Structural transformations in liquid, crystalline and glassy B$_2$O$_3$ under high pressure

We present in situ (X-ray diffraction) and ex situ (quenching) structural study of crystalline, liquid, and glassy B$_2$O$_3$ up to 9 GPa and 1700 K, drawing equilibrium and non-equilibrium phase diagrams of B$_2$O$_3$. Particularly, we have determined the melting curve, the stability regions for crystalline B$_2$O$_3$ I and B$_2$O$_3$ II modifications, the regions of transformations, such as densification or crystallization, for both the liquid and glassy states, including the region of sharp first-order-like transition in liquid B$_2$O$_3$ to a high-density phase nearby 7 GPa. Quenching experiments also show that the transition to the high-density liquid can occur at much lower pressures in non-stochiometric melt with excess of boron. B$_2$O$_3$ is the first glassformer, where transformations in the disordered state have been comparatively studied for both liquid and glassy phases.