Theoretical Modeling of Metallic Compounds with Versatile Properties by Combining First-Principles Calculations and Global Structure Prediction Algorithms

Improving the target properties of existing materials or finding new materials with enhanced functionality for practical applications is at the heart of the materials research. In this respect, the first-principles approaches, which have been successfully integrated into modern high- performance computers, have become an indispensable part of the materials research, providing a better understanding of existing materials and guidance on the design of new materials. Using state-of-the-art computational/theoretical approaches that couple global structure prediction with ab initio density functional theory calculations, we investigate structural and electronic properties of Cs_xO [cesium oxides], Li_1+xMn₂O₄ [lithium manganese oxides], and Y₅Si₃H_x [hydrogenated yttrium silicide]. First, we identify thermodynamically stable crystal structures of Cs_xO (1 ≤ x ≤ 3), Li_1+xMn₂O₄ (0 ≤ x ≤ 1), and Y₅Si₃H_x (0 ≤ x ≤ 8), and analyze their properties that are relevant to practical applications. Cs_xO are known as low work function materials, and it is revealed that a very low work function (0.66 eV) is achievable for CsO compound over a relatively wide range of the oxygen chemical potential. Moreover, atomic adsorption on the surface of Cs₃O results in a decrease in the work function, which stems from the fact that it is an electride. In the case of LiMn₂O₄, a promising cathode material for secondary lithium-ion batteries, it is demonstrated that lithium insertion into the spinel structure results in the emergence of various degrees of freedom for crystal formation such as Li ions occupation sites, oxidation states of Mn ions, and local Jahn-Teller distortions. This finding suggests a possible cause of the cycle capacity fade. Finally, Y₅Si₃ is known as an electride that can improve catalytic reaction and reversibly store hydrogen. Investigation on the anionic electrons demonstrates that Y₅Si₃ contains two times more anionic electrons than reported value and that the anionic electrons are robust against lattice vibrations. We also shed light on the hydrogenation mechanism of Y₅Si₃H_x by identifying favorable hydrogen occupation sites and diffusion paths. It is revealed that the 1D hollow space is the dominant diffusion channel, and fast diffusion is possible in a specific hydrogen content range.