Lee, Kim, and coworkers have recently claimed room-temperature and ambient-pressure superconductivity in a copper-doped lead apatite material named LK-99. However, the polycrystalline material synthesized has a significant fraction of copper(I) sulfide. Copper(I) sulfide has a known phase transition at 104 degrees C from an ordered low-temperature phase to a high-temperature superionic phase. As a result of this phase transition, copper(I) sulfide exhibits sharp transitions in electrical resistivity and heat capacity, which are expected to coincide with the temperature-induced transitions reported for LK-99. This implies that LK-99 must be synthesized without any copper(I) sulfide to allow unambiguous validation of the superconducting properties of LK-99.
@article{arxiv.2308.05222,
title = {Superionic phase transition of copper(I) sulfide and its implication for purported superconductivity of LK-99},
author = {Prashant K. Jain},
journal= {arXiv preprint arXiv:2308.05222},
year = {2023}
}