On the crystal structures of Ln_3MO_7(Ln=Nd, Sm,Y and M =Sb,Ta)-Rietveld refinement using X-ray powder diffraction data

摘要

We have investigated, using X-ray powder diffraction data, the crystal structures of some fluoritederivatives with the formula Ln_3M0_7(Ln lanthanide or Y and M = Sb Ta). In these compoundsordering of Ln and M occurs, leading to a parent structure in Cmmm. Tilting of the MO6 octahedra causesdoubling of one of the cubic axes, leading to a number of non-isomorphic subgroups, e.g. Cmcm, Ccmmand Cccm. We have identified an alternative space group Ccmm instead of C222_1for those compoundscontaining a medium sized lanthanide or Y and M being Sb or Ta. Interestingly this is an alternativesetting for the space group of the structure obtained when Ln is large (Cmcm). However, there tilting ofthe octahedra is around the a-axis of the parent structure, rather than around the b-axis as it is found inthe compounds which we are reporting on here. In one compound, Nd_3TaO_7, both tilts occur. The phase transition between the two possiblestructures is a slow and difficult process above 801<, allowing both phases to coexist.