Ab Initio Investigation of the Physical Properties of Pb-Rich Compounds Pb₂X (X = Rh, Pd, and Au)

In this study, detailed physical properties including electronic, elastic, mechanical, phonon, and electron-phonon interaction properties of three Pb-rich compounds Pb₂X (X = Rh, Pd, and Au) have been researched by performing scalar relativistic and full relativistic ab initio pseudopotential calculations based on density functional theory with its generalized gradient approximation. A comparison of scalar relativistic and full relativistic results reveals that the existence of Pb 6p electrons at the Fermi level increases with the enhanced mass of the X element in Pb₂X compounds, making the effect of spin-orbit coupling on their physical properties more pronounced because Pb 6p electrons are well-known to have a strong spin-orbit interaction. In particular, when spin-orbit coupling is taken into account, the electron-phonon interaction in all the examined compounds becomes stronger with being the strongest for Pb₂Au. As a consequence, the calculation neglecting spin-orbit coupling underestimates the superconducting transition temperatures of the examined compounds, while the calculation including spin-orbit coupling improves the accordance with experiment satisfactorily, signaling the necessity of this coupling in determining the physical properties of the three Pb-rich compounds Pb₂X.