Crystal structures of lanthanum to 230 GPa

The structural behavior of lanthanum metal has been investigated up to 227 GPa using synchrotron x-ray diffraction. Rather than having the rhombohedral or orthorhombic structures previously reported to be stable above 80 GPa, we find that La adopts the tetragonal $tI2$ structure seen previously in Ce and Pr. We show that the recent claims of a rhombohedral structure arose from the misidentification of lanthanum hydride peaks. The revised pressure dependence of the atomic volume of La above 100 GPa further constrains the stoichiometry of the superconducting ${\mathrm{LaH}}_{10}$ superhydride, which is synthesized from La above 160 GPa.