This proposal is to determine high quality structural models using Rietveld analysis for the pyrochlore phases CaLnTr0.5Nb1.5O7 (Ln = La, Nd, Ho, Yb and Tr = Al, Sc, Cr, Mn, Fe, Y), as part of a larger examination of ionic stability in the calcium niobate-based pyrochlore systems. This system is of interest as model compounds for the immobilisation of long-lived actinides, predominantly as they allow a range of compositional modifications to be examined with a minimal change in structure. We request 3 days of beam time to acquire data for Rietveld refinement of 24 compositions.