Ion-beam irradiation of Gd2Sn2O7 and Gd2Hf2O7 pyrochlore:: Bond-type effect

Ceramics with III-IV pyrochlore compositions, A(2)(3+)B(2)(4+)O(7) (A = Y and rare earth elements; B = Ti, Zr, Sn, or Hf), show a wide range of responses to ion-beam irradiation. To evaluate the role of the B-site cations on the radiation stability of the pyrochlore structure-type, Gd2Sn2O7 and Gd2Hf2O7 have been irradiated by I MeV Kr+. The results are discussed in terms of the ionic size and type of bonding of Sn4+ and Hf4+ and compared to previous results for titanate and zirconate pyrochlores. Gd2Sn2O7 is sensitive to ion beam-induced amorphization with a critical amorphization dose of approximately 3.4 displacements per atom (dpa) (2.62 x 10(15) ions/cm(2)) at room temperature and a critical amorphization temperature of approximately 350 K. Gd2Hf2O7 does not become amorphous at a dose of approximately 4.54 displacement per [lattice] atom (3.13 x 10(15) ions/cm(2)) at room temperature, but instead is transformed to a disordered fluorite structure upon ion-beam irradiation. Although the radius ratio of the A- to B-site cations provides a general indication of the type of radiation response of different pyrochlore compositions, the results for Gd2Sn2O7 emphasize the importance of bond type, particularly the covalency of the bond in determining the radiation response.