biostudies-literatureAndriambariarijaona LEuropean Research CouncilDeutsche Forschungsgemeinschaft (German Research Foundation)Helmholtz AssociationHelmholtz Association | GSI Helmholtzzentrum für Schwerionenforschung GmbH (GSI Helmholtzzentrum für Schwerionenforschung)Agence Nationale de la Recherche (French National Research Agency)374https://www.ebi.ac.uk/biostudies/studies/S-EPMC12796318biostudies-literatureUnknown17(1)The study of superionic (SI) water has been a highly active research area since its theoretical prediction. Despite significant experimental and computational efforts, its melting curve and the stability of different oxygen lattices remain debated, impacting our understanding of SI ice's peculiar transport properties. Experimental results at lower pressures show disagreement, whereas data at higher pressures are scarce due to the extreme challenges of such experiments. In this work, we present ultrafast X-ray diffraction results of water compressed by multiple shocks to pressures up to  ~ 180 GPa. At pressures exceeding 150 GPa and temperatures around 2500 K, our diffraction patterns challenge the pure FCC-SI phase model, providing experimental evidence of the mixed close-packed superionicNature communicationsObservation of a mixed close-packed structure in superionic water.PMC12796318495324226SI-URDK2224 withFOR 2440724690ERC-RA-0041VH-NG-1141ANR-236 22-CE92-003Brambrink EPelka AMcBride EEBethkenhagen MCunningham EHoppner HLefevre FBespalov DSchumacher SKonopkova ZGlenn GDToncian TBergermann AVorberger JSchwinkendorf JPFletcher LBFrost MHerbert MLLecherbourg LSchoenwaelder CGlenzer SHRavasio ALynn WDyer GRips JVoigt KRandolph LHe ZNagler BQu CHumphries OSPandolfi SZastrau UFrench MRanjan DPreston TRStevenson MGGleason AEStrohm CTang MBenuzzi-Mounaix AVinci THartley NJHusband RLee HJHernandez JAHeuser BKuhlke JGarcia ALMay PLutgert JOfori-Okai BKAndriambariarijaona LMasruri MCowan TECafiso SDDDescamps AGaltier ELindqvist BGuarnelli YNakatsutsumi MRedmer RKraus DBaehtz CSchuster AKKhaghani DNaedler JPAppel KfalseObservation of a mixed close-packed structure in superionic water.The study of superionic (SI) water has been a highly active research area since its theoretical prediction. Despite significant experimental and computational efforts, its melting curve and the stability of different oxygen lattices remain debated, impacting our understanding of SI ice's peculiar transport properties. Experimental results at lower pressures show disagreement, whereas data at higher pressures are scarce due to the extreme challenges of such experiments. In this work, we present ultrafast X-ray diffraction results of water compressed by multiple shocks to pressures up to  ~ 180 GPa. At pressures exceeding 150 GPa and temperatures around 2500 K, our diffraction patterns challenge the pure FCC-SI phase model, providing experimental evidence of the mixed close-packed superionic2025-01-01T00:00:00Z2025 Dec2026-06-06T14:51:28.374Z2026-06-01T03:07:58.88ZS-EPMC127963184135467210.1038/s41467-025-67063-2