Robust structural stability in tI2-La with constant axial ratio under pressures exceeding 330 GPa

Abstract Using synchrotron x-ray diffraction, this study systematically investigated the crystal structures, equations of state, and compressibility of lanthanum under pressures reaching 337.2 GPa. A sequential phase transition series was observed: dhcp ( hP4 ) → fcc ( cF4 ) → distorted fcc ( hR24 ) → distorted fcc ( oI16 ) → fcc ( cF4 ) → I4/mmm ( tI2 ), with the tI2 phase fully established at 82.8 GPa. Notably, tI2 -La remained stable up to 337.2 GPa with a constant c / a axial ratio above 260 GPa, showing structural similarity to lanthanides Ce, Pr and actinide Th under the highest pressure studied so far. Equation of state analysis indicated that tI2 -La exhibited compressive behavior akin to that of simple metals such as Al, Au and Cu, suggesting it possesses nearly ideal metallic properties under ultrahigh pressures. These findings provide valuable insights into the structural evolution and mechanical properties of lanthanum under extreme conditions, and also shed light on the behavior of other lanthanides and actinides under high pressure.