The aim of this experiment is to determine high quality structural models for pyrochlore and zirconolite phases in the Y2Ti2-2xFexNbxO7 and Y2-xCexTi2-xFexO7 solid solutions, from Rietveld analysis of high resolution neutron diffraction data. These materials are of topical interest for immobilisation of long lived actinides. These solid solutions are unusual, in that: i) the zirconolite structure is stabilised over a wide doping range without intergrowth defects, as determined by TEM/SAED. ii) lanthanide zirconolites are rather rare, with only three other examples reported in the ICSD database. High quality structural models will enable us to understand why the above doping mechanisms lead to a transformation from pyrochlore to zirconolite structure by analysis of bond valence sums and Madelung energy calculations.