Nanosecond X-ray diffraction of shock-compressed superionic water ice

• Published in: Nature (London) Vol. 569; no. 7755; pp. 251 - 255
• Main Authors: Millot, Marius, Coppari, Federica, Rygg, J. Ryan, Correa Barrios, Antonio, Hamel, Sebastien, Swift, Damian C., Eggert, Jon H.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.05.2019; Nature Publishing Group
• Subjects: 639/301/119/2795; 639/33/445/846; 639/766/119/1002; 639/766/119/995; Analysis; Bonding strength; Bridgman method; Compressibility; Computer simulation; CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; Conduction; Conductivity; Confining; Crystal growth; Crystal structure; Crystallinity; Diamonds; Experiments; High temperature; Hugoniot curves; Humanities and Social Sciences; Hydrogen; Hydrogen bonding; Hydrogen bonds; Hydrogen ions; Ice; Ice melting; Ion currents; Laboratories; Lasers; Letter; Melt temperature; Melting; multidisciplinary; Nanotechnology; Numerical simulations; Optical measurement; Oxygen; Phase transitions; Polymorphism; Protons; Science; Science (multidisciplinary); Shock waves; Solidification; Solids; Water; Water analysis; Water ice; Water sampling; X-ray diffraction; X-rays
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-019-1114-6

Since Bridgman’s discovery of five solid water (H 2 O) ice phases 1 in 1912, studies on the extraordinary polymorphism of H 2 O have documented more than seventeen crystalline and several amorphous ice structures 2 , 3 , as well as rich metastability and kinetic effects 4 , 5 . This unique behaviour is due in part to the geometrical frustration of the weak intermolecular hydrogen bonds and the sizeable quantum motion of the light hydrogen ions (protons). Particularly intriguing is the prediction that H 2 O becomes superionic 6 – 12 —with liquid-like protons diffusing through the solid lattice of oxygen—when subjected to extreme pressures exceeding 100 gigapascals and high temperatures above 2,000 kelvin. Numerical simulations suggest that the characteristic diffusion of the protons through the empty sites of the oxygen solid lattice (1) gives rise to a surprisingly high ionic conductivity above 100 Siemens per centimetre, that is, almost as high as typical metallic (electronic) conductivity, (2) greatly increases the ice melting temperature 7 – 13 to several thousand kelvin, and (3) favours new ice structures with a close-packed oxygen lattice 13 – 15 . Because confining such hot and dense H 2 O in the laboratory is extremely challenging, experimental data are scarce. Recent optical measurements along the Hugoniot curve (locus of shock states) of water ice VII showed evidence of superionic conduction and thermodynamic signatures for melting 16 , but did not confirm the microscopic structure of superionic ice. Here we use laser-driven shockwaves to simultaneously compress and heat liquid water samples to 100–400 gigapascals and 2,000–3,000 kelvin. In situ X-ray diffraction measurements show that under these conditions, water solidifies within a few nanoseconds into nanometre-sized ice grains that exhibit unambiguous evidence for the crystalline oxygen lattice of superionic water ice. The X-ray diffraction data also allow us to document the compressibility of ice at these extreme conditions and a temperature- and pressure-induced phase transformation from a body-centred-cubic ice phase (probably ice X) to a novel face-centred-cubic, superionic ice phase, which we name ice XVIII 2 , 17 . The atomic structure of H 2 O is documented at several million atmospheres of pressure and temperatures of several thousand degrees, revealing shockwave-induced ultrafast crystallization and a novel water ice phase, ice XVIII, with exotic superionic properties.