LK-99
A purported room-temperature superconductor that proved to be an insulator.
LK-99, also referred to as PCPOSOS, is a polycrystalline compound that appears gray–black or purple. Chemically, it is a copper-doped lead oxyapatite. Researchers from Korea University, led by Lee Sukbae and Kim Ji-Hoon, first investigated this material as a possible superconductor starting in 1999. In July 2023, they released preprints claiming it functioned as a room-temperature superconductor at up to 400 K (127 °C) under ambient pressure.
Within weeks, many independent teams attempted to replicate the results. By mid-August 2023, the scientific consensus was that LK-99 is not a room-temperature superconductor; in its pure form, it is an insulator. Several replication efforts identified non-superconducting causes—such as ferromagnetism and diamagnetism—for the observations that had been interpreted as signs of superconductivity. A key culprit was a copper sulfide impurity that can form during the synthesis, which may produce resistance drops, a lambda transition in heat capacity, and magnetic responses in small samples.
After the initial preprints appeared, Lee stated they were incomplete, and coauthor Kim Hyun-Tak acknowledged that one of the papers contained flaws.
**Chemical properties and structure** LK-99 has an approximate composition of Pb₉Cu(PO₄)₆O. Compared to pure lead-apatite (Pb₁₀(PO₄)₆O), about one-quarter of the Pb(II) ions in position 2 of the apatite structure are replaced by Cu(II) ions. Its structure is similar to that of apatite, with space group P6₃/m (No. 176).
**Synthesis** Lee and colleagues described a three-step synthesis. First, lanarkite is made by heating a 1:1 molar mixture of lead(II) oxide and lead(II) sulfate powders at 725 °C for 24 hours: PbO + Pb(SO₄) → Pb₂(SO₄)O. Second, copper(I) phosphide is produced by heating copper and phosphorus powders in a 3:1 molar ratio in a sealed evacuated tube at 550 °C for 48 hours: 3 Cu + P → Cu₃P. Finally, the lanarkite and copper phosphide crystals are ground together, sealed in an evacuated tube, and heated at 925 °C for 5–20 hours: Pb₂(SO₄)O + Cu₃P → Pb₁₀₋ₓCuₓ(PO₄)₆O + S(g), where 0.9 < x < 1.1.
The original paper’s synthesis had several issues. The reaction was not balanced, and others reported the presence of copper(I) sulfide as well. For x = 1, a balanced reaction might be: 5 Pb₂SO₄O + 6 Cu₃P → Pb₉Cu(PO₄)₆O + 5 Cu₂S + Pb + 7 Cu. Many syntheses produced fragmentary results
- field
- Materials science, condensed matter physics
- known_for
- Claimed room-temperature superconductor later shown to be an insulator
- composition
- Approximately Pb9Cu(PO4)6O
- structure
- Apatite-like, space group P63/m (No. 176)
- synthesis
- Three-step process involving lanarkite and copper(I) phosphide
Lore & Background
The material was first studied by Lee Sukbae and Kim Ji-Hoon at Korea University starting in 1999, and they founded the Quantum Energy Research Centre (Q-Centre) in 2008. In July 2023, two preprints on arXiv claimed LK-99 was a room-temperature superconductor at up to 400 K. However, the preprints were criticized as incomplete, and coauthor Kim Hyun-Tak said one contained flaws. Within weeks, many researchers attempted replication and by mid-August 2023 the consensus was that LK-99 is not a superconductor; pure samples are diamagnetic insulators. Non-superconducting ferromagnetic and diamagnetic causes, particularly a copper sulfide impurity, were identified as responsible for misleading observations.
Reader's Guide
LK-99 gained widespread attention in July 2023 as a potential breakthrough in room-temperature superconductivity, but the claim collapsed under scrutiny within weeks. The episode highlights the importance of rigorous replication and the need to observe multiple definitive properties of superconductivity—such as zero resistance, the Meissner effect, and flux pinning—none of which were ever demonstrated for LK-99. The material's initial magnetic responses were misinterpreted as partial levitation, but were later explained by ordinary diamagnetism or ferromagnetism from impurities. The controversy also underscored issues in the publication process, including incomplete preprints and a patent history. Ultimately, LK-99 serves as a cautionary tale about the challenges of verifying extraordinary claims in condensed matter physics.
Did You Know?
- The name LK-99 comes from the initials of Lee and Kim, and the year 1999 when they first started working with the material.
- A prominent cause of misleading observations was a copper sulfide impurity that can produce resistance drops and magnetic responses in small samples.
- The first synthesis to produce pure crystals of LK-99 found them to be diamagnetic insulators.
- None of the definitive properties of superconductivity—such as zero resistance, the Meissner effect, or flux pinning—were ever observed by the original experiment or any replications.
More in Discovery and invention controversies 1-24
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record