High-pressure phase transitions and melt structure of PbO2 : An analog for silica

In situ x-ray diffraction measurements and inverse Monte Carlo simulations of pair distribution functions were used to study the structural response of $\mathrm{Pb}{\mathrm{O}}₂$ under pressure and to characterize the local structure of liquid $\mathrm{Pb}{\mathrm{O}}₂$. Two phase transitions are observed upon room-temperature compression of crystalline $\mathrm{Pb}{\mathrm{O}}₂$ up to \ensuremath{\sim}65 GPa. The starting mixture of rutile structured $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Pb}{\mathrm{O}}₂$ and orthorhombic $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Pb}{\mathrm{O}}₂$ undergoes a transition to a ${\mathrm{ZrO}}₂$-type orthorhombic phase with space group $Pbca$ at \ensuremath{\sim}21 GPa. Above 42 GPa, the $Pbca$ phase transforms to a cotunnite-type phase with space group $Pnam$. In this paper, we also report short-range order in liquid $\mathrm{Pb}{\mathrm{O}}₂$, showing \ensuremath{\sim}8--9-fold and 4--5-fold coordination around Pb and O atoms, respectively.