Superionic phase transition of copper(I) sulfide and its implication for purported superconductivity of LK-99

TL;DR

This paper investigates the phase transition in copper(I) sulfide and its implications for claims of room-temperature superconductivity in LK-99, emphasizing the need to eliminate copper(I) sulfide for accurate validation.

Contribution

It reveals that copper(I) sulfide's phase transition could explain LK-99's reported properties, highlighting the importance of pure synthesis for verifying superconductivity.

Findings

Copper(I) sulfide exhibits a phase transition at 104°C.

The phase transition affects electrical resistivity and heat capacity.

LK-99 synthesis must exclude copper(I) sulfide for proper validation.

Abstract

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.