Effect of composition on the behavior of pyrochlores irradiated with swift heavy ions

A2B2O7 pyrochlores (zirconates with A=Gd, Nd, Sm or Eu, and a titanate Gd2Ti2O7) were irradiated with swift heavy ions in order to investigate the effects of the chemical composition on the structural changes induced by high electronic excitation. XRD results show that the structural modifications induced by irradiation with 119MeV U ions are strongly dependent on the sample composition: Gd2Ti2O7 is readily amorphized, whereas Gd2Zr2O7 is transformed into a radiation-resistant anion-deficient fluorite structure. For Sm2Zr2O7, Eu2Zr2O7 and Nd2Zr2O7, a more complex behavior is observed since both a pyrochlore–fluorite phase transformation and amorphization occur. The results suggest that an ion track is composed of an amorphous track core surrounded by an anion-deficient fluorite shell. The diameters of different track regions were estimated by using a direct impact mechanism coupled with a single overlap process to fit the transformation build-ups. These diameters depend on the sample composition, the susceptibility to amorphization by high electronic excitation increasing with the cation radius ratio rA/rB.