Superconductivity and Equation of State of Distorted fcc-Lanthanum above Megabar Pressures

Lanthanum (La) is the first member of the rare-earth series of elements that has recently raised considerable interest because of its unique high-Tc superhydride LaH10. Although several studies have found superconductivity and phase transitions in metallic La, there was a lack of experimental evidence for the equation of state (EoS) and superconductivity above one megabar pressure. Here, we extend the pressure range up to 140 GPa to study EoS and superconductivity of lanthanum via electrical transport and X-ray diffraction measurements. The experimental XRD patterns point to a phase transition sequences R3m-Fm3m-Fmmm above 78 GPa. All the experimental pressure-volume data were fitted by the 3rd order Birch-Murnaghan equation: V0 = 35.2 (4) A^3, B0 = 27 (1) GPa and B0' = 4. Superconducting critical temperature Tc(onset) of lanthanum is 9.6 K at 78 GPa, which decreases to 2.2 K at 140 GPa. The upper critical magnetic field Bc2(0) was found to be 0.32-0.43 T at 140 GPa. Ab initio calculations give predicted Tc(A-D)=2.2 K (mu*=0.195), dTc/dP = 0.11-0.13 K/GPa and Hc=0.4 T at 140 GPa.