Optical properties of LK-99 and Cu2S

We investigated Pb10-xCux(PO4)6 (0.9 < x < 1.1) (LK-99) and Cu2S, presumed to be contained as an impurity in LK-99, in a wide spectral range from far infrared to ultraviolet using optical spectroscopy. The optical conductivity spectra of both samples were obtained from measured reflectance spectra at various temperatures from 80 to 434 K. Both samples showed several infrared-active phonons in the far and mid-infrared regions. LK-99 showed typical insulating features with a band gap of ~1 eV. Cu2S showed a nonmonotonic temperature-dependent trend and two energy gaps: one energy gap of ~93 meV and a band gap of 2.42 eV. Our results indicate that LK-99 cannot be a superconductor because it is an insulator with a large band gap.