We report discovery of new metallic and magnetic phases in the van-der-Waals antiferromagnets MPS3, where M = Transition Metal, form an ideal playground for tuning both low-dimensional magnetic and electronic properties[1-4]. These are layered honeycomb antiferromagnetic Mott insulators, long studied as near-ideal 2D magnetic systems with a rich variety of magnetic and electric properties across the family.

We will present magnetic, structural and electrical transport results and compare the behaviour of Fe-, V-, Mn- and NiPS3 as we tune them towards 3D structures – and Mott transitions from insulator to metal. I will show recent results on record high-pressure neutron scattering, which has unveiled an enigmatic form of short-range magnetic order in metallic FePS3.

We have mapped out the full phase diagram - a first in this crucial family of materials. We observe multiple transitions and new states, and an overall increase in dimensionality and associated changes in behaviour.

[1] G. Ouvrard et al., Mat. Res. Bull., 1985, 20, 1181.

[2] C.R.S. Haines et al., Phys. Rev. Lett. 2018, 121, 266801.

[3] M.J. Coak et al., J.Phys.:Cond. Mat. 2019, 32, 124003.

[4] M.J. Coak, et al., Phys. Rev. X, 11, 011024 (2021)

During last decades, the impact of high-pressure studies on fundamental physics, chemistry, and Earth and planetary sciences, has been enormous. Modern science and technology rely on the vital knowledge of matter which is provided by crystallographic investigations. The most reliable information about crystal structures of solids and their response to alterations of pressure and temperature is obtained from single-crystal diffraction experiments. Advances in diamond anvil cell (DAC) techniques, designs of double-stage DACs, and in modern X-ray instrumentation and synchrotron facilities have enabled structural research at multimegabar pressures.

We have developed a methodology for performing single-crystal X-ray diffraction experiments in double-side laser-heated DACs and demonstrated that it allows the crystal structure solution and refinement, as well as accurate determination of thermal equations of state above 200 GPa at temperatures of thousands of degrees. Application of this methodology resulted in discoveries of novel compounds with unusual chemical compositions and crystal structures, uncommon crystal chemistry and physical properties. Perspectives of materials synthesis and crystallography at extreme conditions will be outlined.

High pressure is a powerful tool to study experimentally the response of selected hydrogen bonds to mechanical stress. Cooling is an alternative method to compress a structure. A comparison of compression on cooling and increasing pressure gives an insight into intermolecular interactions. Guanine and its derivatives, as well as nucleic acids, in general, attract much attention because of their interesting properties. Crystals made of small RNA or DNA fragments can serve as models of the effect of pressure on nucleic acids and oligonucleotides, similar to how the crystals of amino acids are used to model proteins. Nucleobases are the structural elements of nucleic acids. They are widely used as components of some crystalline drugs and molecular materials. Guanine is remarkable for its unique ability to form assemblies. In particular, oligonucleotides enriched with guanine can form quadruplexes in the presence of alkali and earth-alkaline metals. Because of the extremely low solubility of guanine in water and most of the organic solvents at neutral pH, only a few guanine compounds are known. An additional challenge is to obtain single crystals. Crystal structures containing guanine, metal ions and water molecules can also be used, to shed more light on the interactions between the guanine anions, metal cations and water molecules. Potassium cations are of special biological importance because they form natural quadruplexes, which are present in telomeric parts of the chromosome. The hydrates of guanine metal salts are of interest in this respect. In this contribution, the approaches to the crystallization of salts of guanine and alkaline metals from aqueous, alcoholic and aqueous-alcoholic solutions. Two salts of guanine were investigated by single-crystal X-ray diffraction, namely, 2Na⁺·C₅H₃N5O²⁻·7H₂O and K⁺ ∙C₅H₄N₅O^- ∙H₂O. The crystals of K⁺∙C₅H₄N₅O^-∙H2O were obtained for the first time. The structure is quite different from that of the previously documented sodium salt hydrate (2Na⁺·C₅H₃N₅O²⁻·7H₂O) [1]. The crystal structures of both sodium and potassium salt hydrates have channels. However, the structure of the channels, the cation coordination, the tautomeric form of the guanine anions, as well as the role of water molecules in the crystal structure are different for the two salt hydrates. In the potassium salt hydrate, there are two tautomeric forms of guanine anions and two types of potassium ions with different coordination. It is interesting to note, that though no “true” guanine quadruplexes could be found in the crystal structure of the potassium salt of guanine hydrate the “quartets” of guanine connected via hydrogen bonds with each other and two water molecules are present in this crystal structure. The sodium salt hydrate (2Na⁺·C₅H₃N5O²⁻·7H₂O) was characterized by single-crystal X-ray diffraction in the pressure range of 1 atm- 2.5 GPa [2] as well as in the temperature range 100 K - 300 K. The potassium salt of guanine was characterized by single-crystal X-ray diffraction in the temperature range 100 K - 300 K. ThetaToTensor software was used to calculate the coefficients of thermal expansion tensor and create a graphical representation of the characteristic surface [3]. The anisotropy of strain on temperature variation was compared for the two salt hydrates, the similarities and the differences are discussed concerning the intermolecular interactions [4].

[1] Gur D., Shimon L. J. W. (2015) Acta Crystallographica Section E: Crystallographic Communications, 71 (3), 281-283.

[2] A.Gaydamaka et al. (2019) CrystEngComm , 21, 4484-92.

[3] Bubnova, R. S., V. A. Firsova, and S. K. Filatov. (2013) Glass Physics and Chemistry, 39.3, 347-350.

[4] Gaydamaka, A. A., Arkhipov, S. G., Boldyreva, E. V., 2021, Acta Crystallographica Section B, in preparation.

Keywords: IUCr2021; guanine, nucleobase, single crystal X-ray diffraction, XRD, vibrational spectroscopy, high pressure, low temperature, ionic channels.

The research was supported by project AAAA-A21-121011390011-4. The equipment of REC MDEST (NSU) was used.

Carbon dioxide, CO₂, is one of the most important compounds in nature and the second most abundant volatile in the Earth's interior. Its structure and properties at high pressures and temperatures pertaining to geoscience are crucial both to fundamental chemistry and solid state physics.

CO₂ has a very complex phase diagram consisting of a number of crystalline molecular phases below 40 GPa. On further compression it polymerizes forming at moderate temperatures (up to 680 K) amorphous glass with carbon in threefold and fourfold coordination [1], while the laser heating above 1800 K/40 GPa produces a polymeric covalent crystal phase (CO₂-V, space group
I-42d) that can be described as a network of fourfold coordinated carbon atoms interconnected by oxygen bridges resembling structurally β-cristobalite (SiO₂) [2].

The substantial kinetic barrier, reflecting dramatic changes in the bonding scheme on transition to the polymeric phase, led to numerous observations of metastable states in the stability field of CO₂-V, causing controversies. Hence, we have decided to investigate the chemical and phase stability of carbon dioxide at pressures up to 120 GPa [3] and temperatures reaching 6000 K [4], an unexplored range in all the previous reports.

High-pressure high-temperature in situ X-ray diffraction patterns, here reported for the first time, proved that CO₂-V is the only non-molecular form of CO₂ relevant to the Earth's deep interior. Moreover, contrary to the previous findings, no evidences for the decomposition of CO₂-V into the elements have been found. Variation of the Bragg peak distribution on Debye-Scherrer rings at temperatures >4000  K [4] may suggest a further possible extension of the stability field of this polymeric solid toward the pre-melting state. The presented findings play a pivotal role in understanding the behavior of hot dense carbon dioxide and provide a good basis for further experimental studies of CO₂ at extreme pressures and temperatures.

[1] Santoro, M., Gorelli, F.A., Bini, R., Ruocco, R., Scandolo, S. & Crichton, W.A. (2006). Nature 441, 857. [2] Santoro, M., Gorelli, F.A., Bini, R., Haines, J., Cambon, O., Levelut, C., Montoya, J.A. & Scandolo, S. (2012). Proc. Natl Acad. Sci. USA 109, 5176. [3] Dziubek, K.F., Ende, M., Scelta, D., Bini, R., Mezouar, M., Garbarino, G. & Miletich, R. (2018). Nat. Commun. 9, 3148. [4] Scelta, D., Dziubek, K.F., Ende, M., Miletich, R., Mezouar, M., Garbarino, G. & Bini, R. (2021). Phys. Rev. Lett. 126, 065701.

The authors thank the Deep Carbon Observatory initiative (Extreme Physics and Chemistry of Carbon: Forms, Transformations, and Movements in Planetary Interiors, from the Alfred P. Sloan Foundation) that supported this work and the ESRF for granting the beamtime.

The martensitic transformation is a fundamental physical phenomenon at the origin of important industrial applications. However, the underlying microscopic mechanism, which is of critical importance to explain the outstanding mechanical properties of martensitic materials, is still not fully understood. This is because for most martensitic materials the transformation is a fast process that makes in situ studies extremely challenging. Noble solids krypton and xenon undergo a progressive pressure induced fcc to hcp martensitic transition with a very wide coexistence domain. Here, we took advantage of this unique feature to study the detailed mechanism of the transformation by employing in situ X-ray diffraction and absorption. We evidenced a four stages mechanism where the lattice mismatch between the fcc and hcp forms plays a key role in the generation of strain. We also determined precisely the effect of the transformation on the compression behavior of these materials.