Raisins in a hydrogen pie. Ultrastable cesium and rubidium polyhydrides

A new method for synthesis of metal polyhydrides via high-pressure thermal decomposition of corresponding amidoboranes in diamond anvil cells is proposed. Within this approach, molecular semiconducting cesium (P4/nmm-CsH${}_7$, P1-CsH${}_{15+x}$) and rubidium (RbH${}_{9-x}$) polyhydrides with a very high hydrogen content reaching 93 at.% are synthesized. Preservation of CsH${}_7$ at near ambient conditions, confirmed both experimentally and theoretically, represents a significant advance in the stabilization of hydrogen-rich compounds. In addition, two crystalline modifications of RbH${}_{9-x}$ with pseudohexagonal and pseudotetragonal structures identified by synchrotron X-ray diffraction, and Raman measurements are synthesized. Both phases are stable at 8-10 GPa. This is an unprecedentedly low stabilization pressure for polyhydrides. These discoveries open up possibilities for modifying existing hydrogen storage materials to increase their efficiency. (© 2024 Wiley‐VCH GmbH)