Fraction Of Non-bridging Oxygen Research Articles

Comparison of structure of alkali silicate glasses prepared by sol-gel and melt-quench methods

Glasses of composition 15M 2 O-85SiO 2 , where M is Na or Li, were prepared by sol-gel and melt-quench methods. X-ray photoelectron spectroscopy and transmission electron microscopy were used to investigate their structure as the gels approach the glassy state. The fraction of non-bridging oxygens in gels approaches the value found in the melt-quench glass as the gel is heated to the glass transition temperature but does not always reach it. The chemical environment of the alkali species is more heterogeneous in the sol-gel glasses than in the melt-quench glasses. Thermodynamically favored phase separation is more easily achieved during the sol-gel process than in the melt-quench method of glass preparation.

57Fe- and119Sn-Mössbauer effect of oxide glasses

Coordination number of network-former (NWF) and formation of nonbridging oxygen (NBO) at the site neighboring to NWF can be estimated from Mossbauer measurements, since small amounts of Fe3+ and Sn4+ substitute NWF in several oxide glasses. Gamma-ray or thermal neutron irradiation of oxide glasses causes electron or charge transfer from oxygen to the Mossbauer ions, and the probability depends on the fraction of NBO. On the contrary, γ-ray irradiation of phosphate glasses results in oxidation of Fe2+ to Fe3+ since iron plays a role of network modifier (NWM) at interstitial sites. Debye temperature θD obtained from low-temperature Mossbauer measurements reflects the site occupation of Mossbauer ions in glasses. A linear relationship between glass transition temperature (Tg) and quadrupole splitting (Δ) of Fe3+,Tg—Δ rule, is also effective for determining the site occupation of Mossbauer ions.

Glass Transition and Infrared Spectra of Low‐Alkali, Anhydrous Lithium Phosphate Glasses

Anhydrous glasses in the series xLi2O + (1 ‐ x)P2O5 have been prepared and characterized in the range 0 ≤x≤ 0.5. FT‐IR spectroscopy and glass transition temperature measurements have been used to explore the structure and a key physical property of the low‐alkali phosphate glasses. The structure of v‐P2O5 is proposed to consist of a 3‐D network of trigonally connected tetrahedra decorated with a P=O unit. Contrary to what has long been proposed for these glasses, the addition of alkali degrades the 3‐D network through the generation of nonbridging oxygens rather than strengthen the network through the proposed alkali ion bridging. The Tg of v‐P2O5 is ∼653 K and decreases some 130 K with the addition of 10 mol% Li2O. Tg then reaches a minimum value at 20 mol% Li2O and increases with further Li2O additions. The increase in Tg, even though the fraction of nonbridging oxygens is still increasing, is interpreted in terms of an increasing entanglement of long‐chain PO2 groups in the glass. Such a structural transition from a 3‐D network of interconnected PO4 groups for P2O5 to a 1‐D chain structure for LiPO3 is one of the first examples of the importance of intermediate‐range order in governing the properties of glass.

Structure of PbOBi 2O 3Ga 2O 3 glasses

The structure of PbOBiO 1.5GaO 1.5 glasses has been investigated by IR and Raman spectroscopy. An infrared absorption band involving four-coordinated Ga 3+ ions was observed at about 610–650 cm −1. The peak of this band shifts toward higher wavenumbers with increasing GaO 1.5 content, indicating that covalency of GaO bond increases with increasing GaO 1.5 content. The peak deconvolution was made on Raman spectra of the glasses. The Raman bands at about 130 cm −1, 400 cm −1, 550 cm −1 and 650 cm −1, obtained by deconvolution, have been assigned to PbO or BiO, GaOPb or GaOBi, GaOGa, and GaO non-bridging vibrations, respectively. The presence of 550 cm −1- and 650 cm −1-band indicates that Ga 3+ ions are coordinated by four oxygens. The much higher peak wavenumber of the 550 cm −1-band in the spectrum of BiO 1.5GaO 1.5 glass compared with that in the spectrum of PbOGaO 1.5 glass is attributed to the much higher ionic field strength of Bi 3+ ion compared with that of Pb 2+ ion. The fraction of non-bridging oxygens in GaO 4 tetrahedra decreases with increasing PbO and BiO 1.5 contents. It is assumed that most Pb 2+ and Bi 3+ ions are network formers.

Mössbauer and ESR Studies of Potassium Borophosphate Glasses

Abstract A linear decrease in isomer shifts of Fe3+ and Fe2+ was observed with an increase in alkali content of borophosphate glasses. The decrease is attributed to a change in the role of iron from a network modifier to a network former, or a decrease in the interatomic distance between iron and oxygen atoms. A linear decrease in quadrupole splitting and linewidth was also observed with increasing alkali content, and the decrease is attributed to the increased symmetry around iron nucleus. A linear decrease in relative absorption area of Fe2+ in Mössbauer spectra was also observed with increasing alkali content of the glasses. This result is attributed to an increased oxidation atmosphere brought about by alkali oxide into BPO4 units. Gamma-ray irradiation (1×108 R) experiments resulted in no change in these Mössbauer parameters, indicating that the glasses are less sensitive to γ-rays than borate or phosphate glasses. ESR absorption intensity of PO42− observed in γ-ray irradiated, iron-free borophosphate glasses increased linearly with an increase in the alkali content in the lower alkali region, and the increase is attributed to a linear increase in the fraction of non-bridging oxygen in PO4 tetrahedra constituting BPO4 (and PO4) units.