A neutron diffraction study of the hydrides of the early lanthanide elements at room temperature

The order structure of CeD_2.29 at room temperature has been determined by profile analysis of powder neutron diffraction data. A non-stoichiometric tetragonal superstructure is adopted; space group I4₁md, lattice parameters a=5.538(1), c=11.046(2)AA. The ideal composition of the superstructure is CeD_2.5 but the fluorite phase is not ordered at this composition because of a high level of intrinsic disorder. It is presumed that the superstructure exists over a range of composition up to approximately CeD_2.4 and identical order structures have been determined in this range for LaD_2.30 and PrD_2.37.