Boroxol Rings Research Articles

Infrared absorption spectra of evaporated V2O5 and co-evaporated V2O5/B2O3 thin films

The Fourier transform infrared spectra of different compositions of evaporated V2O5/B2O3 thin films have been investigated. Most of the absorption bands corresponding to V2O5 and B2O3 films coincide with those reported by other authors. The short-range order in amorphous V2O5 films is found to be conserved. The absorption spectra indicate a boroxol ring structure for B2O3 films. In co-evaporated V2O5/B2O3 films the boron is observed to substitute in the V2O5 network such that the coordination number of vanadium ion remains unchanged. The presence of a number of bands corresponding to -OH groups indicates the hygroscopic character of the films.

Optical absorption spectra of evaporated V2O5 and co-evaporated V2O5/B2O3 thin films

The Fourier transform infrared spectra of different compositions of evaporated V2O5/B2O3 thin films have been investigated. Most of the absorption bands corresponding to V2O5 and B2O3 films coincide with those reported by other authors. The short-range order in amorphous V2O5 films is found to be conserved. The absorption spectra indicate a boroxol ring structure for B2O3 films. In co-evaporated V2O5/B2O3 films the boron is observed to substitute in the V2O5 network such that the coordination number of vanadium ion remains unchanged. The presence of a number of bands corresponding to -OH groups indicates the hygroscopic character of the films.

Raman study of lead borate glasses

Glasses in the system xPbO: (100 − x)B 2O 3 (22 ≦ x ≦ 85 mol%) have been studied by Raman spectroscopy with a view to understand the effect of PbO on the borate network. The conversion of three-fold to four-fold coordinated boron takes place primarily by incorporation of BO 4 into boroxol rings in the form of pentaborate and diborate units. For high lead content back conversion of four to three coordination of borons occurs and the network consists primarily of pyroborate, orthoborate and metaborate groups.

Nuclear quadrupole resonance of boron in borate glasses

A continuous wave nuclear quadrupole resonance spectrometer that has a high sensitivity even at low frequencies has been built. Boron and aluminum NQR has been detected in the region 200 kHz to 1.4 MHz. For the first time, boron NQR has been detected in a glass. The NQR spectrum of pure B 20 3 glass is consistent with 85 ± 2% of the boron atoms belonging to boroxol rings. In sodium borate glasses, the number of borons in boroxol rings decreases with increasing sodium content, until when sodium oxide comprises 20 mol% of the glass less than 2% of the borons are in boroxol rings.

Pure nuclear quadrupole resonance spectroscopy studies of the structure of glasses

Pure nuclear quadrupole resonance (NQR) studies of 10B and 11B in borate glasses have been accomplished. The data yield values of the quadrupolar parameters (the coupling constant Q cc and the asymmetry parameter ν) that are an order of magnitude more accurate than those obtained from analysis of quadrupolar effects in nuclear magnetic resonance (NMR) spectra. The linewidth of the 11B spectrum (at 1.35 MHz) for crystalline B 2O 3 is 9.6 ± 0.1 kHz, and the value for vitreous B 2 is only 22.3 ± 0.1 kHz. Two 11B responses separated by about 50 kHz are found for vitreous B 2O 3. They arise from borons in boroxol rings (∼ 85%) and chains of BO 3 triangles (∼ 15%). The data are obtained at 77 K with an extremely sensitive regenerative oscillator of the Robinson type.

Calculation of NMR shieldings and other properties for three and five coordinate Si, three coordinate O and some siloxane and boroxol ring compounds

Valuable information on the geometric and electronic structure of both predominant and defect species in the bulk and on the surface of amorphous SiO 2 can be obtained from solid state 29Si and 17O NMR. Most previous NMR.studies have made assignments of structure types to NMR signals by relying on a comparison to crystalline reference materials. Using the quantum mechanical technique of Coupled Hartree-Fock Perturbation Theory (CHFPT) we can directly calculate NMR shielding tensors, σ, for Si and O and electric field gradients tensors, q for O in silicates. Previous CHFPT calculations on (SiH 3) 2O at 〈 Si O Si = 180° to 140° semiquantitatively reproduced observed trends in σ for SiO 2 polymorphs. These calculations have been improved and extended to 〈SiOSi = 120° in (SiH 3) 2O and to the small siloxane rings H 4Si 2O 2, H 6Si 3O 3, H 8,Si 4O 4 which are models for the ring structures which may occur in the bulk and surface of amorphous SiO 2. Thus, predictions can be made of the 17O and 29Si NMR characteristic of such species. We have also calculated NMR shieldings and other properties for three and five coordinate Si and for three coordinate O. The 29Si NMR shielding increases with coordination, as expected, and five coordinate Si shows an anisotropy which although large is still smaller than that inferred from spin-lattice relaxation times in silicate melts. O q values are also calculated for (BH 2) 2O and H 3B 3O 6. The results are consistent with O EFGs and Raman frequencies observed in B 2O 3 glass, supporting a boroxol ring model.

Calculation of the structure, vibrational spectra, and polarizability of boroxine, H3B3O3, a model for boroxol rings in vitreous B2O3

Boroxine, H{sub 3}B{sub 3}O{sub 3}, is a stable but reactive gas-phase molecule which is also a model for the boroxol, B{sub 3}O{sub 6}{sup 3{minus}}, rings which occur in vitreous B{sub 2}O{sub 3}. We have optimized the geometry of H{sub 3}B{sub 3}O{sub 3} at the RHF/3-21G* level and have calculated its vibrational energies and IR intensities. We have also calculated the polarizability of H{sub 3}B{sub 3}O{sub 3} and its polarizability derivative for the Raman-active A{sub 1}{prime} vibrational mode involving symmetric breathing of the oxygens. Our results for H{sub 3}B{sub 3}O{sub 3} are consistent with a boroxol ring model for v-B{sub 2}O{sub 3}.

Structure and Ionic Conductivity of Lithio-Borate Thin Films

AbstractThin films of vitreous solid electrolyte in the boron-oxide/lithium oxide system have been grown using a thermal evaporation technique. Solid electrolyte films of composition B2O3-xLi2O with 0.5≤x≤5 are obtained from a mixture of lithium metaborate and lithium oxide in good proportion. Structure and ionic conduction are studied as a function of the glass modifier concentration and process conditions. The structure is investigated by the mid-infrared absorption spectroscopyof films deposited on a silicon wafer. Comparison with bulk materials whose structures are known shows that the films have a similar structure, and exhibit the transformation of boroxol rings into triborate or di-triborate units as the Li2O concentration increases. The ionic conductivity has been studied over wide frequency and temperatureranges using the complex impedance spectroscopy in a sandwiched geometry. The ionic conductivity increases with increasing Li content and exhibits a maximum value for B2O3-3Li2O.

Structure of alkali borate glasses based on the chemical equilibrium concept

The structure of alkali borate glass has been simulated based on a chemical equilibrium concept among possible structural units. The structural units assumed include boroxol ring, diborate, triborate, di-triborate, metaborate, pyroborate, orthoborate units, “loose” BO 3 triangles, and “loose” BO 4 tetrahedra. As a first step, only one equation was used, and then the number of equations was increased so as to be able to obtain a best fit to the experimental data. It was found that equilibrium constants were almost unanimously determined by a trial and error technique. Assuming ten equilibrium equations and equilibrium constants, it was shown that the variation of coordination number of boron atoms and that of bridging and non-bridging oxygens in the glass as a function of alkali concentration can be reproduced in good agreement with the NMR data and Raman spectra.

Inelastic deformation and structure of borate glasses

The work of fracture for soda borate glasses was measured in kerosene using short-bar specimens with chevron notch. In unloading-loading cycles during stable crack growth, an inelastic phenomenon was observed on the diagram of load versus load-point displacement for B 2O 3-rich glass. From these results, it is suggested that the inelastic behavior is caused by an increase of the plastic deformation with increasing the boroxol rings.

Infrared studies of the structure of borate glasses

Infrared spectroscopy investigations of binary glasses B 2 O 3 xLi 2 O with 0< x1 have been performed in order to understand the glass-modifying properties of Li 2O. The mid-reflectivity spectra show clearly the disappearance of boroxol rings and the formation, in a first step, of tetraborate groups and later of diborate groups as the oxide content increases. For high values of ξ, borate groups with non-bridging oxygen atoms are formed. One band at about 390 cm −1 is attributed to vabration of the lithium ions. Modifications of the boron-oxygen network with the addition of lithium sulphate as a doping salt have also been investigated in the ternary system B 2 O 3 xLi 2 OyLi 2 Si 4. At the same time, by far-infrared absorption on the binary glass, the vibrational motion of the lithium cations has been confirmed.

Vibrations of boron oxide glass

Vibrations of vitreous B 2O 3 are investigated theoretically. Boron oxide glass is considered to be an infinite network of planar boroxol rings B 3O 3 interconnected via oxygen bridges. The Bethe approximation is used to handle this infinite system. We also calculate vibrational spectra of an isolated boroxol molecule. We shall be particularly interested in evolution of molecular (localized) modes into extended (delocalized) modes when an infinite network is formed, while certain modes remain localized, in particular the sharp peak at 800 cm −1.

IR investigation of amorphous BaO-Fe 2O 3-B 2O 3 glasses prepared using 1 mol% TiO 2

An infrared (IR) investigation of 35BaO-25Fe 2O 3-40B 2O 3glasses prepared by rapidly quenching the melts containing 1 mol% of TiO 2 at room temperature (25°C) is reported. The system is composed of BO 3 and BO 4 groups in the form of boroxol ring with tetraborate and diborate units forming the network structure. The iron oxide (Fe 3+) possesses the positions in the network to occupying tetrahedral coordination sites. The TiO 2 (Ti 4+) added by 1 mol% in the system probably maintain a tetrahedral (TiO 4) structure in the quenched glass, and thus induces growth of FeO 4 and BO 4 structural groups in the network. The network with latter groupings enhances the thermal stability as long as the sample does not crystallise significantly by given thermal treatments at temperatures upto 1000°C. A correlation with IR data has been made to account for the factors inhibiting the crystallisation of a system on addition of TiO 2.

Raman and Infra-red Spectroscopy and Some Physical Properties of the Glasses in the MgO-Ga&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;-B&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; System

Glass-forming regions were determined in the MgO-Ga2O3-B2O3 system by melt quenching method. Phase separation occured for B2O3 rich compositions. Raman spectroscopy and IR spectroscopy measurements revealed that boroxol ring groups exist in the B2O3 rich compositions and that diborate groups of 4-fold coordinated boron exist at MgO rich but Ga2O3 poor compositions. Pyroborate or orthoborate groups of 3-fold coordinated boron were found at MgO rich compositions in the ternary system. With the increase in Ga2O3 content the concentration of diborate group decreased and GaO4 tetrahedra were formed together with di-tiborate and metaborate ring groups. Hence, MgO-Ga2O3-B2O3 system as well as GaO-Ga2O3-B2O3 system forms a continuous network of 6-membered rings consisting of GaO4-tetrahedra. A possibility that Ga+3 ion might partially be incorporated in these groups was pointed out from the shifts of the Raman scattering peaks associated with Pyroborate or orthoborate groups. The smooth variation in density and refractive index of the glass with composition indicates that the structural change of glass with composition is continuous.

Phonon spectra of vitreous B2O3

Inelastic neutron scattering measurements from vitreous B2O3 indicate that the Raman matrix elements for the boroxol ring breathing mode are extremely large. Despite this, calculations show that the experimental data are consistent with a model containing a high concentration of boroxol rings.

A constrained molecular dynamics study of B2O3glass

The results of a MD simulation study, employing constraints, of v-B2O3 are presented. Half the number of B-O-B and O-B-O angles are constrained at 120 degrees during the simulation. The simulated glass structure consists of a network of planar BO3 triangles. No boroxol rings are formed, as adjacent BO3 triangles have a preference to be not in the same plane.

New insights into the structure of B2O3 glass

The results of structural investigations of various types of B2O3 glass with different thermal histories are presented. The density and the glass transition temperature Tg depend on the cooling rate at which the glass is quenched. For quickly quenched glass (cooling rate 103 K/s) the density is 1.79 g/cm3 and Tg is 278°C whereas these values for slowly quenched glass (cooling rate 4 × 10−3 K/s) are respectively 1.83 g/cm3 and 298° C. X-ray diffraction and Molecular Dynamics simulations show that the structure of vitreous B2O3 can be explained by a model of randomly connected BO3 triangles. The slower the glass is quenched, the more the BOB angles of adjacent BO3 units approach 120°. We have found no tendency of neighbouring BO3 triangles to form equiplanar complexes if the cooling rate decreases and therefore we have found no evidence for the existence of boroxol rings. The combination of the results of density measurements and X-ray diffraction studies indicates that for the slowly quenched glass the oxygen tend to be out of the plane of adjacent BO3 triangles. This results in a more densely packed glass structure for slowly quenched B2O3 glass.

A Structural Model for B2O3Glass –An Analysis of Neutron and X-Ray Data by a Liquid Model–

Several liquid models considered to have a close connection with the network structure in B 2 O 3 glass were proposed and the neutron and X-ray structure factors for the models were calculated by using the general procedure of analysis of structure factors for molecular liquids. From the agreement between the calculated values and the experimental data it was concluded that the structure of the glass was understood as that of the liquid composed of boroxol rings as `molecules', where each boroxol ring is regarded as a rigid sphere in the hard sphere fluid: the effective hard sphere diameter is 4.18 Å and the packing fraction becomes 0.40. Intermediate range structure between two boroxol rings was also determined.

Ab initio calculation of B and O nuclear quadrupole coupling constants and NMR shielding constants in molecular models for B 2O 3 glass

Ab initio Hartree-Fock-Roothaan calculations with large polarized basis sets yield 17O nuclear quadrupole coupling constants, e 2 q O Q O/ h, in planar H 2BOBH 2, a molecular model for bridging oxygens in B 2O 3, which vary strongly with angle. The calculated e 2 q O Q O/ h values of 4.8 and 6.1 MHz for ∠BOB of 120° and 132°, respectively, are in good agreement with experimental values of 4.7 and 5.8 MHz for the two inequivalent oxygens in vitreous B 2O 3, for which ∠BOB of 120° (boroxol rings) and 128–132° (BO 3 triangles linked to boroxol rings) have been obtained by X-ray and neutron diffraction. The calculated 17O NMR shielding constant, σ, changes from −91 ppm for H 2BO − to 87 ppm for H 2BOBH 2 (BOB = 132°) and increases with ∠BOB at a rate of about 1 ppm/deg. Less accurate smaller basis set calculations on H 3B 3O 6 rings and on the H 2BOBH 2 molecule in lower symmetries indicate that the 17O q values are not changed significantly by ring closure but are affected by nonzero dihedral angles between the BH 2 planes. 17O and central T atom values of σ and q are also calculated for the series BO 3 3−, CO 3 2− and NO 3 − and compared witt experiment. The O in NO 3 − is calculated for the deshielded by 325 ppm with respect to H 2O(g), consistent with an experimental chemical shift of about 384. The calculated anisotropy in σ T increases along the BO 3 3− −NO 3 − series, consistent with experiment. These results indicate that accurate calculations on simple molecular models can reproduce the trends in q and σ observed in borate glasses and other solids.

TEMPERATURE-DEPENDENT 11B NMR STUDIES OF B2O3 GLASS

The vitreous and liquid phase of B2O3 are investigated by the temperature-dependent 11B CW-NMR technique. The results show that there are two processes in the structure changes of B2O3 glass with temperature increasing. Above 430℃, an increasing number of bonds begin to snap off and the number of boroxol rings is reduced accordingly. Above 600℃, a chain-like structure is formed, which is composed of B2O3 bipyramids. Above 930℃, this chain-like structure is broken to give B2O3 molecules.