X-ray study of the crystallographic transition 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 < T* < 1.5 K and 0.48 < chi < 0.7. 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