Stable Structure And Superconductivity Of YBaH_n (n=2-12) System Under High Pressure

Metal hydrogen has potential high temperature superconductivity,but it needs very high pressure.Hydrides can pre adsorb hydrogen,thus reducing the pressure required for the stable existence of the system.Scientists have been trying to find high temperature superconductors,including hydride superconductors under high pressure.BaH₂ is the most stable stoichiometry at various pressures,but the superconducting transition temperature(T_c)of BaH₂ was found in the experiment to be only few m K at 60GPa.On the other hand,Y-H system is a good superconductor system.In order to improve the superconducting critical transition temperature of Ba-H system,we introduced light rare earth Y into Ba-H system and studied the structure,thermodynamic stability and superconductivity of Y-Ba-H system with 11 components.At 50-200 GPa,the stable structure of YBaH_n(n=2-12)was searched by genetic algorithm(GA)combined with first principles structural optimization method,and the structural stability was analyzed.The critical superconducting transition temperature T_c of Y-Ba-H system at high pressure was calculated by Quantum Espressp software,and its band structure,phonon band,electro acoustic coupling and superconductivity were analyzed.The results show that YBaH₅ is the most stable component at 50-100 GPa,and the phase transition occurs from P3m1 to R3m at 73 GPa.At 150-200 GPa,P6/mmm-YBaH₄ is the most stable phase.At 50 GPa,the T_c of P6/mmm-YBaH₄ and R3m-YBaH₅ are 12.9 and6.7 K,respectively.YBaH₈ is not the most stable phase at 50-200 GPa,but a metastable phase.Through the analysis of triangular phase diagram,the possible synthesis path of P4/mmm-YBaH₈ is BaH₂+YH₆?YBaH₈.The T_c of P4/mmm-YBaH₈ is 52.2 and 95.3 K at 50 and 150 GPa,respectively.In addition,we also calculated La BaH₈.It was found that the T_c of La BaH₈ is 39.6 and 48.7 K at 50 and 150 GPa,respectively.YBaH₈ has higher T_cthan La BaH₈.Compared with BaH₂ and YH₆,the T_c of YBaH₈ is between the former two,which is much higher than that of BaH₂ and lower than that of YH₆.However,the minimum stable pressure of YBaH₈ is 50 GPa,which is lower than that of YH₆ at 100 GPa.This study can provide some theoretical information and reference value for the experimental study of Y-Ba-H system.