New Low-Temperature Phase of Molecular Deuterium at Ultrahigh Pressure

Abstract

We have studied ortho-${\mathrm{D}}_{2}$ and para-${\mathrm{H}}_{2}$ at pressures up to 540 kbar (54 GPa) and $1.1 \mathrm{K}<T<300 \mathrm{K}$ by means of Raman scattering in a diamond anvil cell. A phase transition in which the molecules which are spherically symmetric at low pressure go into an orientationally ordered state has been observed in $o$-${\mathrm{D}}_{2}$ at 278\ifmmode\pm\else\textpm\fi{}5 kbar. Line broadening of the roton bands for $p\ensuremath{\gtrsim}200$ kbar is interpreted as a precursor of this transition.