Named after Sukbae Lee, Ji-Hoon Kim, and the year 1999, LK-99 is a hexagonal P63/m lead-oxyapatite ceramic reported in July 2023 preprints as an ambient-pressure superconductor up to 400 K (127 °C).
Worldwide replication studies in August 2023 demonstrated that the apparent resistivity drop and heat-capacity lambda anomaly near 377 K (104 °C) were artifacts of copper(I) sulfide (Cu2S) impurity phase transitions from the unbalanced lanarkite + Cu3P synthesis.
Floating-zone growth of phase-pure single crystals of Pb9Cu(PO4)6O at the Max Planck Institute for Solid State Research proved that intrinsic LK-99 is a transparent purple diamagnetic insulator rather than a superconductor.