Alkali Borate Research Articles

Alkali and halogen dependence of optical absorption bands of alkali-borate glasses

We examined the alkali and halogen ion concentration dependence of the optical absorption in alkali borate glasses. When potassium iodide was added to alkali borate glasses, I 3 − molecules were formed and induced two bands with peaks at 287 and 354 nm. These bands increased with K 2O concentrations until 10 mol%, and decreased with K 2O from 20 to 25 mol%. In glasses with additions of KBr and KCl, the bands were observed at 266 and 259 nm, respectively. These positions were negligibly influenced by the kind of alkali ions. It is confirmed that all of these bands are attributed to the X 3 − ions (X: halogen), by the comparison with absorption spectra of thin films prepared by simultaneous evaporation of alkali halide and a small amount of halogen. Molecular orbital (MO) calculations were also carried out to examine the charge distribution and structures. From the experimental results and MO calculations, we suggest that the X 3 − ion binds to a boroxol ring that is coupled with a triborate group through an alkali ion.

Environment of Ni, Co and Zn in low alkali borate glasses: information from EXAFS and XANES spectra

XANES spectroscopy confirms that transition elements such as nickel, cobalt and zinc are octahedrally co-ordinated in low-alkali borate glasses, a co-ordination state which is unusual in most oxide glasses. EXAFS spectroscopy indicates that, despite their diluted character, transition elements are inhomogeneously distributed, with a medium range order extending up to 6 Å with multiple scattering features characteristic of the presence of collinear cations. This peculiar structure is attributed to the presence of rigid units in these low-alkali borate glasses. The presence of these ordered domains in 0.1Li 2O–0.9B 2O 3 glasses with NiO contents ranging from 0.5 to 2 wt% shows their independence relative to the concentration of the transition element.

Physical properties of alkali and mixed lithium–cesium vanadate glasses prepared over an extended range of compositions

We report glass transition temperatures and density measurements of alkali and mixed alkali vanadate glasses. The glass formation of the alkali vanadate glasses ( RM 2O·V 2O 5), which required rapid cooling, occurred in the range R=0.00–1.50, where M is the alkali. We also studied the mixed alkali vanadate glasses [(( RX)Li 2O+ R(1− X)Cs 2O)·V 2O 5] with R=0.60 and X in the range 0.00–1.00, where we discovered a narrow region of bulk glass formation centered on X=0.60. We also compared molar volumes of the alkali vanadates to the analogous alkali phosphates [1,2]. Predicted molar volumes, determined from densities of the respective oxides, were found to closely agree with experimental values for the glasses. Unlike the alkali borates, silicates, and germanates, lithium phosphates and vanadates exhibited molar volumes indicative of structural expansion.

Short-range structure of alkaline-earth borate glasses by pulsed neutron diffraction and molecular dynamics simulation

The structure of vitreous MO· nB 2O 3 (M=Ca and Ba; n=2, 3 and 4) has been studied by pulsed neutron diffraction measurement with the help of molecular dynamics (MD) simulation. The first and second peaks assigned to the nearest-neighbor B–O and O–O correlations, respectively, in the obtained total pair distribution functions shifted little with increasing MO content, while the asymmetry of the first peak increased significantly with MO content; these results are different from those for potassium borate glasses. It was inferred that the BO 3 and BO 4 units are better defined in these alkaline-earth borate glasses than those in the potassium borate glasses. Both the full-width at half maximum (FWHM) of the first peak and the average co-ordination number of O around B clearly increased as MO content increased which is the same behavior with alkali borate glasses. The fraction of four-co-ordinated B showed a larger deviation from x/(1− x) for CaO–B 2O 3 glasses than that for BaO–B 2O 3 glasses which shows a different dependence of the deviation on cation size from that in alkali borate glasses, and is in good agreement with the results from MD simulation.

Electronic structure of the paramagnetic boron oxygen hole center in B-dopedSiO2

We have studied the ground-state properties of boron-related dia- and paramagnetic point defects in B-doped silica. Hartree-Fock and density functional theory calculations have been performed to determine the structure, charge, and spin distribution of the boron oxygen hole center (BOHC). The currently accepted model of the BOHC is that of a hole localized on a nonbonding $2p$ orbital of an O atom in a bridge position between a B and a Si atom, $\ensuremath{\equiv}{\mathrm{B}\mathrm{---}\mathrm{O}}^{\mathbf{\ensuremath{\cdot}}}---\mathrm{Si}\ensuremath{\equiv}.$ Our calculations do not support this model and show that the structure is not stable and spontaneously evolves into a planar trigonal diamagnetic boron center, $g\mathrm{B}---,$ and a nonbridging oxygen, $\ensuremath{\equiv}{\mathrm{Si}\mathrm{---}\mathrm{O}}^{\mathbf{\ensuremath{\cdot}}}.$ The results of this study suggest that the BOHC consists of a three-coordinated B atom bound to a non bridging oxygen, $g\mathrm{B}---{\mathrm{O}}^{\mathbf{\ensuremath{\cdot}}}.$ The computed hyperfine coupling constants for this model are in quantitative agreement with those measured experimentally for B-doped silica. This assignment is consistent with recent magnetic resonance studies on borosilicates and alkali borate glasses.

Thermochemical studies of nitrides and oxynitrides by oxidative oxide melt calorimetry

High temperature oxidative solution calorimetry is a versatile technique for obtaining enthalpies of formation of nitrides and oxynitrides. In this method, the solid phase is dissolved in a molten oxide solvent (sodium molybdate or an alkali borate) while oxygen gas is bubbled through the melt. The nitrogen is evolved as N 2 and the final state is a dilute solution of dissolved oxides in the solvent. A thermochemical cycle may then be written to obtain the enthalpy of formation. The energetics of silicon nitride and of binary and ternary nitrides are discussed to illustrate the methodology.

Short- and medium-range structural order around cations in glasses: a multidisciplinary approach

The structural environment of cations at a short- and medium-range scale may be investigated either by spectroscopic methods or by radiation diffraction giving either a description of the geometry and symmetry of the cationic site, including the nature of the chemical bond, or a chemically resolved radial distribution function. Cations exhibit several original structural properties in oxide glasses. Short-range order is characterized by unusual coordination numbers, such as five-coordinated sites or tetrahedral sites which are in a network forming position, with the relative proportion of these sites depending on glass composition. Oxide glasses can also exhibit elements with unusual oxidation states, such as pentavalent uranium. The determination of the sites occupied by the elements in their different oxidation states allows to rationalize the chemical dependence of redox equilibria, which is the way to predict Fe behavior in magmatic silicate systems. Several experimental data lend support that cations are located in domains extending up to more than 8 Å radius, in which cationic polyhedra may be linked together either by edges or by corners. In low alkali borate glasses, transition elements such as Co, Ni, Zn exist in peculiar highly ordered domains corresponding to the presence of rigid borate units. Strong differences are observed between modifying and charge compensating cations, either concerning site geometry or medium-range organization. The use of numerical models for experimental data inversion allows to rationalize the structural behavior of the various glass components.

On the structure of trapped holes in borosilicates

Danburite (CaB2Si2O8) is a naturally occurring borosilicate mineral that is a crystalline analog of borosilicate glass. Using two-dimensional correlation electron spin echo envelope modulation spectroscopy, we determined the structure of the prevalent radiation-induced defect, boron oxygen hole center, in this mineral. The hole is trapped by nonbridging oxygen in a strongly distorted O3−B–O––BO3− linkage; the precursor of the hole center is a Ca2+ vacancy. These results and computer modeling suggest that in borosilicate glasses, the hole center is the same >B–O⋅ radical as in alkali borate glasses. We suggest that a >B–O− unit near a cation vacancy is the precursor of hole centers in both types of glasses.

A quantum-chemical study of the mechanism of ionic conductivity in alkali borate glasses

The incorporation of alkali metal oxide molecules into a continuous random network of vitreous boron oxide is considered within the cluster approximation at a semiempirical MNDO level. It is found that the oxygen atom of the Li2O molecule easily forms an additional bond with the threefold-coordinated boron atom with the formation of the BO4 tetrahedron. Different types of metal cation motion in alkali borate glasses are treated. It is revealed that the M+ cation in neutral systems is predominantly located near the edges of a BO 4 - tetrahedron on the outside of the B-O-B corner fragments and can readily change its position. The M+ cation in the neighborhood of the pentaborate grouping makes a complete turn without considerable expenditure of energy. The vibrations with a change in the rotation angle from 180° to 270° are possible in tetraborates, and, only in the neighborhood of triborate, one position is substantially more stable than the other positions. The cation can migrate from center to center along chains of a continuous glass network on the outside of the B-O-B corner fragments. In the pentaborate and tetraborate centers, the cation can migrate into another chain and change its direction of motion.

A study of selected physical properties of alkali germanate glasses over wide ranges of composition

The compositional ranges for various alkali germanate glasses have been explored, with some reaching as high as J=3 (or 75 molar percent alkali oxide), where J is the molar ratio of alkali oxide to germanium oxide. The T g (glass transition temperature) and carbon dioxide retention were found for each sample, while the density was found for the more stable samples. These data were then compared to the previous work that had been done on the alkali silicate and alkali borate families by this lab. It was found that the T g trend in the alkali germanates was similar to that found in the alkali borates due to a change in coordination in each system. Also, the density and T g could be related and explained with an understanding of the coordination of the units present in the glasses. Volumes of the structural units were obtained in the alkali germanate systems and compared with the alkali borates and silicates. In addition, packing fractions of the units were obtained in all three systems. It was found that the silicate units did not have an anomonously large packing fraction of any of its structural units at low alkali concentrations, whereas the germanate octahedral units (N 6) and the borate tetrahedral units (f 2) did. At high alkali contents the packing fractions were similar in all systems indicating a common effect due to both the formation of non-bridging oxygens and the alkali ions filling space in a random manner.

Cationic ordering in oxide glasses: the example of transition elements

AbstractStructural data have been obtained on the cation surroundings in multi-component silicate and borosilicate glasses using chemically selective spectroscopic and scattering methods, such as extended X-ray absorption and neutron scattering with isotope substitution (NSIS). Transition elements such as Ni or Ti may occur in unusual 5-coordinated sites which coexist with other coordination numbers, depending on glass composition. Distribution of cationic sites in the glassy structure is responsible for unusual spectroscopic properties, as shown by Fe2+ Mössbauer spectroscopy. The environment of cations such as Zn, Zr or Mo, has been determined by EXAFS and discussed using the bond valence theory, which predicts the way to charge compensate the oxygen neighbours and which indicates the linkage of cationic sites with the silicate framework. Cation-cation correlations are given by NSIS up to ∼8 Á, indicating an extensive Medium Range Ordering (MRO) with corner- and edge-linked cationic polyhedra, for Ti and Ni-bearing glasses, respectively. This heterogeneous cationic distribution in glasses is consistent with the presence of two-dimensional domains in which cation mixing may occur, as shown in a Ca-Ni metasilicate glass. Three-dimensional domains have also been found by Ni-K edge EXAFS in the case of low alkali borate glasses, with a local structure which mimics some aspects of crystalline NiO. The presence of ordered cationic domains, clearly illustrated by Reverse Monte Carlo simulations helps to rationalize the physical properties of multi-component silicate glasses.

Static 11B NMR studies of the short range order in alkali metal modified B 2S 3 glasses

The 11B NMR spectra of xRb 2S+(1− x)B 2S 3 glasses in the range 0⩽ x⩽0.75 and of xCs 2S+(1− x)B 2S 3 glasses in the range 0⩽ x⩽0.60 are reported. The addition of Rb 2S to B 2S 3 creates on average approximately two and one-half tetrahedral borons for each added sulfur ion, whereas it is found that the addition of Cs 2S creates approximately 2 tetrahedral borons for each added sulfur ion. This behavior while more similar to that seen in the alkali borate glasses, contrasts that seen in the Na and K thioborate glasses, where six to eight and three, respectively, tetrahedral borons are formed for every sulfide anion added to the glass. These findings are supported by the IR and 11B NMR spectra of the di-thioborate polycrystals (c-Rb 2S:2B 2S 3 and c-Cs 2S:2B 2S 3) whose structures appear to be comprised of two BS 4 tetrahedrals and two BS 3 trigonals (N 4∼0.5) like that in the alkali di-borate phases for both Rb and Cs. Unlike the 11B NMR resonances of the sodium thioborate glasses where a single sharp line is observed for the tetrahedral boron site and a single quadrupolar broadened line is observed for all the trigonal sites, a third resonance line is observed at high alkali fractions for the rubidium and cesium thioborate glasses. This new structural feature may arise from asymmetric MBS 2 (meta-thioborate groups) or tetrahedral boron groups possessing a non-bridging sulfur.

Magnetic resonance studies on radiation-induced point defects in mixed oxide glasses. I. Spin centers in B 2O 3 and alkali borate glasses

Radiation-induced spin centers in vitreous boron trioxide and alkali borate glasses were studied using pulsed electron paramagnetic resonance (EPR). It is shown that electrons and holes in these glasses are trapped on valence alternation defects, undercoordinated oxygen (holes) and overcoordinated oxygen (electrons). The local environment around these defects has major effect on spin parameters of the corresponding spin centers. The electronic and atomic structure of spin-1/2 centers and their diamagnetic precursors is analyzed using semiempirical and ab initio calculations.

Structure of M 2O–B 2O 3 (M: Na and K) glasses and melts by neutron diffraction

The structures of alkali borate glasses, M 2O–B 2O 3, and melts with 10 and 30 mol% M 2O (M=Na and K) have been studied by pulsed-neutron total scattering measurements. With an increase of M 2O content, the first peak of the experimental radial distribution function corresponding to the nearest neighbor B–O correlation spreads asymmetrically to the higher- r side, and the coordination number of O atoms around a reference B atom, N B–O, increases from 3.0 to 3.4∼3.5, reflecting a structural change from BO 3 triangle units to BO 4 tetrahedral units. It can be concluded from the neutron results that some of the BO 3 units present are converted to BO 4 units during the addition of up to 30 mol% M 2O to the B 2O 3 and that they melt in a way similar to M 2O–B 2O 3 glasses. XAFS studies in potassium borate glasses have made important contributions to our understanding of the network-modifying structure of glass. The local structure in K 2O–B 2O 3 glasses has been determined in order to elucidate the environment of the network modifying K + ions. The average coordination number, N K–O, of oxygen around K + cations and the mean distance, r K–O, are found to be approximately 6 and 2.83–2.86 Å, respectively.

Temperature induced structural changes and transport mechanisms in borate, borosilicate and boroaluminate liquids: high-resolution and high-temperature NMR results

Temperature dependent structural rearrangements in glass-forming liquids and associated changes in the configurational entropy have been hypothesized to be closely related to the transport processes such as viscous flow. The nature of such structural rearrangements is, however, not well understood. In this study the temperature dependence of the short-range structural changes involving borate, silicate and aluminate speciation reactions in sodium borate, sodium borosilicate and lithium boroaluminate liquids have been investigated with high-resolution and high-temperature (up to 1793 K) 11B, 29Si and 27Al nuclear magnetic resonance spectroscopy. It is shown that such structural changes are the principal source of configurational entropy in binary alkali borate liquids. However, silicate speciation in the borosilicate and borate speciation in the boroaluminate liquids are found to be insensitive to temperature changes. It is speculated that in these liquids the temperature induced changes in the intermediate-range structural order are likely to be an important source of configurational entropy. The inter-connection between temperature dependent speciation reactions and atomic scale mechanisms for viscous flow in these liquids is also established.

Behavior of the packing densities of alkali germanate glasses

Packing densities are more suitable than the mass densities for structural studies. A comparison of the corresponding data on alkali germanate glasses with those of alkali borate glasses corroborates the suggestion of the formation of GeO 5 and/or GeO 6 units in systems (Me 2O) x (GeO 2) 1− x in that range of composition where the maximum packing density occurs. The packing densities of both the alkali germanate and borate glasses strongly obey a common behavior while the data of the related crystals are widely spread. Simple structural rules like those used for the borate glasses are suggested for an explanation of the germanate anomaly. However, a specific characteristic of some germanate crystals is the occurrence of three-fold coordinated O sites accompanied by clusters of GeO 6 units. There is no evidence for a significant role of such clusters in the glass structures though minor indications for such features were detected by Raman spectroscopy.

Silicon nitride: Enthalpy of formation of the α- and β-polymorphs and the effect of C and O impurities

High-temperature oxidative drop solution calorimetry was used to measure the enthalpy of formation of α− and β−Si3N4. Two different solvents, molten alkali borate (48 wt% LiBO2 · 52 wt% NaBO2) at 1043 and 1073 K and potassium vanadate (K2O · 3V2O5) at 973 K, were used, giving the same results. Pure α− and β−Si3N4 polymorphs have the same molar enthalpy of formation at 298 K of −850.9 ± 22.4 and −852.0 ± 8.7 kJ/mol, respectively. The unit cell dimensions of impure α−Si3N4 samples depend linearly on the O and C impurity contents, and so does the molar enthalpy of formation. The energetic stability of the α−Si3N4phase decreases when the sample contains O and C impurities. The experimental evidence strongly suggests that the impurities dissolve into the α−Si3N4 structure to form a (limited) isostructural solid solution series but that this solid solution series is energetically less stable than a mechanical mixture of pure (α or β) Si3N4, SiO2, and SiC. Thus, the α-phase is not stabilized by impurities and is probably always metastable.

Free volumes in several ion-conducting glasses

Studies of the packing of atoms in glass have usually been made by assuming that the atoms are spherical. Since spheres contain large spaces into which no nearby atoms or ions can penetrate, cubes rather than spheres are used in this Letter for representing atoms and the free volume is defined as a volume in which no glass constituents exist. It is concluded that alkali ions in alkali borate or silicate glasses occupy the free volume which the glass network-former provides.

Enthalpy of Formation of Rare-earth Silicates Y2SiO5 and Yb2SiO5 and N-containing Silicate Y10(SiO4)6N2

The enthalpies of formation of two rare-earth silicates (Y2SiO5 and Yb2SiO5) and a N-containing rare-earth silicate Y10(SiO4)6N2 have been determined using high-temperature drop solution calorimetry. Alkali borate (52 wt% LiBO2·48 wt% NaBO2) solvent was used at 800 °C, and oxygen gas was bubbled through the melt. The nitrogen-containing silicate was oxidized during dissolution. The standard enthalpies of formation are for Y2SiO5, Yb2SiO5, and Y10(SiO4)6N2, respectively, –22868.54 ± 5.34, –22774.75 ± 8.21, and –14145.20 ± 16.48 kJ/mol from elements, and –52.53 ± 4.83, –49.45 6 ± 8.35, and –94.53 ± 11.66 kJ/mol from oxides (Y2O3 or Yb2O3, SiO2) and nitride (Si3N4). The silicates and N-containing silicate are energetically stable with respect to binary oxides and Si3N4, but the N-containing silicate may be metastable with respect to assemblages containing Y2SiO5, Si3N4, and SiO2. A linear relationship was found between the enthalpy of formation of a series of M2SiO5 silicates from binary oxides and the ionic potential (z/r) of the metal cation.

Cation Structure in Binary Potassium Borate Glasses

XAFS studies of network modifying cations in alkali borate glasses have made an important contribution to our understanding of the structure of glass. Using the synchrotron XAFS facility, we have performed to determine the local structure in K2O-B2O3 glasses in order to elucidate the environment of B-O network modifying K+ ions. The average coordination number NB-O of oxygen around K+ cations and the mean distance rB-O for K-O correlations are found to be approximately 6 and 2.83∼2.86Å, respectively. The XAFS results are in good agreement with our X-ray diffraction and MD simulation results.