Ion Beam-Induced Amorphization of the Pyrochlore Structure-Type: A Review
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Ion Beam-Induced Amorphization of the Pyrochlore Structure-Type: A Review R. C. Ewing, J. Lian, and L. M. Wang Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109-2104, USA
ABSTRACT This paper reviews the recent developments in the understanding of the radiation-damage processes in A2B2O7 (Fd3m; Z=8) pyrochlore-structure compounds. Pyrochlore structure compounds display a wide range of behaviors in response to ion beam irradiation. Some compositions, such as Gd2Ti2O7, are amorphized at relatively low doses (~0.2 dpa at room temperature) while other compositions, such as Gd2Zr2O7, do not amorphize (even at doses of 36 dpa at 25 K) and instead disorder to a defect fluorite structure. The response to ion beam irradiation is highly dependent on compositional changes that affect both the structural distortion from the ideal fluorite structure and the associated energetics of the disordering process. Generally, the ionic size of the cations plays a dominant role in determining the radiation response of different pyrochlore compositions. However, the cation ionic radius ratio criteria cannot be applied allinclusively in predicting the radiation “tolerance” of a pyrochlore. Systematic irradiation studies of the radiation response of rare-earth (A-site) pyrochlores in which B = Ti, Zr, and Sn have shown that the behavior of the pyrochlore also depends on the cation electronic structure, i.e., the type of bonding, which is closely related to the polyhedral distortion and structural deviation from the ideal fluorite structure. These structural changes affect the dynamic defect recovery process directly linked to the material’s response to and recovery from irradiation.
I. INTRODUCTION Pyrochlore, A1-2B2O6Y0-1, encompasses a wide range of chemistries that are manifest in over 450 synthetic compositions [1]. The diverse chemistry is coupled to a remarkable variation of properties that are important in numerous technological applications, such as catalysis, piezoelectricity, ferro- and ferrimagnetism, luminescence, giant magnetoresistance, and resistance to radiation damage. The electronic properties vary from being metallic or semiconducting to having a high ionic conductivity. There has been an increased interest in pyrochlore because it can be used as a nuclear waste form for the immobilization of actinides, particularly plutonium from dismantled nuclear weapons [2]. Actinides decay by the emission of an alpha-particle, hence there has been considerable effort devoted to understanding the relationship between radiation damage [3] and the composition of the pyrochlore [4-7]. In a related effort, there has been an increasing effort to use ion beam irradiations to manipulate the properties, such as ionic conductivity, of pyrochlore at the nano-scale [8]. In this paper we review the results of systematic ion beam irradiations for pyrochlores in which A = lanthanides and B = Ti, Zr, and Sn.
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II. STRUCTURE AND COMPOSITION Pyrochlore is isometric (Fd3m, Z = 8, a
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