P-Tphase diagram and structural transformations of moltenP2O5under pressure

Abstract

The ${\mathrm{P}}_{2}{\mathrm{O}}_{5}$ compound is an archetypical glass-forming oxide with a record high hygroscopicity, which makes its study extremely difficult. We present the in situ x-ray diffraction study of the pressure-temperature phase diagram of ${\mathrm{P}}_{2}{\mathrm{O}}_{5}$ and, particularly, of the liquid ${\mathrm{P}}_{2}{\mathrm{O}}_{5}$ structure under high pressure up to 10 GPa. Additionally, quenching from the melt has been used to extend the melting curve up to 15 GPa. We found that structural transformation in the liquid ${\mathrm{P}}_{2}{\mathrm{O}}_{5}$ under pressure is unique and includes three stages: first, the disappearance of the intermediate range order of the melt together with a slow increase in the average first-coordination number $\ensuremath{\langle}{N}_{1}\ensuremath{\rangle}$ (P-O and O-P neighbors) up to 4 GPa; second, the ``normal'' compression almost without structural modification at higher pressures up to 8--9 GPa; and, finally, the abrupt change of the short-range order structure of the liquid with the jumplike $\ensuremath{\langle}{N}_{1}\ensuremath{\rangle}$ increase at 9--10 GPa. The last stage correlates with the melting curve maximum (\ensuremath{\approx}1250 \ifmmode^\circ\else\textdegree\fi{}C) at \ensuremath{\approx}10 GPa and can be interpreted as a transformation to the liquid phase with entirely fivefold-coordinated phosphorus and twofold-coordinated oxygen atoms.