First-principles calculation of structural and energetic properties for A 2 Ti 2 O 7 (A = Lu, Er, Y, Gd, Sm, Nd, La)
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Fei Gao and W.J. Weberb) Pacific Northwest National Laboratory, Richland, Washington 99352 (Received 20 June 2008; accepted 12 November 2008)
A first-principles method was used to investigate the structural and energetic properties for A2Ti2O7 (A = Lu, Er, Y, Gd, Sm, Nd, La), including the formation energies of the cation antisite-pair, the anion Frenkel pair that defines anion-disorder, and the coupled cation antisite-pair/anion-Frenkel. It is proposed that the hA–O48fi interaction may have more significant influence on the radiation resistance behavior of titanate pyrochlores, although the hTi–O48fi interactions are relatively stronger than the hA–O48fi interactions. It was found that the defect formation energies are not simple functions of the A-site cation radii. The formation energy of the cation antisite-pair increases continuously as the A-site cation varies from Lu to Gd, and then decreases continuously with the variation of the A-site cation from Gd to La, in excellent agreement with the radiation-resistance trend of the titanate pyrochlores. The band gaps in these pyrochlores were also measured, and the band gap widths changed continuously with cation radius.
I. INTRODUCTION
Pyrochlore structured oxides (A2B2O7), which encompass a remarkable range of compounds, show a variety of physical, chemical, and electrical properties including high ionic conductivity, superconductivity, luminescence, and ferromagnetism.1 This leads to significant potential for their use in a wide range of technical applications, such as solid electrolyte,2,3 oxygen gas sensor,2,4 and fuel cells in space.5,6 In particular, experimental studies have shown that pyrochlore structured oxides can be used as host matrices for immobilization of actinide wastes.7 To explore the mechanism responsible for varying radiation resistance of different pyrochlore oxides, numerous experimental studies have been performed.3–5,8–26 Irradiation-induced amorphization was initially observed in Gd2Ti2O7 pyrochlore due to alpha decay of incorporated 244Cm at room temperature.8,9 Subsequent studies showed that all of the rare-earth titanate pyrochlores can be amorphized even at a relatively low irradiation dose. For example, Gd2Ti2O7 can be amorphized by a)
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs .org/jmr_policy DOI: 10.1557/JMR.2009.0152 J. Mater. Res., Vol. 24, No. 4, Apr 2009
600 keV Ar+ at room temperature at 0.2 dpa.18 In addition, it was found that different pyrochlore compositions have different response behaviors under irradiation,14–17,27,28 since different rare-earth titanate pyrochlores have a different critical amorphization temperature and critical dose along with the substitution of the A-site cations. Lian et al.16 have investigated the irradiation response of A2Ti2O7 (A ▽ Sm to Lu, and Y) pyrochlores by means of in
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