LK-99 (from the Lee-Kim 1999 research) is a gray–black, polycrystalline compound, identified as a copper-doped lead‒oxyapatite. A team from Korea University led by Lee Sukbae () and Kim Ji-Hoon () began studying this material as a potential superconductor starting in 1999. In July 2023, they published preprints claiming that it acts as a room-temperature superconductor at temperatures of up to at ambient pressure.
Many different researchers have attempted to replicate the work, and were able to reach initial results within weeks, as the process of producing the material is relatively straightforward. By mid-August 2023, the consensus was that LK-99 is not a superconductor at any temperature, and is an insulator in pure form.
As of October 1, no replications have gone through the peer-review process of a journal, but some have been reviewed by a materials science lab. A number of replication attempts identified non-superconducting ferromagnetic and diamagnetic causes for observations that suggested superconductivity. A prominent cause was a copper sulfide impurity occurring during the proposed synthesis, which can produce resistance drops, lambda transition in heat capacity, and magnetic response in small samples.
After the initial preprints were published, Lee claimed they were incomplete, and coauthor Kim Hyun-Tak () said one of the papers contained flaws.
Chemical properties and structure
The chemical composition of LK-99 is approximately Pb9Cu(PO4)6O, in which— compared to pure lead-apatite (Pb10(PO4)6O)— approximately one quarter of Pb(II) ions in position 2 of the apatite structure are replaced by Cu(II) ions.
The structure is similar to that of apatite, space group P63/m (No. 176).
Synthesis
Lee et al. provide a method for chemical synthesis of LK-99 in three steps. First they produce lanarkite from a 1:1 molar mixing of lead(II) oxide (PbO) and lead(II) sulfate (Pb(SO4)) powders, and heating at for 24 hours:
PbO + Pb(SO4) → Pb2(SO4)O.
Then, copper(I) phosphide (Cu3P) is produced by mixing copper (Cu) and phosphorus (P) powders in a 3:1 molar ratio in a sealed tube under a vacuum and heated to for 48 hours:
3 Cu + P → Cu3P.
Then, lanarkite and copper phosphide crystals are ground into a powder, placed in a sealed tube under a vacuum, and heated to for between 5‒20 hours:
Pb2(SO4)O + Cu3P → Pb10-Cu(PO4)6O + S (g), where 0.9 < x < 1.1.
There were a number of problems with the above synthesis from the initial paper. The reaction is not balanced, and others reported the presence of copper(I) sulfide () as well. For a balanced reaction might be:
5 .
Many syntheses produced fragmentary results in different phases, where some of the resulting fragments were responsive to magnetic fields, other fragments were not. The first synthesis to produce pure crystals found them to be diamagnetic insulators.
Physical properties
Some small LK-99 samples were reported to show strong diamagnetic properties, including a response confusingly referred to as "partial levitation" over a magnet. This was misinterpreted by some as a sign of superconductivity, although it is a sign of regular diamagnetism or ferromagnetism.
While initial preprints claimed the material was a room-temperature superconductor, they did not report observing any definitive features of superconductivity, such as zero resistance, the Meissner effect, flux pinning, AC magnetic susceptibility, the Josephson effect, a temperature-dependent critical field and current, or a sudden jump in specific heat around the critical temperature.
As it is common for a new material to spuriously seem like a potential candidate for high-temperature superconductivity, thorough experimental reports normally demonstrate a number of these expected properties. not one of these properties had been observed by the original experiment or any replications.
Proposed mechanism for superconductivity
Partial replacement of Pb2+ ions with smaller Cu2+ ions is said to cause a 0.48% reduction in volume, creating internal stress in the material, causing a heterojunction quantum well between the Pb(1) and oxygen within the phosphate ([PO4]3−). This quantum well was proposed to be superconducting, based on a 2021 paper by Kim Hyun-Tak describing a novel and complicated theory combining ideas from a classical theory of metal-insulator transitions, the standard Bardeen–Cooper–Schrieffer theory, and the theory of hole superconductivity by J.E.Hirsch.