Maeno et al. reported it in 1994 in Nature (372, 532–534) as superconductivity in a layered perovskite without copper.
It adopts the K₂NiF₄ body-centered tetragonal structure and superconducts at 0.93 K, rising to about 1.5 K in the best samples.
1998 oxygen-17 Knight-shift work argued for spin-triplet pairing, but Pustogow et al. (Nature, 2019) showed those NMR measurements contained artifacts, reviving even-parity interpretations.