Consequences Of Sp(2)-Sp(3) Boron Isomerization In Supercooled Liquid Borates

Time-resolved high-energy synchrotron x-ray total scattering measurements on supercooled molten lithium metaborate (LiBO2) reveal an isomerization reaction involving conversion of trigonal sp(2) boron to tetrahedral sp(3) boron during quenching and glass formation. Van't Hoff analysis yields an accurate enthalpy change, Delta H=21(1) kJ mol(-1) boron, from which we develop an analytical model for the sp(3) isomer fraction and its contribution to configurational heat capacity (C-p(conf)) and entropy as a function of temperature and composition. Isomerization constitutes 40% of the total calorimetric C-p(conf) at the glass transition for LiBO2 and directly contributes to the observed rise in liquid fragility with the lithium content.