32 / 2023-04-03 13:24:23

Synthesis, X-ray Diffraction and Nuclear Magnetic Resonance studies of Cesium and Rubidium polyhydrides

polyhydrides, high pressure, solid state NMR

高压物理与材料科学 > 氢与高能量密度材料-I

Reaching the metallic state of pure hydrogen by compression is one of the most spectacular challenges in high-pressure physics and chemistry. Despite a significant progress in achieving ultrahigh pressures in the last five years, detecting superconductivity of hydrogen remains an unsolved problem because studies of the electrical conductivity at pressures above 400 GPa remain very challenging. In 2015–2017, it has been shown that lithium and sodium form molecular semiconducting hydrides LiH₆ [1] and NaH₇ [2]. Continuing these studies, we have synthesized a unique barium superhydride BaH₁₂ [3] which is a molecular metal and a superconductor at 120–140 GPa. Recently, an even more interesting compound has been discovered in the Sr-H system: semiconducting SrH₂₂ with a glass-like superionic hydrogen sublattice [4]. And our next goal is the formation of rubidium and cesium polyhydrides.

The ionization potential of Rb and Cs is lower than that of Li and Na, while the radius of Rb⁺ and Cs⁺ is larger than that of Li⁺ and Na⁺. This fact, according to the general concept of tight packing, will promote the formation of higher polyhydrides with a high hydrogen-to-metal ratio. Theoretical calculations show that Rb and Cs polyhydrides can be stabilized at very low pressure: RbH₃, RbH₅ and RbH₉ are thermodynamically stable near 30 GPa, while superhydrides RbH₁₆ can be stabilized at around 100 GPa. Moreover, for CsH₃, CsH₇ and CsH₉ the lower stabilization limit is even lower: these hydrides lie on the convex hull at 10 GPa, but CsH₁₆ appears at 50 GPa which is a very comfortable range for the X-ray diffraction.

In this report I will describe the results of a study of Cs and Rb polyhydrides at pressures up to 30 GPa synthesized from metals (Cs, Rb), their hydrides (CsH) and salts with ammonia borane (CsNH₂BH₃ and RbNH₂BH₃). We chose powder X-ray diffraction at the synchrotron radiation source (Elettra, Italy) as the main method of research. The results obtained were interpreted using the USPEX code for the evolutionary search of thermodynamically stable crystal structures. A joint experimental-theoretical study indicated the formation of Cs, Rb polyhydrides with high hydrogen content already from 5 to 18 GPa: semiconducting  hexagonal and cubic RbH₉, as well as P1-CsH_15-17 were found. In addition, we will discuss studies of the obtained polyhydrides using solid-state nuclear magnetic resonance of ¹³³Cs, ⁸⁷Rb and ¹H nuclei in specially designed high-pressure DACs.



References:

[1] C. Pépin et al. PNAS 112, 7673 (2015).

[2] V. V. Struzhkin et al. Nat. Commun. 7, 12267 (2016).

[3] W. Chen, D.V. Semenok et al. Nat. Commun. 12, 273 (2021).

[4] D. Semenok et al. Advanced Materials, 34, 2200924 (2022).