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‐CsH₇, P1‐CsH_15+x) and rubidium (RbH_9‐x) polyhydrides with a very high hydrogen content reaching 93 at.% are synthesized. Preservation of CsH₇ 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.