M A T T E R A T E X T R E M E S

S C I E N T I F I C H I G H L I G H T S

3 2 H I G H L I G H T S 2 0 2 2 I

stacking faults and regions described as diaphite with graphene layers coherently bonded to the diamond matrix. The complexity reported in the Canyon Diablo sample can occur in a wide range of carbonaceous materials produced by shock and static compression or by deposition from a vapour. The recognition of the various graphene and diamond stackings is relevant for understanding the pressure-temperature conditions that occur during asteroid impacts and the rich variety of diaphites that can form. Through the controlled layer growth of structures, it should be possible to design materials that are both ultra-hard and

also ductile, as well as with adjustable electronic properties. The new discovery has therefore opened the door to new carbon materials with exciting mechanical and electronic properties that may result in new applications ranging from abrasives and electronics to nanomedicine and laser technology.

The researchers are grateful to the late co-author Professor Paul McMillan for bringing the team together, his tireless enthusiasm for this work and his lasting contributions to the field of diamond research.

PRINCIPAL PUBLICATION AND AUTHORS

Shock-formed carbon materials with intergrown sp3- and sp2-bonded nanostructured units, P. Németh (a), H.J. Lancaster (b), C.G. Salzmann (c), K. McColl (d), Z. Fogarassy (e), L.A.J. Garvie (f), L. Illés (e), B. Pécz (e), M. Murri (g), F. Corà (c), R.L. Smith (c), M. Mohamed (h), C.A. Howard (b), P.F. McMillan (c), Proc. Natl. Acad. Sci. USA 119, e2203672119 (2022); https:/doi.org/10.1073/pnas.2203672119 (a) Institute for Geological and Geochemical Research, Research Centre for Astronomy and Earth Sciences (MTA Centre of Excellence), Eötvös Loránd Research Network, Budapest (Hungary) (b) Department of Physics & Astronomy, University College London (UK) (c) Department of Chemistry, University College London (UK) (d) Department of Chemistry, University of Bath (UK) (e) Institute of Technical Physics and Materials Science, Centre for Energy Research (MTA Centre of Excellence), Eötvös Loránd Research Network, Budapest (Hungary) (f) Buseck Center for Meteorite Studies, Arizona State University (USA) (g) Department of Earth and Environmental Sciences, University of Milano-Bicocca (Italy) (h) ESRF

REFERENCES

[1] A.E. Foote, Am. J. Sci. 42, 413-417 (1891). [2] C. Frondel & U.B. Marvin, Nature 214, 587-589 (1967).

Unique clathrate hydrates grown and studied at high pressure and low temperature

In-situ studies at pressures up to 1.2 GPa and temperatures down to 230 K were carried out to grow and structurally characterise four clathrate hydrates of acetone. It is shown that clathrates hosting polar molecules are not as exotic as previously thought and could be stabilised at high pressure conditions through hydrogen bonding.

Clathrate hydrates are crystalline compounds built on the host−guest principle: hydrogen-bonded water molecules are configured as cages that host small molecules of guest substances. Most of the known clathrates are formed with nonpolar guests, i.e., with molecules possessing no partial positive or negative charges (e.g., N2, CH4, CO2). However, several studies over the past two decades have suggested that polar molecules may nonetheless be enclathrated in the presence of clathrate promoters such as CH4 or CO2. Hydrogen bonding between guests and host water can be formed in this case, as has been shown for alcohols, small ethers and ketones. The phenomenon of guest−host hydrogen bonding and its effect on physical properties

have become subjects of extensive research in the field of clathrate hydrate science. So far, host−guest hydrogen bonding has been known only for binary (i.e., having two types of guests) clathrates grown in the presence of a helper gas at ambient or low pressures of a few MPa. Here, in-situ single-crystal X-ray diffraction (SC-XRD) evidence is provided for guest−host hydrogen bonding for several acetone clathrates grown at high-pressure (HP)-low- temperature (LT) conditions.

Single crystals of acetone clathrate hydrates were grown from acetone−water solution in pressure and temperature ranges of 0.25−1.2 GPa and 230−300 K at beamline ID15B. The HP and LT conditions were maintained by a cryostat-cooled diamond anvil cell. The formation of four different clathrate structures was observed: trigonal (sTr), two orthorhombic (sO, sO-II) and tetragonal (sT), all different in the architecture of the host frameworks (Figure 24). The frameworks of clathrates are typically described in terms of polyhedral cages, where each edge represents a pair of hydrogen-bonded water molecules. Commonly, cages are noted as 4x5y6z, where x, y and z are the number of tetra-, penta- and hexagonal faces, respectively. The sTr reveals the most complicated architecture built upon four cage types: 425861 (U), 51262 (T), 51263 (P) and 4151063 (T ).