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 0 H I G H L I G H T S 2 0 2 1 I

Ternary polyhydrides of lanthanum and yttrium: new members of the high-temperature superconductor family

X-ray diffraction was used to study the properties of ternary lanthanum yttrium superhydrides. The results show that alloying is an effective way of stabilising near-room-temperature superconductors YH10 and LaH6.

Recent progress in hydride superconductivity studies has made it possible to reach near-room critical temperatures in many compressed binary hydrides, including, among others, H3S (TC = 203 K) [1], LaH10 (TC = 250 K) [2], ThH10 (TC = 161 K) [3], and YH6 (TC = 226 K) [4]. This progress would not have been possible without the development of modern density functional theory methods and evolutionary algorithms for crystal structure prediction (such as USPEX). On the other hand, experimental studies of small samples (10 30 × 2 3 µm) in high-pressure diamond anvil cells became possible only with the development of synchrotron sources.

In 2014, the first high-temperature superconducting hydride was discovered: unusual sulfur hydride H3S was predicted and later experimentally confirmed to have a critical temperature of ~191 204 K at a pressure of

150 GPa. Following this, many high-pressure research groups focused on superhydrides the hydrides that are abnormally rich in hydrogen and found new compounds that proved to be superconductors at even higher temperatures: LaH10 (predicted and then experimentally shown to exhibit superconductivity at 250 260 K and 190 GPa) and YH10 (predicted to be a room-temperature superconductor). Despite the similarity between yttrium and lanthanum, YH10 later proved to be unstable and, thus far, no one has succeeded in synthesising it in its pure form. In 2020, after more than 100 years of research, scientists synthesised the first room-temperature superconductor a ternary hydride of sulfur and carbon with a critical temperature of +15oC [5]. What is the structure of this compound? What role does carbon play in it? What is the mechanism of superconductivity in CxSyHz? Many questions brought about by this discovery remain unanswered.

Having reached the upper limit of critical temperatures for binary hydrides, chemists turned to ternary hydrides, which are regarded as the most promising avenue for high- temperature superconductivity below 100 GPa (a million atmospheres) pressure. In one of the approaches expected to be successful, a combination of well-known binary metal superconductors LaH10 and YH10 was investigated at beamline ID15B using powder X-ray diffraction in diamond anvil cells (Figure 18).

PRINCIPAL PUBLICATION AND AUTHORS

Experimental evidence of mosaic structure in strongly supercooled molecular liquids, F. Caporaletti (a,g) S. Capaccioli (b,c), S. Valenti (d), M. Mikolasek (e), A.I. Chumakov (e,f), G. Monaco (a,h), Nat. Commun. 12, 1867 (2021); https:/doi.org/10.1038/s41467-021-22154-8 (a) Dipartimento di Fisica, Università di Trento, Povo (Italy) (b) Dipartimento di Fisica, Università di Pisa (Italy) (c) CISUP, Centro per l Integrazione della Strumentazione dell , Universitá di Pisa (Italy) (d) Department of Physics, Universitat Politécnica de Catalunya, Barcelona (Spain) (e) ESRF (f) National Research Center Kurchatov Institute , Moscow (Russia) (g) Present address: Van der Waals-Zeeman Institute, Institute of Physics/Van t Hoff Institute for Molecular Sciences, University of Amsterdam (Netherlands) (h) Present address: Dipartimento di Fisica ed Astronomia, Universitá di Padova (Italy)

REFERENCES

[1] C.A. Angell, Science 267, 1924 (1995). [2] G.P. Johari & M. Goldstein, J. Chem. Phys. 53, 2372 (1970). [3] A.Q.R Baron et al., Phys. Rev. Lett. 79, 2823 (1997). [4] J.D. Stevenson & P.G. Wolynes, Nat. Phys. 6, 62-68 (2010).

place. Combining their data with the literature [3], the researchers were able to provide a new picture for the microscopic dynamics in the supercooled liquid state. The molecules participating in the Johari-Goldstein relaxation form a percolating cluster that spans the whole sample (Figure 17). The bJG process thus marks the development of a mosaic state in the supercooled liquid phase characterised by patches of less mobile molecules

that are separated by an ever-changing network of more mobile ones. These observations provide new insights into the heterogeneous dynamics of glasses. The appearance of such an infinite cluster, related to the bJG- process, supports the idea that a dynamical transition is taking place in the supercooled liquid phase at low enough temperatures, as suggested by some theoretical models of the glass transition [4].