Temperature-induced structural modifications between alkali borate glasses and melts

High-temperature neutron diffraction and Raman spectra have been obtained on M2O-2B(2)O(3) (M = Li, Na, K) glasses and melts. Both techniques indicate a coordination change of boron atoms: the tetrahedral boron sites present in the glasses are converted into triangular boron sites. These changes of the borate network yield modifications of the alkali environment, as assessed for Li using the isotopic substitution technique. We observe that Li atoms are in a charge-compensating position in the glass and in a modifying position in the liquid. These structural modifications have important implications toward understanding the physical properties of borate melts.