Zirconate and titanate pyrochlores were subjected to 1 MeV of Kr+ irradiation. Pyrochlores in the Gd2(ZrxTi1-x)2O7 system (x = 0, 0.25, 0.5, 0.75, 1) showed a systematic change in the susceptibility to radiation-induced amorphization with increasing Zr content. Gd2Ti2O7 amorphized at relatively low dose (0.2 displacement per atom at room temperature), and the critical temperature for amorphization was 1100 K. With increasing zirconium content, the pyrochlores became increasingly radiation resistant, as demonstrated by the increasing dose and decreasing critical temperature for amorphization. Pyrochlores highly-enriched in Zr (Gd2Zr2O7, Gd2Zr1.8Mg0.2O6.8, Gd1.9Sr0.1Zr1.9Mg0.1O6.85, and Gd1.9Sr0.1Zr1.8Mg0.2O6.75) could not be amorphized, even at temperature as low as 25 K.

The isometric pyrochlore structure, A 2 B 2 O 7 , is generally susceptible to radiation damage, but certain compositions are remarkably resistant to radiation damage. In the binary system Gd 2 (Ti 2Ϫx Zr x )O 7 , the radiation resistance increases dramatically with the substitution of Zr for Ti, until the pure end member Gd 2 Zr 2 O 7 cannot be amorphized, even at doses as high as ϳ100 dpa. Although zirconate pyrochlores are generally considered to be radiation resistant, we report results for the amorphization of a zirconate pyrochlore La 2 Zr 2 O 7 by ion beam irradiation ͑ϳ5.5 dpa at room temperature͒. The critical amorphization temperature T c is low, ϳ310 K. The susceptibility to ion-beam-induced amorphization and structural disordering for zirconate pyrochlores is related to the structural deviation from the ideal fluorite structure, as reflected by the x parameter of the O 48f .

Zirconate pyrochlores, A2Zr2O7, are important potential nuclear waste forms for Puimmobilization. The binary Gd2(Ti2-xZrx)O7 has been shown to have increasing resistance to ionirradiation damage with the increasing Zr content, and Gd2Zr2O7 is radiation resistant to a 1 MeV Kr+ ion irradiation at 25 K to a dose of 5 dpa. In this study, a 1.5 MeV Xe+ irradiation was completed for zirconate pyrochlores A2Zr2O7 (A=La, Nd, Sm, Gd). The radiation resistance decreases with an increase of the ionic radius of A-site cation. La2Zr2O7 is the first zirconate pyrochlore to be amorphized by ion beam irradiation, and the critical amorphization temperature, Tc, is ∼310 K. The susceptibility of La2Zr2O7 to ion beam damage is related to its structure, which shows the largest deviation from the ideal fluorite structure. These results are also consistent with calculations of the cation antisite formation energy in the pyrochlore structure. The ion irradiation-induced pyrochlore-to-fluorite transformation occurred in all of the irradiated zirconate pyrochlore phases. Based on the results for Gd2Ti2-xZrxO7 and A2Zr2O7, the defect fluorite structures are stable when the ionic radii ratio rA/rB≤1.54; beyond this limit, the defect fluorite structure becomes increasingly unstable relative to the amorphous state.

The pyrochlore structure-type is a proposed host phase for the immobilization of plutonium. Previous studies have shown that a wide variety of actinide pyrochlores can be synthesized/ Gadolinium zirconate with the pyrochlore structure has been shown to be remarkably radiation resistant. We report additional results of ion-beam irradiation studies. Samples of Gd 2 Zr 2 O 7 and three variations from the ideal composition (Gd 2 Zr 18 Mg 02 O 68/ Gdj 9 Sr 0 ^Zr^Mgo-A.85/ and Gd 2 9 Sr 0 jZ^ 8 Mg 02 O 675) were prepared by solution combustion synthesis. All samples were confirmed to have the pyrochlore structure by electron diffraction. Irradiation experiments with 1.5 MeV Xe+ and 1 MeV Kr + were performed using the HVEM-Tandem Facility at the Argonne National Laboratory. All samples, irradiated to a dose of 8.9 x 10 15 ions/cm 2 (or 16 dpa, assuming E d = 50 eV) with 1.5 MeV Xe + at room temperature, showed no evidence of amorphization. A similar irradiation series with 1.5 MeV Xe+ on the four samples was repeated at a temperature of 20 K, and no amorphization was observed after a dose of 5 dpa. Clearly, compositions in this regime will be extremely stable and resistant to amorphization under repository conditions, even after reaching high radiation doses. All the samples experienced an irradiation-induced pyrochloreto-fluorite structural transformation, shown by HREM and SAED (Fig. 1).

The zinc contents of a series of zinc‐aluminium binary alloys were determined by wavelength‐dispersive x‐ray fluorescence spectrometry using very small samples. The Zn Kα radiation from the alloy undergoes an apparent enhancement due to the widely different absorption coefficients of zinc and aluminium for radiation close to the short‐wavelength side of the K absorption edge of zinc. This was corrected mathematically by using mass absorption coefficients from the literature. The method is comparable in accuracy to wet chemical analysis. The precision obtained is of the order of 0.25% relative at 6 atom‐% of zinc. The method of fitting the measured Zn Kα intensity to a non‐linear function of composition is also examined.