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 measurements have been used to explore the structure and a key physical property of the low-alkali phosphate glasses. The structure of nu-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 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 increases with further Li2O additions. The increase in T(g), even though the fraction of nonbridging oxygens is still increasing, is interpreted in terms of an increasing entanglement of long-chain PO, 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.