Structural phase transitions and superconductivity of YC₂ from first-principles calculations

The transition metal carbides have attracted considerable attentions in recent years since they exhibit superconductivity behaviors under high pressure. The structural, electronic and lattice dynamic properties of YC₂ at high pressures are essential to understand the superconducting behavior and its underlying mechanisms. Currently, these properties remain unclear. Here, we systematically study these properties over a wide range of pressure and temperature conditions by using evolutionary structure search methods combined with first-principles calculations. Four new crystalline phases of YC₂ are identified, and a series of structural phase transitions driven by temperature or pressure are revealed in our calculations. At atmospheric pressure, the C2/m phase is energetically more stable than the experimentally observed α phase, and thus the C2/m could be ground-state structure of YC₂. The superconducting critical temperature T_c for the P6/mmm phase is predicted to be as high as 10.5 K, which is around three times larger than the existing α phase. Further study shows that the softening vibrational modes induced by fluctuant sp² hybridization in the graphene-like layer of the YC₂ should be responsible for the large electron-phonon coupling parameter λ and thus this large λ leads to the high T_c. Current predictions about these new properties call for further experimental exploration and verification.