We report synchrotron x-ray-diffraction measurements on low-density hcp-solid ${}_{}{}^4{}_{}{}^{}$ He at a temperature of ∼0.7 K. The root-mean-square deviation of the atoms from their lattice positions 〈${\mathrm{u}}^2$ ${\mathrm{〉}}^{1\mathrm{/}2}$ is determined from measurements of the Debye-Waller factor at the (002), (004), and (006) Bragg peaks. We find 〈${\mathrm{u}}^2$ ${\mathrm{〉}}^{1\mathrm{/}2}$ =0.96±0.02 Å along the c axis. Our results are consistent with Green's function–Monte Carlo calculations. Weak reflections at the hcp-forbidden (001) and (003) Bragg peaks were also observed, probably indicating structural defects in the crystals.

DOI:https://doi.org/10.1103/PhysRevB.55.5767