A high-pressure ten-fold coordinated structure in hafnium dihydrogen with increasing superconducting transition temperature induced by the enhancive pressure

Abstract High pressure is an effective method to cause structural and electronic variations, further inducing novel high-pressure structures with excellent physical and chemical properties. Herein, we investigate the structural phase transition of hafnium dihydrogen (HfH2) in the pressure range of 0-500 GPa through the first-principles calculations and CALYPSO crystal structure prediction. The high-pressure phase transition sequence of HfH2 is I4/mmm →Cmma →P-3m1 and the two phase transition pressure points are 220.21 and 359.18 GPa, respectively. A newly trigonal P-3m1 structure with 10-fold coordination firstly appears as an energy superior structure under high pressure. These three structures are all metallic with the internal ionic bonding of Hf and H atoms. Moreover, the superconducting transition temperature (T c) values of Cmma at 300 GPa and P-3m1 at 500 GPa are 3.439 K and 19.737 K, respectively. Interestingly, the superconducting transition temperature of the P-3m1 structure presents an upward trend as the pressure climbs, which can be attributed to the increment of electron-phonon coupling eventuated by the enhanced Hf-d electronic density of states at Fermi level under high pressure.