Physical properties and structural studies of lithium borophosphate glasses containing TeO2

Abstract

This work presents a study on the structure, thermal behavior and properties of (100-x)[0.5Li2O-0.1B2O3-0.4P2O5]-xTeO2 glasses prepared within the concentration range of x = 0–80 mol% TeO2. The structure of the glasses was studied with a combined Raman, 31P and 11B MAS NMR spectroscopy. 31P MAS NMR and Raman spectrum of the pure 50Li2O-10B2O3-40P2O5 glass indicates the co-existence of Q2 and Q1 phosphate units in the glass structure. Raman spectra of TeO2 containing glasses revealed that TeO2 is incorporated in the structural network in the form of TeO3 and TeO4 (TeO3+1) units. The ratio of TeO4/TeO3 increases with rising TeO2 content. Apart from the formation of P‒O‒Te and Te‒O‒Te bonds, the formation of B‒O‒Te bonds in the form of mixed tetrahedral units B(OP)4-n(OTe)n also takes place as shown by 11B MAS NMR spectroscopy. The structural studies were correlated with the thermal behavior and physical properties of glasses (density, molar volume, chemical durability and refractive index). DSC studies showed that all glasses crystallize on heating within a temperature region of 400–550 °C and the surface nucleation mechanism prevails over the internal one. The crystallization temperature decreases with increasing TeO2 content. The compounds formed by crystallization were LiPO3, Li4P2O7, BPO4 and α-TeO2. Glass transition temperature and dilatometric softening temperature, estimated from thermodilatometry curves, decrease with increasing TeO2 content, whereas the thermal expansion coefficient increases.