Novel intermetallic hydrides

The paper focuses on structural chemistry of novel intermetallic hydrides with unusual structural properties. In such “anisotropic” hydrides, a huge expansion proceeds in a sole crystallographic direction and leads to a dramatic differentiation of the properties of the hydrides along the direction of the expansion and normal to it. The behaviour of the “anisotropic” hydrides is dominated by the metal–hydrogen and hydrogen–hydrogen interactions in contrast to the “conventional” intermetallic hydrides where the metal–metal interactions are the most important ones. In sharp contrast to the known crystal structures of intermetallic hydrides, in “anisotropic” hydrides deuterium atoms do not fill initially existing interstices but, instead, attract rare earth atoms into their surrounding and form new D-occupied sites. This paper will summarise our recent research on the “anisotropic” hydrides with a particular focus on two groups of materials: (a) RENiIn-based deuterides (RE = rare earth metal) containing the shortest known separation of hydrogen atoms in the structures of metal hydrides and (b) RENi3–(CeNi3) and RE2Ni7–(La2Ni7)-based deuterides which develop unusually large (59–63%) expansion of the constituent RENi2 layers.