X-ray diffraction probes superconductivity in yttrium hydrides

Several rare-earth superhydrides have garnered attention for their potential as near-ambient-temperature superconductors at high pressures. In most cases, they are synthesized in diamond anvil cells at extreme pressures and temperatures, resulting from chemical reactions. Their phase and chemical compositions are often unknown, which make claims of superconductivity uncertain. The measurable critical temperature T_c,  which indicates superconductivity, depends on many factors, including the purity of the sample and its hydrogen content. Thus, the existence of near-room-temperature high-pressure superconductors remains under scrutiny.

To shed light on the phase composition and crystal chemistry of hydrides at extreme conditions, researchers from the University of Bayreuth investigated the products of chemical reactions in diamond anvil cells. They subjected yttrium to laser heating with two hydrogen-rich precursors – ammonia borane or paraffin oil – reaching temperatures up to 3500 K at pressures of about 170 GPa. Synchrotron single-crystal X-ray diffraction (SCXRD) experiments were conducted at ESRF beamlines ID11, ID27 and ID15B to structurally characterise the multiphase, microcrystalline samples. Additional experiments were carried out at Petra III in Germany and the Advanced Photon Source in the USA.

Fig. 1: The crystal structures of two yttrium hydrides with the chemical formulas Y₄H₂₃ (top) and Y₃H₁₁ (bottom). Yttrium atoms are green; interconnected hydrogen atoms are light pink.

The research yielded several discoveries, including five novel yttrium hydrides (Y₃H₁₁, Y₂H₉, Y₄H₂₃, Y₁₃H₇₅ and Y₄H₂₅), two new yttrium allotropes and an yttrium carbide (YC₂), alongside the already known cubic and tetragonal YH₃ yttrium compounds. While the positions of non-hydrogen atoms in the crystal structures could be determined from synchrotron SCXRD data, the positions of hydrogen atoms could not be, necessitating the estimation of hydrogen content from empirical relations for the volume per yttrium atom. The Endeavour software aided in analysing possible hydrogen arrangements in unknown hydrides, further refined by ab initio calculations (Figure 1). The structural compositions revealed for each compound indicate the complexity and diversity of yttrium hydrides under high-pressure conditions. Notably, hydrogen content in the discovered hydrides increased with pressure, from 1:3 in YH₃ at 87 GPa to 1:6.25 in Y₄H₂₅ at 171 GPa. 

This study emphasizes the intricate nature of the yttrium-hydrogen system and its multiphase character under high pressure. Complex phase diversity, variable hydrogen content in yttrium hydrides, and their metallic nature as revealed by ab initio calculations represent some of the challenges in identifying superconducting phases and understanding electronic transitions in high-pressure synthesized materials. The results significantly contribute to understanding the behaviour of materials at extreme conditions, and shed light on the nature of potentially superconducting hydrides.

Principal publication and authors
Diverse high-pressure chemistry in Y-NH₃BH₃ and Y–paraffin oil systems, A. Aslandukova (a), A. Aslandukov (a), D. Laniel (b), Y. Yin (a,c,d), F. I. Akbar (a), M. Bykov (e), T. Fedotenko (f), K. Glazyrin (f), A. Pakhomova (g), G. Garbarino (g), E.L. Bright (g), J. Wright (g), M. Hanfland (g), S. Chariton (h), V. Prakapenka (h), N. Dubrovinskaia (a,d), L. Dubrovinsky (a), Sci. Adv. 10, 47, eadl5416 (2024); https://doi.org/10.1126/sciadv.adl5416
(a) University of Bayreuth, Bayreuth (Germany)
(b) University of Edinburgh, Edinburgh (UK)
(c) Shandong University, Jinan (China)
(d) Linköping University, Linköping (Sweden)
(e) University of Cologne, Cologne (Germany)
(f) DESY, Hamburg (Germany)
(g) ESRF
(h) University of Chicago, Chicago (USA)