Structure property correlation in lithium borophosphate glasses

Abstract

To investigate the influence of cation mobility variation due to the mixed glass former effect, 0.45Li(2)O-(0.55-x)P(2)O(5)-xB(2)O(3) glasses (0≤x≤0.55) are studied keeping the molar ratio of Li(2)O/(P(2)O(5)+B(2)O(3)) constant. Addition of B(2)O(3) into lithium phosphate glasses increases the glass transition temperature (T(g)) and number density, decreases the molar volume, and generally renders the glasses more fragile. The glass system has been characterised experimentally by XRD, XPS and impedance studies and studied computationally by constant volume molecular dynamics (MD) simulations and bond valence (BV) method to identify the structural variation with increasing the B(2)O(3) content, its consequence for Li(+) ion mobility, as well as the distribution of bridging and non-bridging oxygen atoms. These studies indicate the increase of P-O-B bonds (up to Y=[B(2)O(3)]/([B(2)O(3)]+[P(2)O(5)])≈0.5 and B-O-B bonds, as well as the decrease of P-O-P bonds and non-bridging oxygens (NBOs) with rising B(2)O(3) content. The system with Y≈0.5 exhibits maximum ionic conductivity, 1.0×10(-7) S cm(-1), with activation energy 0.63 V. Findings are rationalised by a model of structure evolution with varying B(2)O(3) content Y and an empirical model quantifying the effect of the various structural building blocks on the ionic conductivity in this mixed glass former system.