  Lee et al. reported that the compound LK99, 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 $\beta$ phase at high temperature to the $\gamma$
phase at low temperature.
