| Abstract
| - The lanthanide stannates, Ln2Sn2O7, Ln = La−Lu and Y, have the isometric pyrochlore structure, A2B2O7,and their structural properties have been refined by Rietveld analysis of powder neutron and synchrotronX-ray diffraction data. In this study, the enthalpies of formation of selected stannate pyrochlores, Ln = La,Nd, Sm, Eu, Dy, and Yb, were measured by high-temperature oxide melt solution calorimetry. Their radiationresponse was determined by 1 MeV Kr2+ ion irradiation combined with in situ TEM observation over thetemperature range of 25 to 1000 K. The enthalpy of formation from binary oxides of stannate pyrochloresbecame more endothermic (from −145 to −40 kJ/mol) as the size of the lanthanide in the A-site decreases.A more exothermic trend of the enthalpy of formation was observed in stannate pyrochlores with largerlanthanide ions, particularly La, possibly as a result of increased covalency in the 〈Sn−O〉 bond. In contrastto lanthanide titanate pyrochlores, Ln2Ti2O7, that are generally susceptible to radiation-induced amorphizationand zirconate pyrochlores, Ln2Zr2O7, that are generally resistant to radiation-induced amorphization, thelanthanide stannate pyrochlores show a much greater variation in their response to ion irradiation. La, Nd,and Gd stannates experience the radiation-induced transformation to the aperiodic state, and the criticalamorphization temperatures are ∼960, 700, and 350 K, respectively. Y and Er stannate pyrochlores cannotbe amorphized by ion beam irradiation, even at 25 K, and instead disorder to a defect fluorite structure.Comparison of the calorimetric and ion irradiation data for titanate, zirconate, and stannate pyrochlores revealsa strong correlation among subtle changes in crystal structure with changing composition, the energetics ofthe disordering process, and the temperature above which the material can no longer be amorphized. Insummary, as the structure approaches the ideal, ordered pyrochlore structure, radiation-induced amorphizationis more easily attained. This is consistent with an increasingly exothermic trend in the enthalpies of formationof pyrochlores from the oxides, that is, the greater the thermochemical stability of the pyrochlore structure,the more likely it will be amorphized upon radiation damage rather than recover to a disordered fluoritestructure.
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