Bandgap and Raman Signature of Undoped and Cu-doped Lead Phosphate Apatite Pb₁₀(PO₄)₆O

Recent claims of a room-temperature ambient-pressure superconductor by Korean scientists have ushered in a worldwide race to replicate the superconductivity of a new compound Pb_10-xCu_x(PO₄)₆O called LK-99. In the meantime, many theory papers have appeared to calculate the band structure of LK-99 and explore its potential superconductivity. Despite these efforts, the basic properties of LK-99 and its parent compound Pb₁₀(PO₄)₆O are still missing. Here we first synthesized Pb₁₀(PO₄)₆O, and obtained its Raman spectrum and electronic bandgap using UV-Vis spectroscopy, confirming that Pb₁₀(PO₄)₆O is a wide bandgap insulator. We then synthesized two types of Cu-doped Pb₁₀(PO₄)₆O and used Raman and UV-Vis to investigate the effect of doping. Raman spectra reveal that LK-99 synthesized using the original Korean recipe is highly inhomogeneous, while Cu-doped Pb₁₀(PO₄)₆O with our own recipe has a relatively pure single phase. Despite the evidence of incorporation of Cu in Pb₁₀(PO₄)₆O from EDX mapping, no Raman shift due to doping is observed in both samples. On the other hand, both Cu-doped samples exhibit slightly lower bandgaps than Pb₁₀(PO₄)₆O, in agreement with reported calculations. Proper doping of Cu is responsible for the possible transition of Pb₁₀(PO₄)₆O from insulating to metallic and even superconducting, Raman signature and bandgap of Pb₁₀(PO₄)₆O pave the way for further investigation and replication of LK-99.