X-ray study of the crystallographic transformation in hydrogen below 1.5 K

The fcc to hcp and hcp to fcc phase transformations in single crystals of hydrogen have been studied using x-ray diffraction techniques. The transformation temperature $T^{*}$ has been mapped as a function of orthohydrogen concentration $\chi$ in the region of $0.17<\mathrm{}{T}^{*}<1.5\mathrm{}$ K and $0.48<\mathrm{}\chi <0.7\mathrm{}$. The fcc to hcp transformation temperature extrapolates to zero near a concentration of 0.49. The transformation temperatures and widths agree with other experimental results at higher temperatures, but exclude low-concentration NMR data in the literature from being associated with the crystallographic transformation in bulk samples.