Boron coordination change in barium borate melts and glasses and its contribution to configurational heat capacity, entropy, and fragility

High-energy x-ray diffraction from molten and glassy BaB2O4 and BaB4O7 has been performed using aerodynamic levitation and laser heating over a wide range of temperatures. Remarkably, even in the presence of a heavy metal modifier dominating x-ray scattering, it was possible to extract accurate values for the tetrahedral, sp(3), boron fraction, N-4, which declines with increasing temperature, using bond valence-based mapping from the measured mean B-O bond lengths while accounting for vibrational thermal expansion. These are used within a boron-coordination-change model to extract enthalpies, Delta H, and entropies, Delta S, of isomerization between sp(2) and sp(3) boron. The results for BaB4O7, Delta H = 22(3) kJ mol(-1) boron, Delta S = 19(2) J mol(-1) boron K-1, agree quantitatively with those found previously for Na2B4O7. Analytical expressions for N-4(J, T) and associated configurational heat capacity, CPconf(J, T), and entropy, S-conf(J, T), contributions are extended to cover a wide composition range 0 <= J = BaO/B2O3 <= 3 using a model for Delta H(J) and Delta S(J) derived empirically for lithium borates. Maxima in the CPconf(J, T-g) and fragility index contributions are thereby predicted for J less than or similar to 1, higher than the maximum observed and predicted in N-4(J, T-g) at J similar or equal to 0.6. We discuss the utility of the boron-coordination-change isomerization model in the context of borate liquids containing other modifiers and the prospect of neutron diffraction to aid in empirical determinations of modifier-dependent effects, illustrated by new neutron diffraction data on (BaB4O7)-B-11 glass, its well-known alpha-polymorph, and lesser-known delta-phase.