THERMAL, PHYSICAL, ELECTRICAL AND XPS STUDIES OF THE LI2O-P2O5-MOO3 GLASS SYSTEM

The glass-forming region in the Li2O:P2O5:MoO3 system has been determined. Three series of glasses with 40, 50 and 60 mol% Li2O have been studied. Maxima in glass transition temperature were observed around x = [MoO3]/([MoO3] + [P2O5]) = 0.4. This was attributed to a convolution of the increasing cross-link density and decreasing mean bond strength with increasing x. DC conductivity, sigma, was found to increase with x for a given Li2O content, with the composition 0.5Li2O:0.5(0.6P2O5:0.4MoO3) having the highest sigma at 25-degrees-C. Linear correlation between the activation energy for conduction, E(act), and logarithm of the pre-exponential factor, sigma-0, in the Arrhenius equation was attributed to strong entropy-enthalpy interaction. Binding energies of the Li1s, P2p, Mo3d5/2 and O1s core-levels have been determined from the respective XPS spectra. Upon deconvolution, the XPS O1s spectrum was found to consist of two peaks. Phosphate groups in the glasses were found to be preferentially modified by Li2O compared with molybdate groups, consistent with the order of acidity, P2O5 > MoO3, of glass-forming oxides in melts. The presence of Li2O is also believed to have promoted the formation of P-O-Mo units at the expense of P-O-P and Mo-O-Mo units.