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 <Sn-O> bond in determining the radiation response.