The Systematic Study on the Stability and Superconductivity of Y-Mg-H Compounds under High Pressure

Structural stabilities of high-pressure YM gH, phases (x = 2 - 10, 12, 14, and 16) and their superconductivities are investigated by employing evolutionary-algorithm-based crystal search combined with first-principles calculations. For predicted candidate structures of YMgHx, the convex hull and phonon analyses reveal seven stable and two metastable phases. For all the predicted phases, superconducting transition temperatures (T-c) are also predicted by using the McMillan formula. P4/mmm-YMgH6 is found having T-c = 76 K at 300 GPa comparable to the boiling temperature of liquid nitrogen, and high-T-c (>= 77 K) being predicted for the H-richer phases, P4/mmm-YMgHg (124 K at 300 GPa), Cmmm-YMgH12 (152 K at 250 GPa), and Fd (3) over barm-YMgH12 (190 K at 200 GPa), which possess clathrate structures composed of H-14, H-18, H-24, and H-24 cages, respectively. To elucidate why the H-rich phases attain high-T-c, electronic and phonon band structures as well as electron-phonon coupling strength are analyzed based on Eliashberg spectral functions. The clathrate structures exhibit both a larger H-driven electronic density of states at the Fermi level and a denser H-driven phonon density of states, correlating with larger EPC constants. These structural and chemical bonding analyses reveal that the highest-T-c phase Fd (3) over barm-YMgH12 has H-4 units formed in the sodalite cage.