Investigation on the mechanism of electronic structure and superconductivity of cubic X2BH6 at ambient pressure

Abstract

The search for high T_c materials under ambient pressure remains a central objective in materials science. This study investigates doping the BH₆ units (B as the primary transition metal) using Group 2 and Group 3 elements (X) as dopants, focusing on charge transfer between dopants and the BH₆ units and its effects on bonding and superconductivity. Doping modulates the E_F position and influences electron pairing. Magnesium is unique due to its electride properties under ambient pressure, generating localized anionic electrons in interstitial sites. These electrons facilitate charge transfer between the dopant and BH₆ units, enhancing the DOS at the E_F from hydrogen, whereas other dopants with d-orbital electrons suppress the contribution of hydrogen. In contrast to high-pressure hydrides, where covalent bonding stabilizes structures and superconductivity, the appearance of positron cluster and ionic bonds within the BH₆ units in the X₂BH₆ system strongly supports the structure exhibiting significant ionic character and charge redistribution. This ionic nature increases the free electron count, enhancing Cooper pair formation and superconductivity. Particularly in Mg₂IrH₆, Mg's localized electrons are readily accepted by hydrogen, boosting the hydrogen-derived DOS at the E_F. This activates mid-to high-frequency phonon modes, driving large electron-phonon matrix elements and inducing strong electron-phonon coupling, resulting in a high T_c of 103.4 K under ambient pressure.