In situ Investigations on the Formation and Decomposition of KSiH3 and CsSiH3

The system KSi‐KSiH₃ stores 4.3 wt % of hydrogen and shows a very good reversibility at mild conditions of 0.1 MPa hydrogen pressure and 414 K. 1 We followed the reaction pathways of the hydrogenation reactions of KSi and its higher homologue CsSi by in situ methods in order to check for possible intermediate hydrides. In situ diffraction at temperatures up to 500 K and gas pressures up to 5.0 MPa hydrogen gas for X‐ray and deuterium gas for neutron reveal that both KSi and CsSi react in one step to the hydrides KSiH₃ and CsSiH₃ and the respective deuterides. Neither do the Zintl phases dissolve hydrogen (deuterium), nor do the hydrides (deuterides) show any signs for non‐stoichiometry, i.e. all phases involved in the formation are line phases. Heating to temperatures above 500 K shows that at 5.0 MPa hydrogen pressure only the reaction 2CsSi + 3H₂ = 2CsSiH₃ is reversible. Under these conditions, KSiH₃ decomposes to a clathrate and potassium hydride according to 46KSiH₃ = K₈Si₄₆ + 38KH + 50H₂.