New Low-Temperature Phase of Molecular Deuterium at Ultrahigh Pressure

We have studied ortho-${\mathrm{D}}_2$ and para-${\mathrm{H}}_2$ at pressures up to 540 kbar (54 GPa) and $1.1 \mathrm{K}<T<\mathrm{}300\mathrm{} \mathrm{K}$ 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±5 kbar. Line broadening of the roton bands for $p\gtrsim 200$ kbar is interpreted as a precursor of this transition.