Structural Transformations and Anomalous Viscosity in the ${\mathbf{B}}_2{\mathbf{O}}_3$ Melt under High Pressure

Liquid ${\mathrm{B}}_2{\mathrm{O}}_3$ represents an archetypical oxide melt with a superhigh viscosity at the melting temperature. We present the results of the in situ x-ray diffraction study and the in situ viscosity measurements of liquid ${\mathrm{B}}_2{\mathrm{O}}_3$ under high pressure up to 8 GPa. Additionally, the ${}^{11}\mathrm{B}$ solid state NMR spectroscopy study of ${\mathrm{B}}_2{\mathrm{O}}_3$ glasses quenched from the melt at five different pressures has been carried out. Taken together, the results obtained provide understanding of the nature of structural transformations in liquid ${\mathrm{B}}_2{\mathrm{O}}_3$. The fraction of the boroxol rings in the melt structure rapidly decreases with pressure. From pressures of about 4.5 GPa, four-coordinated boron states begin to emerge sharply, reaching the fraction 40%–45% at 8 GPa. The viscosity of the ${\mathrm{B}}_2{\mathrm{O}}_3$ melt along the melting curve drops by 4 orders of magnitude as the pressure increases up to 5.5 GPa and remains unchanged on further pressure increase.

Figure 1

Examples of the energy-dispersive x-ray diffraction (EDXD) data for the liquid ${\mathrm{B}}_2{\mathrm{O}}_3$ under pressure (a) measured at the two angles of the detector and (b) calculated from the experimental EDXD data and smoothed total interference functions for different pressures. (c) $Q$ position of the first peak of the structure factor, (d) amplitudes of the first three maxima of the structure factor, and (e) radial distribution functions [notations are the same as in (b)] of liquid ${\mathrm{B}}_2{\mathrm{O}}_3$, where the inset shows the pressure dependence of the radius of the 1st coordination sphere.

Figure 2

Experimental ${}^{11}\mathrm{B}$ MAS NMR spectra for ${\mathrm{B}}_2{\mathrm{O}}_3$ glasses, quenched from the melt at different pressures $P_{\mathrm{syn}}$, and their simulations with decomposition (a) and the population vs $P_{\mathrm{syn}}$ dependences for different boron sites (b). The NMR fitting parameters, such as isotropic chemical shift ${\delta }_{\mathrm{iso}}$ [ppm], quadrupolar coupling constant $C_q$ [MHz], and quadrupolar asymmetry parameter $\eta$, for the boroxol ring ${}^{[3]}\mathrm{B}$ and nonring ${}^{[3]}\mathrm{B}$ are the following, respectively: 18.3, 2.61, 0.22 and 15.9, 2.76, 0.17 for $P_{\mathrm{syn}}=1\mathrm{atm}$; 18.3, 2.66, 0.22 and 14.5, 2.65, 0.27 for $P_{\mathrm{syn}}=2.0\mathrm{GPa}$; 18.2, 2.67, 0.24 and 14.5, 2.68, 0.32 for $P_{\mathrm{syn}}=3.2\mathrm{GPa}$; 18.8, 2.62, 0.04 and 15.0, 2.62, 0.31 for $P_{\mathrm{syn}}=4.7\mathrm{GPa}$; 18.3, 2.69, 0.23 and 14.5, 2.67, 0.20 for $P_{\mathrm{syn}}=5.8\mathrm{GPa}$. The position of ${}^{[4]}\mathrm{B}$ peak for $P_{\mathrm{syn}}=5.8\mathrm{GPa}$ is equal 0.2 ppm.

Figure 3

Experimental phase diagram of ${\mathrm{B}}_2{\mathrm{O}}_3$ with the approximated color mapping of the melt viscosity (in Pa s units) (a) and pressure dependence of viscosity along the melting curve (b). The experimental pressure-temperature points, where viscosity was measured, are marked in (a) by the measured values of viscosity.