One-step synthesis of Cu-doped Pb10(PO4)6Cl2 apatite: a wide-gap semiconductor

Abstract

The recent claim of potential room-temperature superconductivity in Pb_10−xCu_x(PO₄)₆O has attracted widespread attention. However, the signature of superconductivity is later attributed to the Cu₂S impurity formed during the multiple-step synthesis procedure. Here we report a simple one-step approach for synthesizing single-phase chloride analogue Cu-doped Pb₁₀(PO₄)₆Cl₂ using PbO, PbCl₂, CuCl₂, and NH₄H₂PO₄ as starting materials. Irrespective of the initial stoichiometry, Cu doping always leads to a lattice expansion in Pb₁₀(PO₄)₆Cl₂. This indicates that Cu prefers to reside in the hexagonal channels rather than as substitutes at the Pb sites, and the chemical formula is expressed as Pb₁₀(PO₄)₆Cu_xCl₂. All the Pb₁₀(PO₄)₆Cu_xCl₂ (0 ≤ x ≤ 1.0) samples are found to be semiconductors with wide band gaps of 4.46–4.59 eV, and the Cu-doped ones (x = 0.5 and 1.0) exhibit a paramagnetic behavior without any phase transition between 400 and 1.8 K. Our study calls for a reinvestigation of the Cu location in Pb_10−xCu_x(PO₄)₆O and supports the absence of superconductivity in this oxyapatite.