  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.
