Low Alkali Borate Glasses Research Articles

Role of alkali field strength on the speciation of Ni2+ in alkali borate glasses: comparison with crystalline Ni-borates

This work presents a comparative spectroscopic study of Ni speciation in binary alkali borate glasses, with up to 35 mol% alkali oxides, using optical absorption spectroscopy and Ni K-edge X-ray Absorption Near Edge Structure (XANES) and taking advantage of comparison with previous structural information obtained with Ni K-edge Extended X-ray Absorption Spectroscopy (EXAFS) on the same samples. Nickel-bearing K, Na or Li borate glasses exhibit an exceptional diversity of coloration (from green to yellow, brown, and purple). This is due to a progressive coordination change of Ni2+ with increasing alkali content, from 6-, to 5- and 4-Ni coordination, each coordination state having a peculiar structural significance. [6]Ni2+ is predominant in low alkali borate glasses, where Ni octahedra cluster in ordered Ni-rich domains. [4]Ni2+ is formed at low alkali cation field strength: it predominates in high potassic glasses but it is only a minor coordination state in Na-borate glasses and it is absent in Li-borate glasses, in which, by contrast, [5]Ni2+ prevails. For these three coordination states, the comparison with the structure of crystalline borates suggests a diversity of linkages between Ni2+ and the borate polymeric network. Among the alkali borate glasses investigated, each Ni-coordination state shows similar spectroscopic properties. This implies that the modification of Ni speciation with glass composition arises only from a variation of the relative contribution of 6-, 5- and 4-coordinated Ni2+ without significant modification of the spectroscopic parameters characteristic of each site. The coordination number of Ni2+ is correlated with the field strength of the modifier cations present in the glass: higher the field strength of the modifier cations, higher the nickel coordination number. High-K borate glasses favor the formation of [4]Ni2+, as this coordination state is not observed in Li- and is only minority in Na borate glasses, because of Li and Na preference for charge compensating [4]B instead of [4]Ni2+. Due to the similarities between Ni2+ and Mg2+, the diversity of the structural scenarios around Ni2+ in alkali borate glasses may shed light on the structural behavior of Mg in the same glasses, providing additional evidence of the control of Mg coordination by the field strength of the alkalis.

Viscosity properties of sodium borophosphate glasses

The viscosity behavior of (1 − x)NaPO 3− xNa 2B 4O 7 glasses ( x = 0.05–0.20) have been measured as a function of temperature using beam-bending and parallel-plate viscometry. The viscosity was found to shift to higher temperatures with increasing sodium borate content. The kinetic fragility parameter, m, estimated from the viscosity curve, decreases from 52 to 33 when x increases from 0.05 to 0.20 indicating that the glass network transforms from fragile to strong with the addition of Na 2B 4O 7. The decrease in fragility with increasing x is due to the progressive depolymerization of the phosphate network by the preferred four-coordinated boron atoms present in the low alkali borate glasses. As confirmed by Raman spectroscopy increasing alkali borate leads to enhanced B–O–P linkages realized with the accompanying transition from solely four-coordinated boron (in BO 4 units) to mixed BO 4/BO 3 structures. The glass viscosity characteristics of the investigated glasses were compared to those of P-SF67 and N-FK5 commercial glasses from SCHOTT. We showed that the dependence of the viscosity of P-SF67 was similar to the investigated glasses due to similar phosphate network organization confirmed by Raman spectroscopy, whereas N-FK5 exhibited a very different viscosity curve and fragility parameter due to its highly coordinated silicate network.

Chemical stability of Ni-enriched nanodomains in alkali borate glasses

The stability of Ni nanodomains observed in low alkali borate glasses (10 mol% alkali oxide) has been investigated at higher alkali contents using Ni K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. EXAFS data indicate that the Ni nanodomains disappear progressively when adding alkali oxide, whatever the nature of the alkali. This medium range reorganization around Ni is associated with a change of the local site from 6- to 5-fold coordination at 27–30 mol% alkali oxide and 4-fold coordination at higher alkali oxide content. Ni–Ni connections are still present for [5]Ni but no second neighbors are observed for [4]Ni in the EXAFS signal. The changes at both the local and medium range around Ni are likely associated to the disappearance of the large, rigid borate groups with the addition of alkali oxide.

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.

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.

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.

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.

Magnetic moments and ESR spectra of Co 2+ ions in alkali borate glasses

Measurements of magnetic moments and ESR spectra at room temperature and at 4.2 K, respectively, have been made on the R 2OB 2O 3 glasses containing Co 2+ (RLi,K). For purposes of comparison the measurements of ESR spectra have also been made on the high alkali-borate glass (70Na 2O30B 2O 3) and the sodium-borosilicate glass (10Na 2O60SiO 230B 2O 3), both of which contain Co 2+. The magnetic moments calculated from the susceptibility data in this paper are considerably smaller in comparison with those of Juza et al.; this finding also indicates that the dependences of the magnetic moments on the glass composition differ from the result of Juza et al. The ESR spectra indicate that Co 2+ is predominantly octahedral in low alkali-borate glasses, whereas in high alkali-borate glasses Co 2+ is located in distorted tetrahedral sites.

The3 A 2g ?3 T 1g(F) transition in the spectrum of some oxygen-ligated octahedral nickel(II) species

In sodium phosphate glasses, nickel(II) is octahedrally coordinated by the oxygens of the phosphates with Δ0 = 7600 cm−1 and B′ = 920 cm−1. Contrary to the case of many other oxygen-coordinated systems including [Ni(H2O)6]2+ and low alkali borate glasses, the second spin-allowed transition to3T1g(F) appears as a single-peaked band well-separated from the spinforbidden transition to1Eg(D). Variation of the glass composition does not change the main features of the spectrum. Dissolving the glasses in water results in a shift of all the bands to higher energies with the second spin-allowed band becoming triple-peaked. This supports Liehr and Ballhausen's explanation of the structure of this band.

Effect of quenching rate and annealing on ESR of Cu 2+ and γ-induced Cd + in low alkali borate glasses

A new ESR hyperfine structure (hfs) of Cu 2+ was detected in moderately (cooling rate ≈ 3 K/s) and rapidly (10 3–10 5K/s) quenched borate glasses with Na 2O content smaller than 5 mol% in addition to the already reported three patterns, spectra I, II and III, for the moderately quenched borate glasses with 5 ⪅ [Na 2O]⪅13, 20 ⪅ [Na 2O] ⪅37 and 55 ⪅ [Na 2O] ⪅ 75, respectively. The ESR parameters of this spectrum were much the same as those of II, but a significant difference was observed between thermal stabilities of these spectra. The newly observed spectrum was named spectrum II′. A superposition of spectra II′ and I was seen in low alkali borate glasses ([Na 2O] ⪅ 5 mol%). The relative intensity of II′ to I increased remarkably with increasing quenching rate of the melts and with decreasing alkali content under the same quenching condition. In addition, spectrum II′ was found to relax into I upon thermal annealing of the glasses. Similar changes were observed also in ESR of the γ-induced Cd + with 5s 1 electronic configuration. A tentative model to account for the spectral changes was proposed on the basis of the characteristics of B—O bonding.

Coordination of Pb2+ in oxide glasses determined by ESR and properties of binary lead glasses.

ESR spectra of paramagnetic Pb3+ with 6s1 electronic configuration induced by γ-irradiation at 77K were measured for a wide variety of oxide glasses. It has been found that the Pb 6s-character of the magnetic electron, evaluated from the analysis of the hyperfine structure due to a 207Pb nucleus (nuclear spin 1/2, abundance 21.7%) highly depends on the stereochemistry of precursor Pb2+, i.e., the s-character of the Pb3+ produced from Pb2+ having a nearly spherical-symmetric ligand field (coordination I) is fairly greater than that produced from Pb2+ with a trigonal- or tetragonal-pyramidal ligand field (coordination II). The determination of stereochemistry of Pb2+ in oxide glasses has been attempted by measuring ESR spectra of as-induced Pb3+ and by using the relation between the s-character of Pb3+ and the coordination of Pb2+ (i.e., coordination I or II). The results obtained are summarized as follows:(1) In phosphate glasses and low alkali borate glasses, most of Pb2+'s exist in coordination I.(2) Lead ions take predominantly coordination II in silicate and high alkali borate glasses. The relation has been also considered between the properties of binary lead glasses and the stereochemistry of Pb2+. A marked difference of limiting PbO concentrations for glass-formation in phosphate system from those in silicate and borate systems has been interpreted by considering the dissimilarity of the Pb2+-coordination structure. It has been found for binary silicate, borate and phosphate glasses that the partial molar refractivity of PbO increases with decreasing the s-character of Pb3+. This relation leads to a conclusion that the polarizability of Pb2+ taking coordination II is considerably greater than that having coordination I.

ChemInform Abstract: VISIBLE SPECTROSCOPY OF IRRADIATED HIGH‐ALKALI BORATE AND MIXED‐ALKALI PHOSPHATE GLASSES

AbstractIn dem stark basischen Glas 69% Na2O‐3 1% B203 (Tg = 367°C) wird ein Elektronenzentrum (Ämax = 619 nm) identifiziert, dessen thermische Stabilität der von Elektronenüberschußzentren in den Systemen Na2O‐B2O3 mit mäßigem (20 bis 35%) Alkalimetalloxidgehalt vergleichbar ist.

Ultraviolet absorption of cerium(III) and cerium(IV) in some simple glasses

The optical absorption spectra of cerium (III) and cerium (IV) in Na2O-B2O3, Na2O-SiO2, Na2O-P2O5 glasses, and in H2O-H2SO4 and H2O-H3PO4 solutions have been studied. Individual molar extinction coefficients of cerium (III) and cerium (IV) at different wavelengths (350 to 200 nm) have been estimated. In all the glasses and solutions, cerium (IV) produces a very strong and broad charge transfer band around 250 nm; the intensity, half-width, and position of this band change appreciably with glass composition. Cerium (III) in glass and in aqueous solution produces a number of absorption bands in the ultraviolet region corresponding to the f → d transitions. The cerium (III) bands are sharp and well resolved in Na2O-P2O5 and in low-alkali borate glasses; the sharpness of resolution deteriorates in Na2O-SiO2, and in high-alkali borate glasses. The intensity of cerium (III) absorption also changes with glass composition. In all the glasses the molar extinction coefficient of cerium (IV) is 5 to 10 times stronger than that of cerium (III).

EPR and optical absorption of Cr3+ in binary Na2O-B2O3 glasses

The optical absorption and EPR spectra of Cr3+ in binary sodium borate glasses have been studied as functions of chromium concentration and Na2O/B2O3 ratio in the glass; the ligand field and EPR parameters have been calculated and were found to be independent of Cr3+ concentration in any particular glass. In low-alkali borate glasses (Na2O = ∼ 11 or ∼ 14 mol %) a single symmetrical EPR line was observed with g ¦=g⊥ =1.984±0.001 corresponding to perfect octahedral symmetry of the Cr3+ ion in these glasses. With increasing Na2O content of the glass, the EPR line becomes more asymmetric (characteristic two-peaked pattern); this has been explained as being due to axial elongation of the six co-ordinated Cr3+-complex in these glasses.