Synthesis and structure determination of complex zinc hydrides .3. Dirubidium- and dicaesiumtetrahydridozincate(II), Rb2ZnH4 and Cs2ZnH4

Rb(2)nH(4), Cs2ZnH4 and the corresponding deuterides were prepared by sintering stoichiometric amounts of the binary alkali hydrides (deuterides) and zinc powder under hydrogen (deuterium) pressure of 80 bar at 650 K. X-ray and neutron powder diffraction on the deuterides show that the compounds are isotypic with K2ZnH4 and crystallize in the beta-K2SO4 structure type, space group Pnma (No. 62), Z = 4, Zinc is tetrahedrally coordinated by deuterium with average Zn-D bond distances of 1.673 Angstrom (Rb) and 1.682 Angstrom (Cs). The decomposition temperature at 1 bar hydrogen pressure is 640(15)K for the Rb and 645(15) K for the Cs compound. The hydrogen storage efficiencies are 1.7 wt.% (50.7 g H-2 l(-1)) for Rb2ZnH4 and 1.2 wt.% (44.4 g H-2 l(-1)) for Cs2ZnH4.