Correlating the structure and dynamics of borophosphate glasses: molecular dynamics approach

Abstract

Employing molecular dynamics simulations, we investigate the structural evolution in sodium borophosphate glasses (\(35\text{Na}_2\text{O}-65[x\text{B}_2\text{O}_3-(1-x)\text{P}_2\text{O}_5]\)). Our simulations reveal that boron coordination shifts from predominantly four-fold (\(\text{B}^{[4]}\)) to mixed three/four-fold (\({B}^{[3]}/{B}^{[4]}\)) configurations with increasing boron content. The dominant \(P{-}O{-}B^{[4]}\) linkages facilitate homogeneous network integration at intermediate compositions (\(x=0.4{-}0.5\)), while higher boron concentrations promote \(B^{[4]}{-}O{-}B^{[3]}\) connectivity and polymerization. Crucially, radial distribution functions and coordination analysis demonstrate that charge-compensating \(\text{Na}^+\) ions exhibit delocalized bonding environments around \(\text{B}^{[4]}\) units, with \(Na{-}O\) coordination increasing from 5.4 (\(x=0\)) to 7.9 (\(x=1\)). This structural reorganization reduces non-bridging oxygens and creates new hoping site that enhance ionic conductivity. Our atomistic insights establish a direct correlation between \(P{-}O{-}{B}\) connectivity, sodium-ion delocalization, and macroscopic ion transport, providing a foundation for designing optimized borophosphate glass electrolytes.