High pressure structure of lanthanum

The behaviour of the lanthanum (La) metal at high pressure is of great interest because it provides insight into the behaviour of f electrons in high density metals, and also because high-pressure lanthanum hydrides have some of the highest superconducting temperatures known. Understanding the high-pressure phases of La is then vital in identifying the hydrogen content of these super-hydrides. However, the structure of La above 70 GPa is uncertain and a number of different, but related, structural forms have been reported in the literature. We have collected new, high-quality diffraction data to 230 GPa (2.3 Mbars) using the sub-micron diameter x-ray beam now available at the PETRA-III synchrotron. Analysis of this new data using Rietveld and Le Bail analysis reveals that La transforms to a body-centred tetragonal structure above ~80 GPa, a structure long-known in neighbouring element Ce, and confirming a previous unpublished experimental result from our group and a recent computational prediction.