In the search for novel ceramics for use within nuclear fuel–related applications and nuclear waste-form matrices, a major focus has been on the development of radiation-tolerant materials. Of particular interest in this field have been numerous compounds with either pyrochlore or the related fluorite-type structures. In this study, we look to expand the family of compounds with defect fluorite–type structure. We have fabricated three new compounds; Yb2Sn1.125O5.25, Yb2Sn1.25O5.5, and Yb2Sn1.375O5.75. The compound Yb2Sn1.125O5.25 was determined, via x-ray diffraction, to have the long-range defect fluorite structure, Fm-3 m symmetry, with cell parameter a = 5.17233(1). Further to this, Sm2ZrO5 and Yb2TiO5 compounds were also fabricated and crystal structures characterised. The use of transmission electron microscopy has revealed a much more complex crystal structure than that of the relatively high symmetry fluorite, with the presence of structural modulations being detected. The ion-irradiation response of these compounds was tested via bulk specimen irradiation using 15-MeV gold ions with grazing incidence x-ray diffraction characterisation. The results show that both Sm2ZrO5 and Yb2Sn1.25O5.5 are highly tolerant to ion-irradiation exposure whilst Yb2TiO5 is susceptible to amorphisation. [ABSTRACT FROM AUTHOR]