LK-99, also called PCPOSOS, is a gray–black (or purple), polycrystalline compound, identified as a copper-doped lead‒oxyapatite. A team from Korea University led by Lee Sukbae (이석배) and Kim Ji-Hoon (김지훈) began studying this material as a potential superconductor in 1999, and in July 2023 published preprints claiming that it acted as a room-temperature superconductor at temperatures of up to 400 K (127 °C; 260 °F) at ambient pressure. Many different researchers attempted to replicate the work, and were able to reach initial results within weeks. By mid-August 2023, the consensus was that LK-99 is not a superconductor at room temperature, and is an insulator in pure form. 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 (김현탁) 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, heated at 725 °C (1,000 K; 1,340 °F) for 24 hours:
PbO + Pb(SO4) → Pb2(SO4)O. Second, 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 550 °C (820 K; 1,000 °F) for 48 hours:
3 Cu + P → Cu3P. Finally, lanarkite and copper phosphide crystals are ground into a powder, placed in a sealed tube under a vacuum, and heated to 925 °C (1,200 K; 1,700 °F) for between 5‒20 hours:
Pb2(SO4)O + Cu3P → Pb10-xCux(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 (Cu2S) as well. For x = 1 {\displaystyle x=1} a balanced reaction might be:
5 Pb2SO4O + 6 Cu3P → Pb9Cu(PO4)6O + 5 Cu2S + Pb + 7 Cu. Many syntheses produced fragmentary results in different phases, where some of the resulting fragments were responsive to magnetic fields, and 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 is a sign of regular diamagnetism or ferromagnetism, however it was misinterpreted by some as a sign of superconductivity. While initial preprints claimed the material was a room-temperature superconductor, they did not report observing any definitive properties 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. Because 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 properties. None of these properties was ever 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−). Kim Hyun-Tak proposed that this quantum well could be superconducting, in a 2021 paper 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. On 31 July 2023, Sinéad Griffin of Lawrence Berkeley National Laboratory analyzed LK-99 with density functional theory (DFT), showing that its structure might have correlated isolated flat bands, which might contribute to superconductivity. However, while other researchers agreed with the DFT analysis, a number suggested that this was not compatible with superconductivity, and that a structure different from what was described in Lee, et al. would be necessary. In August, a study by Alexandru Georgescu at Indiana University did not find flat bands at Fermi level, concluding that they related to an unfavored high-symmetry structure.
Proposed absence of superconductivity Analyses by industrial and experimental physicists noted experimental and theoretical shortcomings of the published works. Shortcomings included the lack of phase diagrams spanning temperature, stoichiometry, and stress; the lack of pathways for the very high Tc of LK-99 compared to prior heavy fermion superconductors; the absence of flux pinning in any observations; the possibility of stochastic conductive artifacts in conductivity measurements; the high resistance and low current capacity of the alleged superconducting state; and the lack of direct transmission electron microscopy (TEM) of the materials.
Compound name The name LK-99 comes from the initials of Lee and Kim, and the year they first started working with the material (1999). The pair had worked with Tong-Seek Chair (최동식) at Korea University in the 1990s. In 2008, they founded the Quantum Energy Research Centre (퀀텀 에너지연구소; also known as Q-Centre) with other researchers from Korea University. Lee would later become CEO of Q-Centre, and Kim would become director of research and development.
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