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In developing improvements in nuclear fuels, work has been conducted in replacing boron with lanthanides to act as neutron absorbent materials. To further this development for the first time the three major polymorphs of Dy 2 TiO 5 have been fabricated as bulk, single-phase materials in a systematic study. Crystal structure refinement has been carried out from powder x-ray diffraction data, with further structural detail attained via electron diffraction. A cubic phase, defect-pyrochlore, Fd-3m space group, was found with cell parameter a = 10.2996 (1) Å and oxygen x48f positional parameter = 0.331 (1). Dy 2 TiO 5 was also fabricated as a bulk, single-phase compound with hexagonal symmetry, P6 3 /mmc space group, with cell parameters a = b = 3.6307(2), c = 11.8963(8) Å. The radiation response for each of the polymorphs has been investigated in-situ via 1 MeV Kr – ion-irradiation and transmission electron microscopy characterisation at the IVEM-TANDEM facility, Argonne National Laboratory. Critical temperatures, T c , for maintaining crystallinity during irradiation were determined and showed Dy 2 TiO 5 with either orthorhombic or cubic symmetry to perform the best, with the lowest T c values of 711 and 761 K respectively when compared with the hexagonal form. These relatively low T c values may be attributed to different characteristics for each phase; high anti-site defect formation energy for the orthorhombic symmetry, and the ability to accommodate disorder for the cubic symmetry. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Abstract: Pyrochlores based on the general composition Ln2TiO5 (Ln2.67Ti1.33O6.67) and Ln2Ti2O7, where Ln=Y or Yb, have been irradiated through the crystalline–amorphous transition with 1MeV Kr ions at the IVEM-TANDEM facility, Argonne National Laboratory. The results show that the Tc (critical temperature for amorphization) differs significantly between each series, e.g. for Y2TiO5 it is 589±18K and for Y2Ti2O7 665±33K. The difference suggests that recovery from damage is more rapid with increasing Ln content, i.e. a lower Tc for amorphization. These results are discussed in the context of the melting points of each phase, atomic disorder, the pyrochlore–fluorite order–disorder transition and the implications for oxide dispersion-strengthened additives. [Copyright &y& Elsevier]