MD structure significantly deviates from the random connectivity model. The deviations were quantified by means of the quasi-chemical approach. The interchange energy between Q3 and Q4 units was found to be negative, indicating preferential formation of Q3–Q4 connections. This means that no tendency to phase separation was observed. Although alkali cations were apparently non-homogeneously distributed in space, regions abounding in Q3–Q3 and Q4–Q4 connections were not formed. It was suggested that grouping of alkali cations can be also mediated by the Q4,4433 groups.

Group connectivity in binary silicate glasses

The medium range order of binary alkali-silicate glass is studied in terms of the groups Q n , a 1 , … , a n , where n stands for the number of bridging oxygen atoms, and a 1, … , a n indicate type of the connected Q n units. The Random Connectivity Model describing interconnections of Q n units in silicate glass was proposed and compared with the available experimental DQ 29Si nuclear magnetic resonance (NMR) data and with data obtained from molecular dynamics (MD) simulations. Analysis of NMR revealed that basic connectivity conditions are not fulfilled. The connectivity distribution in glass prepared by MD was found to be random.

Nuclear magnetic resonance studies of 0.139MO (or M ′2O) · 0.673SiO 2 · (0.188− x)Al 2O 3 · xB 2O 3 (M=Mg, Ca, Sr and Ba, M ′=Na and K) glasses

The 11B, 27Al and 29Si magic angle spinning (MAS) NMR spectra of 0.139MO (or M ′ 2O) · 0.673SiO 2 · (0.188− x)Al 2O 3 · xB 2O 3 · (M=Mg, Ca, Sr and Ba, M ′=Na and K) glasses were examined. When three network forming oxides such as SiO 2, Al 2O 3 and B 2O 3 coexist, Al 2O 3 reacts preferably with network modifier such as M ′ 2O or MO. The reactivity of residual network modifier with SiO 2 was high compared with B 2O 3 in the order of K 2O < Na 2O < BaO < SrO < CaO < MgO. The apparent equilibrium constants, K app, of the reactions, Si(Q 3) + B(Q 3)=Si(Q 4) + B(Q 4), were determined. Here, the Si(Q n ) or B(Q 3) and B(Q 4) represent the SiO 2 structural units or three- and four-coordinated boron atoms, respectively. The subscript are the number of bridging oxygen atoms. The K app values indicate the reactivity of MO or M ′ 2O with B 2O 3 and SiO 2 in the MO(M ′ 2O) · SiO 2 · Al 2O 3 · B 2O 3 glasses. The reaction proceeds to the left direction in the order of MgO > CaO > SrO > BaO > Na 2O > K 2O. The order of the reactivity is the same as that of the electronegativities among alkali and alkaline earth metals.

Structural aspects of PbO–P 2O 5 glasses containing ThO 2

PbO–P 2O 5 glasses in which part of PbO/P 2O 5 was replaced by ThO 2 have been prepared by the conventional melt quench method and their structural aspects have been studied by 31P MAS NMR, Raman and FTIR techniques. Binary PbO–P 2O 5 glass is characterized by mainly Q 2 and Q 1 structural units of P (Q n represents the phosphorus structural units having ‘ n’ number of bridging oxygen atoms). When part of the PbO has been replaced by ThO 2, the phosphorus structural units are not significantly affected, indicating that Th 4+ replaces Pb 2+ at the interstitial positions in the glass network. Unlike this when part of P 2O 5 has been replaced by ThO 2 extensive depolymerisation of the phosphate network resulted in the conversion of Q 2 to Q 1 structural units of P. Above 10 mol% of ThO 2 replacement in either case resulted in the partial devitrification of the glass.

Combined [formula omitted] double quantum NMR and MD simulation studies of network connectivities of binary Na 2O·SiO 2 glasses: new prospects and problems

Quantitative results of structural studies of binary xNa 2O·(1− x)SiO 2 glasses ( x=0.15, 0.20, 0.25), using both experimental (double quantum NMR) and theoretical methods (molecular dynamics simulation), are presented. The primary goal is to verify and quantify connectivities of the various SiO 4 species in terms of small clusters of Q n, ijkl units, with n being the number of bridging oxygen atoms of the `central' Q unit. The additional superscripts denote the Q character of the bonded adjacent SiO 4 tetrahedra. It is the aim of the paper to compare these DQ NMR results with theoretical predictions by the most recent MD simulations. The current problems and limitations of this new approach are discussed in detail.

Evidence for a residual elastic modulus in inorganic glasses by mechanical spectroscopy

The dynamic modulus of various oxide glasses has been investigated over a large temperature and frequency range, using mechanical spectroscopy experiments. No pseudo-rubbery plateau is apparent in silica; in contrast, with mixed oxide glasses, the existence of a residual modulus after the principal α relaxation is clearly evidenced under appropriate conditions. The magnitude of this residual modulus depends on applied stress, indicating therefore a non-linear behavior in these oxide glasses at low stress fields. A physical model has been proposed. This model is based on a process in which one Si–O bond instantaneously ‘flips’ between positions by a stress assisted movement. The number of cross-links is involved in the expression of the residual modulus; therefore the composition of the glass is taken into account, since the number of bridging oxygen atoms is introduced in this model. Agreement between computation and experimental data is very good, using a biparabolic expression for the dynamic modulus.

The Molecular Dynamics Study of Lithium Ion Conduction in Phosphate Glasses and the Role of Non-Bridging Oxygen

Molecular dynamics (MD) simulation of lithium phosphate (Li2O-P2O5) glasses with varying Li2O content has been carried out. Two different P-O distances corresponding to phosphorus coordination with bridging oxygen (BO) and non-bridging oxygen (NBO) were identified in the simulated glasses. NBO-BO interconversion or bond switching was noted, which results in a dynamic equilibration of the tetrahedral phosphate units (P-n, n = 1,3 indicates the number of bridging oxygen atoms in the coordination of phosphorus). The NBO-BO bond switching is mildly activated with an effective activation barrier of 0.03-0.05 eV. Lithium ion jumps do not appear to be strongly coupled to bond switching. But the number of Li+ ions coordinated to an optimum number of NBOs and the number of Li+ ions jumping out of their sites appear to be correlated. Detailed analysis was made of the dynamics of P-n species and new insights have been obtained regarding ion migration in network-modified phosphate glasses.

The structural groups of alkali silicate glasses determined from 29Si MAS-NMR

Lithium, sodium and potassium silicate glasses containing 20–56 mol% alkali oxide were investigated by 29Si nuclear magnetic resonance (MAS-NMR) spectroscopy. In the spectrum of each sample, at least two to four distinct peaks were identified. The distributions of SiO 4 structural units, Q n , where n is the number of bridging oxygen atoms bound to other Si atoms, were determined as a function of composition. The equilibrium constants of the reactions, 2Q n ⇌ Q n−1 + Q n+1 ( n = 3, 2, 1), were determined. The reaction proceeds to the right direction as cationic power of alkali ion ( Z/ r) increases (Li +>Na +>K +) at the same alkali oxide concentration. The apparent equilibrium constants of the above reactions are discussed along with a proposed thermodynamic model. The 29Si chemical shifts assigned to each structural unit increase linearly with alkali oxide contents. The slope of these lines decreases as the numbers of attached bridging oxygen (BO) atoms decrease. The average chemical shifts also increase linearly with an increase of alkali content. A close relationship between the average chemical shifts and the theoretical optical basicity was observed.

X-Ray Photoelectron Spectroscopy Studies of Silicate Glasses: Implications to Bonding and Leaching

X-ray photoelectron spectroscopy can be applied to the study of bonding properties of metal oxides in silicate glasses and to the analysis of water-leached glass surfaces. The addition of CaO to Na2O-2SiO2 results in a decrease in the number of bridging oxygen atoms in the glass matrix as monitored by oxygen 1s peak intensities. In sodium disilicate glasses containing dissolved uranium, significantly different leaching behavior can occur, depending on the oxidation state of the uranium ions.

Metal Oxide Trialkylsilyloxide Polymers. Part VI. Aluminum Oxide Trimethylsilyloxides

Tris(trimethylsilyloxy)aluminum, [Al(OSiMe3)3]2, was hydrolyzed under controlled conditions in dioxane at 25 °C. The soluble initial products [AlOx(OH)z(OSiMe3)3−2x−z]n, with x = 0.03–0.65, underwent disproportionation when heated at 120–150 °C under vacuum to give final products with x = 0.8–1.1. Molecular weights were determined by cryoscopy in cyclohexane, and the linear dependence of 1/n on x was combined with an extension of the theory of regular polymer series to show that the number of OH groups, z, was small compared to the number of bridging oxygen atoms, x, per Al atom. The initial products consist of dimeric units, each linked to an adjacent unit by one or two oxygen bridges between aluminum atoms. The degree of polymerization is 1 to 4, based on the dimer unit. The final products have much higher degrees of polymerization (10–40, based on the dimer), and appear to contain a higher proportion of Al—O—Al bridges.