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 1 I

Fig. 19: a) Pressure dependence of the interatomic distance R(Y0)-(Y(1)) for fcc YH3 obtained from XRD versus EXAFS data.

b) Structure of the metal hydride YH3.

Towards a better understanding of high-temperature superconductivity in superhydrides at ultrahigh pressures

The search for room-temperature superconductors is one of the most important problems in physics. Recently, high-temperature superconductivity was demonstrated in superhydrides at ultrahigh pressures. XAS, XRD and Raman measurements at 180 GPa were combined to elucidate the nature of pressure-induced rearrangements in the electronic and atomic structure in YH3.

Searching for new high-temperature superconductors is one of the most important problems in physics and chemistry. For decades after the discovery of cuprates in 1986, only unconventional superconductors (SC) that do not follow the theory developed by J. Bardeen, L. Cooper, and R. Schrieffer (BCS) had shown high-temperature superconductivity. In 2015, the record critical temperature (Tc) for SC of 203 K in hydrogen sulfide at 160 GPa was observed [1], suggesting that room-temperature superconductivity described by the BCS-Eliashberg theory is realistic. Soon afterwards, nearly room-temperature SC at 250 K in LaH10 [2] and at 243 K

in YH9 [3] was discovered at a pressure above 200 GPa. Even higher Tc above room temperature is predicted for YH10 [4] and Li2MgH16 [5].

These new findings urged for a better understanding of hydrogen interaction mechanisms with the heavy atom sublattice in metal hydrides under high pressure at the atomic scale. Therefore, the pressure-induced rearrangements of the electronic and atomic structure at the local and bulk scale, which may inhibit or favour superconductivity, were studied in the archetypal metal hydride YH3 (Figure 19).

X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) measurements carried out at beamline BM23 were combined with Raman spectroscopies to study YH3 under ultrahigh pressures up to 180 GPa, and also at low temperatures (10-300 K, 39 GPa). High-pressure Y K-edge extended X-ray absorption fine structure (EXAFS) data on YH3 were collected up to 180 GPa in a diamond anvil cell equipped with nano-polycrystalline diamonds, ensuring glitch-free signals with a high signal-to-noise ratio up to 16.5 Å−1 in momentum space. Complementary XRD data were acquired sequentially to probe the phase transitions and lattice parameter evolutions (Figure 20).