Physical Origin of Color Changes in Lutetium Hydride under Pressure

Recently,near-ambient superconductivity was claimed in nitrogen-doped lutetium hydride(LuH_(3-δ)N_(ε)).Unfortunately,all follow-up research still cannot find superconductivity signs in successfully synthesized lutetium dihydride(LuH_(2)) and N-doped LuH_(2±x)N_(y).However,a similar intriguing observation was the pressure-induced color changes(from blue to pink and subsequent red).The physical understanding of its origin and the correlation between the color,crystal structure,and chemical composition of Lu–H–N is still lacking.In this work,we systematically investigated the optical properties of LuH_(2) and LuH_(3),and the effects of hydrogen vacancies and nitrogen doping using the first-principles calculations by considering both interband and intraband contributions.Our results demonstrate that the evolution of reflectivity peaks near blue and red light,which is driven by changes in the band gap and Fermi velocity of free electrons,resulting in the blue-to-red color change under pressure.In contrast,LuH_(3) exhibits gray and no color change up to 50 GPa.Furthermore,we investigated the effects of hydrogen vacancies and nitrogen doping on its optical properties.Hydrogen vacancies can significantly decrease the pressure of blue-to-red color change in LuH_(2) but do not have a noticeable effect on the color of LuH_(3).The N-doped LuH_(2) with the substitution of a hydrogen atom at the tetrahedral position maintains the color change when the N-doping concentration is low.As the doping level increases,this trend becomes less obvious,while other N-doped structures do not show a blue-to-red color change.Our results can clarify the origin of the experimental observed blue-to-red color change in lutetium hydride and also provide a further understanding of the potential N-doped lutetium dihydride.