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Recently, near-ambient superconductivity was claimed in nitrogen-dopedlutetium hydride (LuH$_{3-\delta}$N$_{\epsilon}$) . Unfortunately, allfollow-up research still cannot find superconductivity signs in successfullysynthesized lutetium dihydride (LuH$_2$) and N-doped LuH$_{2\pm x}$N$_y$.However, a similar intriguing observation was the pressure-induced colorchanges (from blue to pink and subsequent red). The physical understanding ofits origin and the correlation between the color, crystal structure, andchemical composition of Lu-H-N is still lacking. In this work, we theoreticallystudy the optical properties of LuH$_2$, LuH$_3$, and some potential N-dopedcompounds using the first-principles calculations by considering both interbandand intraband contributions. Our results show that LuH$_2$ has an opticalreflectivity peak around blue light up to 10 GPa. Under higher pressure, thereflectivity of red light gradually becomes dominant. This evolution is drivenby changes in the direct band gap and the Fermi velocity of free electronsunder pressure. In contrast, LuH$_3$ exhibits gray and no color change up to 50GPa. Furthermore, we considered different types of N-doped LuH$_2$ and LuH$_3$.We find that N-doped LuH$_2$ with the substitution of a hydrogen atom at thetetrahedral position maintains the color change when the N-doping concentrationis low. As the doping level increases, this trend becomes less obvious. Whileother N-doped structures do not show significant color change. Our results canclarify the origin of the experimental observed blue-to-red color change inlutetium hydride and also provide a further understanding of the potentialN-doped lutetium dihydride.

Physical origin of color changes in lutetium hydride under pressure