Spectroscopic Studies on the Structures of Phosphate Sealing Glasses
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MRS BULLETIN/NOVEMBER 1998
polyphosphate glasses do not react with the ambient atmosphere as rapidly. These compositions can be made using conventional, open-crucible melting procedures. We review here some recent spectroscopic studies of the structures of phosphate glasses. Because of the disorder in the polyhedral arrangements that constitute the structure of a glass, comprehensive bonding information cannot be obtained by diffraction techniques— except for the simplest compositions. A variety of spectroscopic probes must be used to solve different pieces of the terminal (nonbridging) oxygens
t? Q3 tetrahedron, [O]/[P]=2.5
Q1 dimer, [O]/[P]=3.5
• bridging oxygens
Q2 tetrahedra, [O]/[P]=3.0
Isolated Q°, [O]/[P]=4.0
Figure 1. Tetrahedral phosphate units that are present in phosphate glasses.
structural puzzle, to characterize cation and anion coordinations, to reveal nextneighbor linkages, and to describe longer range molecular organization. With such structural information, an understanding of the relationships between glass properties and composition can be developed. We will show how such spectroscopic information can be used to guide the development of compositions for specialty sealing applications. Structural Chemistry of Binary Phosphate Glasses Distributions of P-tetrahedra The network structures of simple phosphate glasses consist of P-tetrahedra linked to neighboring tetrahedra through bridging oxygens (BOs). In vitreous P2O5, each tetrahedron has three BOs and so is identified as a Q 3 tetrahedron. The fourth P-oxygen bond is very short compared to the P-BO bond lengths (1.432 A versus 1.581 A),5 reflecting the localization of electron density to this terminal oxygen bond. An increase in the [O]/[P] ratio of the glass by the addition of a modifying oxide (e.g., alkali, R2O, or alkaline earth, R'O) disrupts the Q3 network of P2O5 by converting BOs to nonbridging oxygens (NBOs), thus replacing Q3 tetrahedra by Q2 tetrahedra (Figure 1). At [O]/[P] = 3.0 (50-mol% R2O or R'O), only Q2 tetrahedra are present in the glass network— either in long chains or rings.6 Further increases in [O]/[P] ratio create Q1 tetrahedra that terminate progressively shorter chain elements. At [O]/[P] = 3.5, the pyrophosphate stoichiometry, Q'-Q 1 dimers dominate the glass network structure. Isolated orthophosphate units (Q°) are expected when [O]/[P] > 3.5. In practice, glass formation in binary xR2O (or R'O) (1 - x)P2O5 systems is limited to x < 0.55-0.60 (mole fraction) for conventional melt processing. Rapidquenching techniques have yielded Li-phosphate glasses with x as large as 0.70, possessing structures based on Q1 and Q° tetrahedra. 7 Pyrophosphate glasses based on a variety of other lowcoordinated metal oxides—including SnO, ZnO, CdO, PbO, and Fe2O3—can be made by conventional melt techniques. These latter glasses are often characterized by unusually good (for phosphates) chemical durability and are currently under development for a variety of applications, some of which are described in the following. The Q" tetra
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