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 less-than-or-equal-to x less-than-or -equal-to 0.5. FT-IR spectroscopy and glass transition temperature mea surements have been used to explore the structure and a key physical p roperty of the low-alkali phosphate glasses. The structure of nu-P2O5 is proposed to consist of a 3-D network of trigonally connected tetrah edra decorated with a P=O unit. Contrary to what has long been propose d for these glasses, the addition of alkali degrades the 3-D network t hrough the generation of nonbridging oxygens rather than strengthen th e network through the proposed alkali ion bridging. The Tg of nu-P2O5 is almost-equal-to 653 K and decreases some 130 K with the addition of 10 mol% Li2O. T(g) then reaches a minimum value at 20 mol% Li2O and i ncreases with further Li2O additions. The increase in T(g), even thoug h the fraction of nonbridging oxygens is still increasing, is interpre ted in terms of an increasing entanglement of long-chain PO, groups in the glass. Such a structural transition from a 3-D network of interco nnected 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.