Cation dynamics in lithium borate glasses

The dynamic properties of lithium cations in the 0.3Li 2O–0.7B 2O 3 glass are investigated by molecular dynamics simulations at T=1250 K by means of the van-Hove space-time correlation functions. Li cations can be distinguished into Li b and Li nb types, if they reside predominantly in sites formed by bridging oxygen atoms (b-type) or sites involving both bridging and non-bridging oxygen atoms (nb-type), respectively, with Li nb cations being more mobile than the Li b ones. The self-van-Hove correlation functions have shown that at times shorter than 15 ps both types of Li cations are characterized by a non-Gaussian behavior, implying a jump-type rather than a random-walk diffusion mechanism. However, whereas for Li nb cations a Gaussian distribution of jump lengths is restored at times longer than 20 ps, this is not the case for Li b cations. Examination of the distinct-van-Hove correlation functions has shown that Li nb ions migrate by hopping into nb-type sites, whereas hopping into a b-type site is a process of negligible probability. On the contrary, the hopping process of Li b cations involves jumps into both nb- and b-type sites, as well as into interstitial sites.