A neutron-diffraction study on the structural relationships of RCo5 hydrides

The hexagonal RCo₅ compounds form several orthorhombic hydrides which are structurally closely related. A neutron-diffraction study of β^I, β^II and β^III types, as found for e.g. PrCo₅D_3·6, PrCo₅D_2·9 and CeCo₅D_2·55 respectively, revealed that the deuterium atoms occupy both octahedral and tetrahedral interstices. In the simplest of these, β^I, the occupancy of the interstices is equal; its space group is Cmmm. In both β^II and β^III the c-axis is doubled compared with β_I. In β^II, space group Im2m, the octahedral positions have a uniformly reduced occupancy, which leads to appreciable ordered shifts of the metal atoms. In β^III (Cccm) half of the octahedral positions are vacant, giving a limiting stoichiometric composition CeCo₅H₃.The magnetizations of the (ferromagnetic) hydrides decrease with increasing hydrogen content. Results of neutron-diffraction analysis are in fair agreement with the magnetization measurements.