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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi>t</a:mi><a:mi>I</a:mi><a:mn>2</a:mn></a:mrow></a:math> 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 <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>LaH</b:mi><b:mn>10</b:mn></b:msub></b:math> superhydride, which is synthesized from La above 160 GPa. Published by the American Physical Society 2024