Ion-Irradiation-Induced Amorphization Of Cadmium Niobate Pyrochlore
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Ion-Irradiation-Induced Amorphization Of Cadmium Niobate Pyrochlore A. Meldrum,1 K. Beaty,1 L. A. Boatner,2 and C. W. White2 1 Dept. of Physics, University of Alberta, Edmonton AB, T5N 2A3, Canada;, Solid State 2 Division, Oak Ridge National Laboratory, Oak Ridge TN 37831 USA ABSTRACT Irradiation-induced amorphization of Cd2Nb2O7 pyrochlore was investigated by means of in-situ temperature-dependent ion-irradiation experiments in a transmission electron microscope, combined with ex-situ ion-implantation (at ambient temperature) and RBS/channeling analysis. The in-situ experiments were performed using Ne or Xe ions with energies of 280 and 1200 keV, respectively. For the bulk implantation experiments, the incident ion energies were 70 keV (Ne+) and 320 keV (Xe2+). The critical amorphization temperature for Cd2Nb2O7 is ~480 K (280 keV Ne+) or ~620 K (1200 keV Xe2+). The dose for in-situ amorphization at room temperature is 0.22 dpa for Xe2+, but is 0.65 dpa for Ne+ irradiation. Both types of experiments suggest a cascade overlap mechanism of amorphization. The results were analyzed in light of available models for the crystalline-to-amorphous transformation and were compared to previous ionirradiation experiments on other pyrochlore compositions. INTRODUCTION Pyrochlores represent a large class of compounds with the general chemical formula A2B2O7. The pyrochlore structure consists of interconnected AO8 and BO6 cation polyhedra [1] where the BO6 octahedra are slightly distorted and form a continuous corner-sharing network. The large, 8-fold coordinated A-site cations and the seventh oxygen occupy channels in the (B2O6)∞ network of octahedra [1,2]. The structure is an anion-deficient MX2 fluorite structure in which the A and B cations are ordered on the M sites. One eighth of the X anions are absent in the pyrochlore structure, and the resulting oxygen vacancies are ordered on the anion sublattice [2]. Recent in-situ ion irradiation experiments have focused on the effects of chemical composition and incident ion mass and energy on the irradiation-induced amorphization of several pyrochlore compositions. Gd2Ti2O7 has been the subject of considerable study as an actinide-bearing phase in SYNROC, – a polyphase ceramic waste form proposed for the disposition of high-level nuclear waste [3]. Replacement of the Gd with other lanthanide elements or Ca has been found to have a relatively minor effect on the kinetics of irradiationinduced amorphization; however, the substitution of Ti with Zr in increasing concentrations sharply increases the resistance to amorphization even at cryogenic temperatures [4]. Pyrochlores containing a variety of A-site cations, including mixtures of Ca, U, [5,6] and various lanthanides [7] are easily amorphized at room temperature, and in fact, amorphization can be induced at temperatures as high as 1030 K. The previous studies of radiation damage effects in pyrochlores have employed either impure natural crystals with relatively uncertain geological histories or polycrystalline synthetic, pressed an
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