Lithium Borates from the Glass to the Melt: A Temperature-Induced Structural Transformation Viewed from the Boron and Oxygen Atoms.

A multiedge study of the local structure of lithium borate glasses and melts has been carried out using X-ray Raman scattering (XRS) as a function of temperature. Thanks to a wide range of compositions, from pure B2O3 up to the metaborate composition, we are able to finely interpret the modifications of the local environment of both the boron and oxygen atoms in terms of boron coordination number, formation of nonbridging oxygens (NBOs), and polymerization degree of the borate framework as a function of temperature and composition. A temperature-induced [4]B to [3]B conversion is observed above the glass transition temperature (Tg) from the glass to the melt from the triborate composition up to the metaborate composition. Two distinct melt structures are reported: a well-polymerized borate network—with few NBOs—below the triborate composition and a depolymerized borate network above the diborate composition with a rapid increase of the number of NBOs when Li2O is added. These two structurally distinct melts allow explaining the two dynamic regimes observed for lithium ion diffusion.