Investigating Copper Doping in Lead Oxyapatite: Flux Growth of Copper-Containing Pb₁₀(PO₄)₆O Single Crystals

A flux growth method for the high-yield synthesis of Pb10(PO4)6O single crystals was developed. This optimized synthesis method was utilized for the systematic doping of copper into the lead apatite structure, where the availability of single crystals allowed for a detailed structural analysis via single crystal X-ray diffraction. Our developed synthesis method resulted in a different type of copper doping in lead apatite where instead of Cu(II) doping onto the lead sites, doping occurred via Cu(I) doping into the disordered channels at the center of the Pb(1)/Pb(2) columns in the form of CuO2 linear units. The presence of Cu(I) as opposed to Cu(II) in the structure was shown via magnetic susceptibility data which also indicated a lack of superconductivity in the modified lead apatite at all achieved doping levels. The knowledge gained from this study enabled a reanalysis of the initial diffraction data reported for LK-99. While this work further supports the lack of superconductivity in the modified lead apatites, it does allow us to suggest a more accurate formula for LK-99: Pb10(PO4)6(CuO)2O1–1.5x.