Single-crystal x-ray diffraction of $n$-${\mathrm{H}}_2$ at high pressure

X-ray diffraction from a single crystal of solid $n$-${\mathrm{H}}_2$ has been obtained at high pressure in a diamond-anvil cell, and the crystal structure has been determined. Nineteen diffraction maxima, corresponding to the hexagonal-close-packed structure ($\frac{P{6}_3}{\mathrm{mmc}}$, $Z=2\mathrm{}$), were observed. Unit-cell dimensions at 5.40±0.03 GPa and 300±1 K are $a=2.659\mathrm{}\pm 0.002$ Å, $c=4.334\mathrm{}\pm 0.003$ Å, and $V=26.55\mathrm{}\pm 0.05$ ${\mathrm{\AA }}^3$. The structure determination and volume measurement provide the most accurate checks to date on the equation of state and phase diagram of hydrogen in this pressure range.