Temperature-induced boron coordination change in alkali borate glasses and melts

Neutron diffraction experiments have been performed on alkali diborate glasses and melts, M₂O-2B₂O₃ with M=Li, Na, K, in order to investigate the structural changes above the glass transition. In the glassy state, the contributions of fourfold- and threefold-coordinated boron could be clearly distinguished on the first peak of the correlation functions, which has been fitted with two Gaussian components. The fraction of boron in fourfold coordination (N₄) is 0.46, 0.43, and 0.40 for the Li-, Na-, and K-bearing glasses, respectively, in agreement with previous NMR studies. In the liquid state at ~1200K, a one-component fit of the peak yields an N₄ value of about 0.30, which means that one-third of the BO₄ tetrahedra in the glass converts to BO₃ triangles in the melt. The estimated change of the mean heat capacity for this conversion is in the range 27--73Jmol^-1K^-1. This value accounts for the major part of the total heat capacity change at Tg, suggesting that the BO₄ to BO₃ conversion is mainly responsible for the fragile behavior of the viscosity of alkali borate melts. Indeed it is closely related to the dynamic processes of the viscous flow, which involve both the akali migration and B-O bond breakening and reforming.