Diborate Groups Research Articles

Spectroscopic investigation of the influence of uranium ions on the structure of borate glasses

The (100 − x)Na 2B 4O 7 · xUO 3 borate glass system with x ⩽ 10 was investigated using infrared Raman and optical spectroscopy techniques. Changes of the spectroscopic features as a function of the UO 3 content indicate that uranium ions act as network modifiers in the glasses studied. Increasing UO 3 content determines the conversion of diborate structural groups into orthoborate structural groups. The UO 3 content of glasses also modifies the redox equilibrium between different valence states of the uranium ions.

Raman scattering study of lithium borate glasses

Raman spectra of lithium borate glasses in the system xLi 2O. (1− x)B 2O 3 for various values of x (0.10 ⩽ x ⩽ 0.50) in the temperature range 25–400°C are reported. It has been observed that with increasing x, the progressive formation of pentaborate, tetraborate, diborate, metaborate, pyroborate and orthoborate groups takes place along with the destruction of some of these units. Addition of Li 2O to v-B 2O 3 gives rise to a marked decrease in the value of the structural correlation range down to ~4.20 Å at the x = 0.50 concentration of Li 2O. Thus short-range ordering in xLi 2O.(1 − x)B 2O 3 glasses does not exceed the dimension of one six-membered boroxol ring. The intensity and position of the 130 cm −1 band is not influenced by the addition of lithium oxide. This band probably arises due to the librational modes of clusters of B and O atoms. Increasing the temperature of the glass samples results in an increasing disorder and the consequent thermal expansion of the glass. The increase of the disorder gives rise to an increase in the Raman bandwidth, whereas the thermal expansion of the glass is responsible for the decrease of the frequencies.

Thermodynamic properties of borate glasses by molecular dynamics simulations

The molecular dynamics method was used to examine the B2O3 and borate glasses. The discontinuity in each of the thermodynamic factors such as the transition temperature, specific heat, thermal expansion and thermal compressibility is detected and checked by the so-called Prigogine-Defay ratio and it is found that this ratio exceeds unity for the systems studied. This behaviour indicates that multiple order parameters are required to characterize the glassy state. Also it is found that the formation of diborate groups in borate glasses reduces the value of the Prigogine-Defay ratio Pi , while the formation of groups with non-bridging oxygen ions increases the value of Pi , i.e. increases the number of order parameters.

Model for the vibrational spectra of B2O3-xLi2O.

A nested coherent-potential approximation in the Bethe lattice is used to model the vibrational spectra in the vitreous compound ${\mathrm{B}}_{2}$${\mathrm{O}}_{3}$-x${\mathrm{Li}}_{2}$O. A local-polarizability model is applied to calculate the Raman and infrared activity of the modes, and the results are compared with experimental data. The agreement is not complete due to the presence of rather complicated local configurations in the glass, as boroxol rings, diborate and tetraborate groups, not taken into account in the theory. However, this calculation is useful to identify the main features of the experimental spectra, and to corroborate the hypothesis of structural changes that occur when doping the pure boron oxide with an alkali-metal oxide.

Phonon sideband of Eu 3+ in sodium borate glasses

The phonon sideband (PSB) of Eu 3+ associated with the 7F 0- 5D 2 transition was investigated for the B 2O 3Na 2O glasses doped with 1 mol% Eu 2O 3. It was found that the coupling of the phonon mode with Eu 3+ ions in the glasses was through B-O − or B-O stretching vibration of BO 3 units and vibrations of BO 4 − units in tetra- and diborate groups. The results showed that B-O − bonds present around Eu 3+ ions, which have the highest phonon energy, contributed to the nonradiative decay by multiphonon relaxation of Eu 3+ ions even in low-alkali borate glasses. Moreover, the electron-phonon coupling strength of each borate group was found to be influenced by the fraction present and by the site selectivity of Eu 3+ ions for the group.

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.

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.

ChemInform Abstract: Molecular Dynamics Simulation of the Structure of Borate Glasses.

The structures of borate glasses have been simulated by use of molecular dynamics. A new type of potential calculated by means of the intermediate neglect of differential overlap (INDO) method and a three-body potential were introduced into the molecular dynamics simulation. Boroxyl rings and diborate groups have been successfully reproduced in the structure models obtained from the present molecular dynamics simulation. Calculated radial distribution functions agreed well with the observed one obtained from X-ray diffraction data. The distribution of O–B–O and B–O–B bond angles is also given.

Molecular Dynamics Simulation of the Structure of Borate Glasses

The structures of borate glasses have been simulated by use of molecular dynamics. A new type of potential calculated by means of the intermediate neglect of differential overlap (INDO) method and a three‐body potential were introduced into the molecular dynamics simulation. Boroxyl rings and diborate groups have been successfully reproduced in the structure models obtained from the present molecular dynamics simulation. Calculated radial distribution functions agreed well with the observed one obtained from X‐ray diffraction data. The distribution of O–B–O and B–O–B bond angles is also given.

X-ray evidence for the existence of diborate groups in sodium-borate glass

According to Krogh-Moe's conceptions the structural units of borate glasses are not BO3 triangles and BO4 tetrahedrons but rather groups of a more complex arrangement. The network of sodium-borate glass with 33 mol% Na2O is supposed to consist exclusively of diborate groups. Since pyroborax, a non-crystalline material prepared from sodium-tetraborate-decahydrate by dehydration, essentially consists of diborate groups, it can be regarded as a model for the short-range order of this glass. Accurate measurements of the scattering curves of pyroborax, of a glass melted from borax at 800 °C. and of normally prepared sodium-borate glasses lend new support to Krogh-Moe's group model of borate glass structure.

Infrared spectroscopic study of mixed-alkali effect in borate glasses

Infrared spectra of mixed-alkali diborate glasses, (Li, Na) 20.2B 2O 3 and (Li, K) 20.2B 2O 3, have been investigated. BO stretching and BOB bending frequencies exhibit nonlinear shifts which can be described as a mild mixed-alkali effect. Both shifts and nonlinear variations of frequencies may be explained on the basis of alteration of the structure of the diborate groups in the presence of Li + ions.

Micro-heterogeneities in R 2OROB 2O 3 glasses

ESR of incorporated Cu 2+ was used in the detection of micro-inhomogeneity in xR 2O · yMgO · (100 − x − y) B 2O 3 glasses. The inhomogeneous region determined by ESR was found to be far wider than that obtained by opalescence. It was concluded that in the glasses of x+y≲15, Mg 2+ tends to accelerate the simultaneous formation of boroxol and diborate groups and in the glasses of x+y≳15, boroxol group and pyrobate ion. The conclusion was confirmed by laser Raman scattering. Molecular volume was measured over the whole glass-forming region to explore the correlation between the micro-structure and a macroscopic property of the glasses. It showed no marked change at the specified composition where micro-structure changed.

Electronic structure and properties of oxide glasses (I) π-Electron distribution on alkali borate glass networks

Basicities of an oxygen in alkali borate glasses were estimated by means of SCF INDO LCAO-MO calculation for molecular models of various borate groups. The non-bonding levels of an oxygen with higher orbital energies were classified according to symmetry properties of the levels. One was the local π type (out-of-plane) molecular orbital primarily populating on a particular planar structure. The other was an in-plane lone pair. The degree of delocalization of the out-of-plane non-bonding level (π character) decreased in the composition region of 15–20 mol% alkali oxide, while in-plane basicity was nearly constant over the composition of 0–35 mol% alkali oxide. The π-type basicity of non-bridging oxygens in several kinds of borate ions was nearly equal to or smaller than those of bridging oxygens in tetraborate and diborate groups. Borate glasses were found to be inorganic π-electron systems, whose properties are largely controlled by the distribution of π electrons on their networks.

Structure and Properties of Silver Borate Glasses

Clear glasses form in the system Ag2O-B2O3 up to about 35 mol% (65 wt%) Ag2O. Infrared absorption, thermal expansion, and density data indicated an analogy to the Na2O-B2O3 system. Pentaborate-triborate group pairs appear to be formed on addition of Ag2O to B2O3 up to 20 mol% Ag2O and diborate groups from 20 to 33 mol% Ag2O. This interpretation is supported by the comparison of the infrared absorption spectra of quenched and crystallized glasses. One crystallization product, Ag2O-4B2O3, was identified previously. A new compound starts to appear at 28 mol% Ag2O. The theory that silver is generally present as a network modifier like sodium was substantiated by the comparison of the molar volume of sodium and silver borate glasses. Above 27 mol% Ag2O some atomic silver is assumed to be present; below 15 mol%, exploratory studies indicate a two-phase structure within an immiscibility gap. A low-temperature internal friction peak in the glasses up to 28 mol% Ag2O corresponds to the alkali peak in other glasses; a high temperature peak appearing in the 34 mol% Ag2O glass is associated with the appearance of nonbridging oxygen in the system.