Structure prediction, and the theoretical computation of reliable superconducting transition temperatures, have undoubtedly played a major role in the discovery of novel high temperature superconductivity in dense hydrides.[1] While the field has delivered room temperature superconductivity,[2] the technological relevance will be limited while the phenomenon is restricted to extremely high pressures. Furthermore, the number of experimental research groups that can study the properties of these compounds at megabar pressures is limited, restricting the potential scientific impact. Rightly, the field is focusing on identifying compounds that superconduct at high temperatures, but much lower pressures. But there are considerable obstacles to progress. The number of theoretical candidates far exceed those experimentally confirmed, suggesting more attention should be paid to predicting synthesisability. It is becoming clear that metastability favours high temperature superconductivity, but how should we choose from the multitude of metastable candidates? Experimentally determined structures are frequently not found to be dynamically stable in static calculations, but full dynamics is computationally expensive, and difficult to account for in high throughput searches. At the same time, it is not clear how to compute superconducting transition temperatures in highly dynamic systems. So far, most attention has been paid to perfect crystals. Doping and deviation from perfect stoichiometry, and well as defects (both point, and extended, such as grain boundaries and interfaces) are likely to be important to the detailed properties of these materials. Finally, as we turn to exploring a broader range of compounds, in the ternaries and beyond, structure prediction becomes more challenging, not least in terms of the management of the quantities of data generated, and the computation of large numbers of superconducting transition temperatures. I will show some recent results which go some way to addressing this.

[1] Pickard, Chris J., Ion Errea, and Mikhail I. Eremets. "Superconducting hydrides under pressure." Annual Review of Condensed Matter Physics 11 (2020): 57-76.

[2] Snider, Elliot, Nathan Dasenbrock-Gammon, Raymond McBride, Mathew Debessai, Hiranya Vindana, Kevin Vencatasamy, Keith V. Lawler, Ashkan Salamat, and Ranga P. Dias. "Room-temperature superconductivity in a carbonaceous sulfur hydride." Nature 586, no. 7829 (2020): 373-377.

The remarkable high-temperature superconducting behavior of H₃S (T_C=200 K, [1]) and LaH₁₀ (T_C=250 K [2]) at about 150 GPa catalyzed the search for superconductivity in compressed ternary hydrides. The highest critical temperature of 288 K at 275 GPa has been found recently in the C-S–H system [3]. High-temperature superconductivity in these compounds is due to the formation of metallic hydrogen sublattice, which is obtained by pulsed laser heating of various elements with hydrogen at extremely high pressures achieved during compression on diamond anvils. In this report we will present new results of studies of high-pressure chemistry, magnetic and superconducting properties of YH₆, UH₇, ThH₁₀, CeH_9-10, PrH₉, NdH₉, EuH₉ and BaH₁₂ binary and (La,Y)H₁₀ ternary polyhydrides discovered in the last 2 years by collaboration of IC RAS, LPI, Skoltech and Jilin University (China). Perspectives of design of light and magnetic sensors (SQIUDs) based on superhydrides synthesized in miniature diamond anvil cells will be discussed.

References:
[1] Drozdov A P, Eremets M I, Troyan I A, Ksenofontov V and Shylin S I, 2015, Nature, 525, 73–76.
[2] Drozdov A P, Kong P P, Minkov V S, Besedin S P, Kuzovnikov M A,Mozaffari S, Balicas L, Balakirev F F, Graf D E, Prakapenka V B,Greenberg E, Knyazev D A, Tkacz M and Eremets M I, 2019, Nature 569, 528–531.
[3] Snider E, Dasenbrock-Gammon N, McBride R, Debessai M, Vindana H, Vencatasamy K, Lawler K, Salamat A et al., 2020, Nature, 586, 373–377.

Artem R. Oganov

Skolkovo Institute of Science and Technology, 3 Nobel St., 121205 Moscow, Russia

Chemical behavior of the elements can be rationalized and anticipated based on a few properties, such as electronegativity, radius (atomic, ionic, van der Waals radii), polarizability, valence state. Among these, electronegativity plays perhaps the most important role – chemical reactivity of the elements, bond energies, directions and heats of reactions, and many properties of molecules and solids are related to electronegativities of the elements. The oldest and the most widely used is Pauling’s scale of electronegativity, developed in 1932 (see [1]) and based on bond energies. However, later it was found (e.g., [2]) that Pauling’s formula, relating bond energies with electronegativity differences, is very inaccurate for significantly ionic bonds. We have proposed [3] another formula, which works well for bonds with any degree of ionicity, and obtained a new thermochemical scale of electronegativities for all elements. New electronegativities better follow chemical intuition than traditional Pauling’s values (e.g. charge transfer in transition metal borides and hydrides is described qualitatively better, and so are oxyacids).
We have also studied another important concept – Mendeleev numbers, introduced in 1984 by Pettifor [4]. Pettifor showed that chemical behavior of the elements can be approximately characterized by just one number, which he called the Mendeleev number; he assigned these numbers to all elements, but the physical meaning of these remained unclear. We have shown [5] that Mendeleev number can be obtained by principal components analysis (PCA) or even a simple linear correlation applied to the set of points “atomic radius – electronegativity – polarizability” of all elements. This dimensionality reduction gives a single variable giving mathematically the best one-parameter description of the chemistry of the elements – the Mendeleev number. We have shown [5] that thus defined Mendeleev numbers perform better than those proposed by Pettifor [4]. Accurate representations of the chemical space require all key properties of the atoms to be explicitly used, but reduced-dimensionality representations (such as representation by Mendeleev numbers) allow easier visualization of big data.

This work is funded by Russian Science Foundation (grant 19-72-30043).

[1] Pauling, L. The Nature of the Chemical Bond 3rd edn (Cornell University Press, 1960).
[2] Matcha, R. L. (1983). Theory of the chemical bond. 6. Accurate relationship between bond energies and electronegativity differences. J. Am. Chem. Soc. 105, 4859–4862.
[3] Tantardini C., Oganov A.R. (2021). Thermochemical electronegativities of the elements. Nature Communications 12, 2087.
[4] Pettifor D.G. (1984). A chemical scale for crystal structure maps. Solid State Commun. 51, 31-34.
[5] Allahyari Z., Oganov A.R. (2020). Nonempirical definition of Mendeleev numbers: organizing the chemical space. J. Phys. Chem. C124, 23867-23878.