  We proposed a new method for synthesis of metal polyhydrides via
high-pressure thermal decomposition of corresponding amidoboranes in diamond
anvil cells. Within this approach, we synthesized molecular semiconducting
cesium ($\textit P4/nmm$-CsH$_7$, $\textit P$1-CsH$_{15+x}$) and rubidium
(RbH$_{9-x}$) polyhydrides with a very high hydrogen content reaching 93 at%.
Preservation of CsH$_7$ at near ambient conditions, confirmed both
experimentally and theoretically, represents a significant advance in the
stabilization of hydrogen-rich compounds. In addition, we synthesized two
crystal modifications of RbH$_{9-x}$ with pseudo hexagonal and pseudo
tetragonal structures identified by synchrotron X-ray diffraction and Raman
measurements. Both phases are stable at 8-10 GPa. This is an unprecedented low
stabilization pressure for polyhydrides. These discoveries open up
possibilities for modifying existing hydrogen storage materials to increase
their efficiency.
