A comparative first-principles study of the structural and electronic properties of the liquid Li-Si and Li-Ge alloys

We have performed a comparative first-principles study on the structural and electronic properties of the liquid Li_1-xSi_x and Li_1-xGe_x alloys over a range of composition from x = 0.09 to 0.50. Our calculations showed that Si and Ge atoms can exhibit very distinct local bonding characteristics as they were alloyed with the Li atoms in the liquid state, where Si atoms tended to form a variety of covalent bonding configurations while Ge atoms predominantly appeared as the isolated anions in the liquid alloys. These differences in bonding characteristics were reflected in their electronic density of states, in which the liquid Li_1-xGe_x alloys have a lower degree of s-p hybridization with narrower distributions of the 3s and 3p states than the liquid Li_1-xSi_x alloys. Our calculations also showed that the optical conductivities of these two liquid alloys can undergo a transition from the Drude-like metallic nature to the semiconductor-like character as the Si/Ge content increases from 0.09 to 0.22. However, as the Si/Ge content further increases to 0.50, the liquid Li_1-xGe_x alloys may transit to exhibit the Drude-like metallic nature, while the liquid Li_1-xSi_x alloys can still hold the semiconductor-like character. Moreover, our calculations revealed that the dc conductivities of these liquid alloys are predominantly determined by the number of total electronic states at the Fermi level. As the liquid Li_1-xSi_x alloys are within the composition range between 0.20 and 0.50, the increment of the states at the Fermi level with increasing the Si content is nearly identical to the amount of the Li states decreased, leading to an almost unchanged number of total electronic states at the Fermi level. However, since Ge atoms do not favor forming covalent bonding in the liquid alloys to keep the Fermi level at a minimum of the density of states, the liquid Li_1-xGe_x alloys would have more electronic states at the Fermi level and thereby higher dc conductivities than the liquid Li_1-xSi_x alloys within the same composition range.