Metastable structures of solid hydrogen

Abstract

The J=0\ensuremath{\rightarrow}2 Raman signal from solid J=0 ${\mathrm{D}}_{2}$ or ${\mathrm{H}}_{2}$ reveals the hcp structure when deposited at a rate 0.1\ensuremath{\le}R(\ensuremath{\mu}m/min)\ensuremath{\le}40 onto ${\mathrm{MgF}}_{2}$ at ${\mathit{T}}_{\mathit{d}}$/${\mathit{T}}_{\mathrm{tp}}$\ensuremath{\gtrsim}0.3, a mixture of hcp and fcc crystals at 0.2${\mathit{T}}_{\mathit{d}}$/${\mathit{T}}_{\mathrm{tp}}$0.3 and possibly a randomly stacked close-packed structure at ${\mathit{T}}_{\mathit{d}}$/${\mathit{T}}_{\mathrm{tp}}$0.2, where ${\mathit{T}}_{\mathrm{tp}}$ is the triple point temperature. Non-hcp crystals transform to hcp continuously and irreversibly with increasing T. Finally, the crystal size decreases with decreasing ${\mathit{T}}_{\mathit{d}}$ and increasing R, from \ensuremath{\sim}1 mm at ${\mathit{T}}_{\mathit{d}}$\ensuremath{\sim}0.8${\mathit{T}}_{\mathrm{tp}}$ and R\ensuremath{\sim}2 \ensuremath{\mu}m/min to \ensuremath{\sim}1 \ensuremath{\mu}m at 0.25 ${\mathit{T}}_{\mathrm{tp}}$ and R\ensuremath{\sim}40 \ensuremath{\mu}m/min. \textcopyright{} 1996 The American Physical Society.