Microscopic Characterization of Pb10−xCux(PO4)6O by 31P and 63/65Cu NMR Measurements

The report of the first room-temperature, ambient-pressure superconductivity in copper-doped lead apatite Pb10−xCux(PO4)6O has attracted lots of attention. However, subsequent studies revealed the presence of numerous impurity phases in the polycrystalline sample, and the sharp superconducting-like transition is not due to a superconducting transition but most likely due to a reduction in resistivity caused by the first-order structural phase transition of Cu2S at around 385 K from the β phase at high temperature to the γ phase at low temperature. Before now, only bulk measurements have been performed on a Pb10−xCux(PO4)6O powder sample, which could be affected by the impurity phases, masking the intrinsic properties of Pb10−xCux(PO4)6O. In this study, 31P and 63/65Cu nuclear magnetic resonance (NMR) measurements have been performed on a Pb10−xCux(PO4)6O powder sample to investigate its physical properties from a microscopic point of view. Our NMR data evidence the non-magnetic insulating nature of Pb10−xCux(PO4)6O without any trace of electron correlation effects. Furthermore, the 63/65Cu NMR results suggest that no copper or very little copper is substituted for Pb in Pb10(PO4)6O prepared by sintering Pb2SO5 and Cu3P.