Zirconolites based on the replacement of Ti4+ with equimolar amounts of Nb5+and Fe3+, CaZrTi2−2x NbxFexO7, have been jointly refined using high-resolution neutron powder diffraction and resonant X-ray powder diffraction data, with extra information provided from 57Fe Mössbauer spectroscopy, to determine cation disorder. The results indicate that the CaZrTi2−2xNbxFexO7 series adopts the zirconolite-2M polytype across the range, with the replacement of Ti4+ by Fe3+ and Nb5+ located within the hexagonal tungsten bronze (HTB) layers. Mössbauer spectroscopy shows that Fe3+ preferentially fills the Ti split (C2) site, with secondary filling of the C1 site and no observable occupancy of the C3 site. This has been confirmed by neutron and resonant X-ray diffraction. Niobium has been found to occupy both the C1 and C3 sites with no evidence for occupancy of the C2 site.

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