First-order transition in LK-99 containing Cu2S

Lee et al. report that the compound LK-99, with a chemical formula of Pb_10-xCu_x(PO₄)₆O (0.9 < x < 1.1), exhibits room temperature superconductivity under ambient pressure. In this study, we investigated the transport and magnetic properties of pure Cu₂S and LK-99 containing Cu₂S. 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 Cu₂S at around 385 K, from the hexagonal structure phase at high temperature to the monoclinic structure phase at low temperature.