Lee et al. report that the compound LK-99, with a chemical formula of Pb 10-x Cu x (PO 4 ) 6 O (0.9 < x < 1.1), exhibits room temperature superconductivity under ambient pressure. In this study, we investigated the transport and magnetic properties of pure Cu 2 S and LK-99 containing Cu 2 S. We observed a sharp superconducting-like transition and a thermal hysteresis behavior in the resistivity and magnetic susceptibility. However, we did not observe zero resistivity below the transition temperature. We argue that the so-called superconducting behavior in LK-99 is most likely due to a reduction in resistivity caused by the first-order structural phase transition of Cu 2 S at around 385 K, from the hexagonal structure phase at high temperature to the monoclinic structure phase at low temperature.
We successfully synthesized polycrystalline LK-99-like ceramic samples with a solid-state-sintering method. Powder X-ray diffraction shows that the main contents are Pb 10− x Cu x (PO 4 ) 6 O and Cu 2 S, consistent with recent reports (arXiv: 2307.12037; arXiv: 2308.01192). In some small flaky fragments, we successfully observed “half levitation” atop a Nd 2 Fe 14 B magnet. Using magnetization measurements on such small pieces, as well as on a large piece which does not exhibit the half levitation, we show that the samples ubiquitously contain weak yet definitive soft ferromagnetic components. We argue that, together with the pronounced shape anisotropy of the small fragments, the soft ferromagnetism is sufficient to explain the observed half levitation in strong vertical magnetic fields. Our measurements do not indicate the presence of the Meissner effect, nor zero resistance, in our samples, leading us to believe that our samples do not exhibit superconductivity. The precise chemical composition and the physics behind the ferromagnetic component remain outstanding questions to be addressed in future research.
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