Richard K. Brow

Review: The Structure of Simple Phosphate Glasses

Recent developments of phosphate glasses for a variety of technological applications, from rare-earth ion hosts for solid state lasers to low temperature sealing glasses, have led to renewed interest in understanding the structures of these unusual materials. In this review, spectroscopic and diffraction studies of simple phosphate glasses, including v-P2O5 and binary phosphate compositions, are described. Special attention is given to the structures of anhydrous ultraphosphate glasses, which have received close attention from the glass community only in the past six years.

The short-range structure of zinc polyphosphate glass

Abstract 31 P magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy and Raman spectroscopy have been used to examine the polyhedral arrangements in x ZnO · (1 − x )P 2 O 5 (0.50 ≤ x ≤ 0.71) glasses. The depolymerization of P metaphosphate chains by the addition of ZnO is quantitatively described by the increase in the concentration of Q 1 -phosphate sites, determined from the 31 P MAS-NMR spectra. When x > 0.60, the NMR and Raman spectra exhibit peaks due to Q 0 and Q 2 tetrahedra, indicating that structures disproportionate in glass melts near the pyrophosphate composition. The splitting of the Raman peak due to the Q 1 terminal oxygen stretching mode indicates that a variety of P-O-Zn bonds participate in the polyphosphate glass structure. The complex mixture of P and Zn polyhedra contributes to the glass-forming tendency of the high ZnO (> 60 mol%) compositions.

Properties and Structure of Sodium-iron Phosphate Glasses

Abstract Selected properties of phosphate glasses, containing from 14 to 43 mol% Fe2O3 and up to 13 mol% Na2O, have been measured. With increasing Fe2O3 and Na2O content, the density and dilatometric softening temperature increased, whereas, the thermal expansion coefficient and dissolution rate in water or saline at 90°C decreased. Glasses containing more than 25 mol% Fe2O3 had an exceedingly good chemical durability. Their dissolution rate at 90°C in distilled water or in saline solution was up to 100 times lower than that of window glass. Mossbauer and X-ray photoelectron spectroscopy indicate that iron(II) and iron(III) were both present in the glasses and the chemical durability improved with increasing iron(III) concentration. The outstanding chemical durability of these glasses was attributed to the replacement of POP bonds by more chemically resistant POFe(II) and POFe(III) bonds.

Raman Spectroscopy Study of the Structure of Lithium and Sodium Ultraphosphate Glasses

Anhydrous binary phosphate glasses containing from 0 to 50 mol% Li2O or Na2O have been prepared and examined by Raman scattering spectroscopy. The unpolarized Raman spectrum of vitreous P2O5 has intense bands near 640 cm−1, attributed to the symmetric stretching mode of POP bridging oxygens, (POP)sym, between Q3 phosphate tetrahedra, and at 1390 cm−1 due to the symmetric stretch of the PO terminal oxygens, (PO)sym. With the addition of alkali oxide to P2O5, a new feature appears in the Raman spectra near 1160 cm−1 indicating the formation of Q2 phosphate tetrahedra with two bridging and two non-bridging oxygens. The increase in relative amplitude of this new (PO2)sym band with increasing modifier content is consistent with a simple depolymerization of the phosphate network. From 20 to 50 mol% alkali oxide, the position of the (PO)sym Raman band decreases by ∼ 130 cm−1 whereas the frequency of the (POP)sym band increases by ∼ 60 cm−1. These frequency shifts are the result of π-bond delocalization on Q3 species that effectively lengthens the PO terminal oxygen bond and strengthens the POP linkages with increasing alkali oxide content. The compositional dependence of the π-bond delocalization on Q3 tetrahedra is described by considering the interconnections between neighboring Q3 and Q2 tetrahedra. The onset of π-bond delocalization on Q3 species corresponds with the anomalous Tg minimum at 20 mol% alkali oxide in alkali ultraphosphate glasses. The increase in Tg between 20 and 50 mol% alkali oxide is attributed to the increased ionic interconnection of what becomes a chain-like phosphate network at higher alkali contents. Finally, the Raman spectra of several alkali ultraphosphate glasses show high frequency shoulders on the Raman bands attributed to the (PO2)sym and (PO2)asym vibrational modes. These shoulders represent the presence of strained structural units, possibly three- or four-membered rings.

Structural design of sealing glasses

Requirements for enhanced component performance and reliability have led to the development of novel glass compositions for a variety of hermetic sealing applications. The development of technologically useful glass compositions was based on an understanding of the relationships between the molecular-level glass structure and important physical properties. The properties of the alkaline earth aluminoborate glasses for lithium batteries are sensitive to changes in B- and Al-coordination number, characterized by solid state nuclear magnetic resonance (NMR) spectroscopy. In general, the most useful compositions have structures that are dominated by tetrahedral Band Al-sites. Mixed alkali aluminophosphate glasses were developed for aluminum electrical connectors. The properties of sodium aluminophosphate glasses depend on the O/P ratio and significant property changes (e.g. maxima in T g and refractive index) occur when O/P exceeds the pyrophosphate limit at 3.5. Associated with these property changes is a decrease in the average Al-coordination number, from six to four, at O/P > 3.5. Raman spectroscopy provides additional information about the aluminophosphate network. Finally, zinc borophosphate glasses are developed for seals in flat panel displays. Boron-11 NMR shows that tetrahedral borons are preferred in xB 2 O 3 (1-x)(PO 3 ) 2 and in yB 2 O 3 (1 - y)Zn 2 P 2 O 7 glasses for x < 0.4 and y < 0.2. Raman spectroscopy reveals the concomitant evolution from a phosphate to a borophosphate network with increasing x and y.

Nuclear magnetic resonance investigation of the structures of phosphate and phosphate-containing glasses: a review

This paper presents a review of the nuclear magnetic resonance (NMR) data for phosphate and phosphate-containing glasses obtained primarily within the past 10 years and of the structural interpretations based on those data. Compositions discussed include P2O5, alkali and alkaline earth phosphates, aluminophosphates, borophosphates, fluorophosphates, and phosphate-containing silicate and aluminosilicate glasses. 31P NMR data, in conjunction with 27Al, 29Si, 11B, 7Li, and 23Na data if appropriate, have proven very powerful in providing direct evidence about the local structural environments present in the these materials and in many cases have allowed interpretation of the physical and chemical behavior of these glasses in terms of polyhedral structures.

The short range structure of sodium phosphate glasses I. MAS NMR studies

A series of x(Na2O + H2O)·(1 − x)P2O5 glasses have been characterized by 31P and 23Na magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. High-resolution 31P NMR spectra reveal the presence of Q2-(2 bridging oxygen/tetrahedron) and Q3-(3 bridging oxygen/tetrahedron) tetrahedral sites in glasses with x 0.5. Quantitative measurements of the respective NMR site populations are in excellent agreement with Van Wazers predictions for the structures of ionic phosphates and illustrate the depolymerizing effects of residual H2O on the glass structure. The systematic change in the Q231P chemical shift is consistent with an increase in the average π-bond character of the phosphorus-nonbridging oxygen bond as x decreases.

An XPS study of oxygen bonding in zinc phosphate and zinc borophosphate glasses

Abstract X-ray photoelectron spectroscopy has been used to determine oxygen bonding in a series of x ZnO · (1 − x )P 2 O 5 (0.50 ≤ x ≤ 0.67) glasses. Curve fitting of the O 1s spectra leads to a quantitative measure of the bridging-to-non-bridging oxygen ratio which is shown to depend on composition according to a simple structural depolymerization model. A third peak, present in the O 1s spectra collected from y B 2 O 3 · (1 − y )Zn(PO 3 ) 2 (0.00 ≤ y ≤ 0.40) glasses, is due to oxygens which link borate and phosphate tetrahedra. The relative concentrations of POP, POZn and POB bonds are shown to be in good agreement with a structural model which assumes that borophosphate units (BPO 4 ) form when B 2 O 3 is added to Zn metaphosphate glass.

The short-range structure of sodium ultraphosphate glasses

Anhydrous sodium ultraphosphate glasses were prepared with Na2O contents between 0 and 50 mol% and were characterized by several structurally sensitive spectroscopic probes to determine the nature of the phosphate tetrahedra that constitute the short-range glass structure. Solid state 31P magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy reveals that Na2O depolymerizes the branched (Q3) P-O network of P2O5 to form metaphosphate (Q2) sites, in quantitative agreement with Van Wazers ‘chemically simple’ model. X-ray photoelectron spectroscopy reveals that the concomitant increase in non-bridging oxygen with increasing Na2O content is also in quantitative agreement with this structural model. Raman spectroscopic analyses of glasses with approximately 40 mol% Na2O suggest that some intermediate-range order, perhaps associated with strained rings, also exists within the glass network. Strained sites are eliminated when the solid glass is heated to melt temperatures.

Iron Redox Equilibrium, Structure and Properties of Zinc Iron Phosphate Glasses

Abstract Iron redox equilibrium, structure and properties were investigated for 40Fe2O3–60P2O5 (mol%) glasses melted at different temperatures. The Fe2+/(Fe2++Fe3+) ratio increased from 17% to 50% as the melting temperature changed from 1150°C to 1400°C. The equilibrium constant, K, for the reaction of Fe3+ being reduced to Fe2+ varied with temperature as lnK=9.40–1.58×104/T. The Raman and infrared spectra indicated that the basic iron pyrophosphate structure of the 40Fe2O3–60P2O5 (mol%) glasses did not change as the Fe2+/(Fe2++Fe3+) ratio changed. All of the properties did not change to any major degree with increasing the melting temperature. The molar volume decreased while the density increased with increasing Fe2+/(Fe2++Fe3+) ratio. It was found by DTA and XRD that two phases, Fe3(P2O7)2 and Fe4(P2O7)3, crystallized from the glass when the glass was heated in nitrogen. The crystallization behavior suggested that the amount of the crystal, Fe3(P2O7)2, may increase with increasing Fe2+/(Fe2++Fe3+) ratio, which supported the opinion that there are some structural similarities between the iron phosphate glass and the crystalline Fe3(P2O7)2 in terms of the iron coordination number and bonding of the phosphate groups. The decrease in dc resistivity and increase in dielectric constant and dielectric loss tangent, which occurred with increasing the Fe2+/(Fe2++Fe3+) ratio, were attributed to the increase of the electronic hopping from Fe2+ ions to Fe3+ ions.