Synthesis and Structure of Novel A 2 BO 5 Compounds Containing A = Y, Yb, Gd, Sm, and La and B = Zr, Ti, and Sn
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.210
Synthesis and Structure of Novel A2BO5 Compounds Containing A = Y, Yb, Gd, Sm, and La and B = Zr, Ti, and Sn R. Newman,1 R.D. Aughterson,2 and G.R. Lumpkin2 1
Imperial College, London, UK, 2Australian Nuclear Science and Technology Organisation
With the aim of creating novel ceramics for applications in nuclear materials with high radiation tolerance, multiple samples with A-B-O stoichiometries ranging from 215 to 227 were synthesized and characterized by a combination of SEM, XRD, and TEM methods. Single-phase defect-fluorite-type compounds with A = Sm or Yb and B = Ti, Zr, and/or Sn are reported; whereas, pyrochlore structured compounds and lanthanide sesquioxide phases were found as major phases in numerous samples. A series of Y-b-Sn-O samples was successfully prepared as nearly single phase or single phase materials. These are essentially defect fluorites with extra weak peaks, most likely due to X-ray scattering from oxygen or oxygen and metal cations. We describe some interesting TEM data and describe selected area diffraction patterns with complex modulations.
INTRODUCTION Rare earth based oxides, in particular pyrochlores and fluorites (or defect fluorites), have been the subject of much ongoing interest to both nuclear and electrochemical researchers for their potential radiation resistance with regard to use as nuclear waste forms [1] and for their high ionic conductivity and transport properties [2]; features that can be conjectured to be highly correlated. The precise features responsible for all the properties of such rare earth based oxide materials have yet to be comprehensively determined, but many influencing factors have been established to general satisfaction. By using a chemical formula of the form A2BO5 on the underlying A2B2O7 pyrochlore-defect fluorite stoichiometry, where A3+ is a lanthanide or Y cation, B4+ is a smaller metal cation, and A and B represent distinct lattice sites, potential solid-solutions from A2BO5 toward A2B2O7 can be represented by the general expression A2(B2-xAx)O7-x/2 (0 ≤ x ≤ 0.67) [3]. This expression reflects the ability of the pyrochlore (and disordered, defect fluorite structures) to accommodate a certain level of cation disorder and anion vacancies on the lattice, but does not account for the possibility to incorporate cation vacancies found in certain pyrochlore systems. In the cubic A2BO5 compounds, this means that some of the larger A-site cations partially occupy the smaller B-site and extra oxygen atoms enter the anion lattice, resulting in the ‘stuffed pyrochlore’ structures with variable A/B cation ratios and oxygen contents, with the potential to affect the defect energetics and ionic mobility of the system. These factors have been determined to contribute to radiation resistance as demonstrated in numerous experimental and computational studies. Although much research remains to be done on the many possible compositions that are possible in these systems, one of the underlying
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