Superconductivity and equation of state of lanthanum at megabar pressures

Lanthanum (La) is the first member of the lanthanoid that has recently raised considerable interest because of its unique La superhydride ${\mathrm{LaH}}_{10}$ and its own superconducting properties. There has been a lack of experimental evidence for the equation of state (EOS) and superconductivity of La at pressures exceeding one megabar. Here, we extend the pressure range up to 140 GPa to explore the EOS and superconductivity of La via the electrical resistance and x-ray diffraction measurements. We identified the phase transition sequences by the La Bail refinements of the experimental XRD patterns and discovered a distorted fcc-La phase (space group Fmmm) above 78 GPa with nonhydrostatic pressure transmitting media. All the experimental pressure-volume data were fitted by the third-order Birch-Murnaghan equation: ${V}_{0}=37.5\phantom{\rule{0.16em}{0ex}}{\AA{}}^{3}, {B}_{0}=14.5\phantom{\rule{0.16em}{0ex}}(1)\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, and ${B}_{0}^{\ensuremath{'}}=5$. Superconductivity still existed in the distorted fcc-La with an onset critical temperature ${T}_{c}$ of 9.6 K at 78 GPa, which decreases to 2.2 K at 140 GPa. We calculated the superconducting parameters of La at several pressure points, and discussed the difference from experimetal ${T}_{c}$.