Phase transitions and properties of lanthanum under high pressures.

Lanthanum (La), the first member of the lanthanide elements, recently aroused interests due to the discovery of high- T c superconductor LaH 10 under high pressures and its unique superconducting properties. Here, we study the phase transitions, superconductivity and mechanical properties of metallic La under high pressures by first-principle calculations. The known face-centered cubic (fcc) phase with space groupFm3¯mstill exists above 100 GPa. And it transforms into an unprecedented body-centered tetragonal (bct) phase with space group I 4/ mmm above 180 GPa, which expands the high pressure phase transition sequence. Further calculations show that the superconducting transition temperature T c of fcc phase decreases with increasing pressure with the rate of -0.13 K GPa -1 , in good agreement with the experimental results. For the bct phase, the estimated superconducting transition temperature is very low with T c of 0.7 K at 200 GPa. The calculations of mechanical properties show that both of fcc and bct phases are compressible and brittle.