It was noted that the isometric pyrochlore, A2B2O7, with compositions in the Y2(Ti,Sn,Zr)2O7 ternary system, were of particular interest because of their marked changes in properties (as ionic conductivity) and response to radiation damage; as a function of the disorder of the A- and B-site cations and O vacancies. First-principles calculations, using density functional theory and the plane-wave pseudopotential method, predicted lattice constants (1.0049 to 1.0463nm), atomic coordinates and bulk moduli (176 to 205GPa) that were linearly dependent upon the B-site cation radius (0.062 to 0.072nm). However, the energetics of formation of cation-antisite (0 to 2eV) and Frenkel-pair (4 to 11eV) defects did not correlate with cation size. This indicated the importance of the specific electronic configuration of the B-site cation. The greater degree of covalent bonding between <Sn4+-O>, as compared with <Ti4+-O> or <Zr4+-O>, resulted in defect formation energies that were otherwise unexpected; due to the radius ratios of the cation species. Here, Y2Sn2O7 exhibited 2 to 4eV greater defect formation energies than were predicted from mean B-site cation sizes. Relaxed calculations of coupled cation-antisite and Frenkel-pair defects showed that cation-antisite reactions probably drove the O-Frenkel pair defect formation process that eventually led to increased O mobility and completely aperiodic structures. The total charge and partial density of state calculations exhibited strikingly different behavior for O on 2 different crystallographic positions; thus emphasizing the need for a full account of the electronic structure.

First-Principles Calculation of Defect-Formation Energies in the Y2(Ti,Sn,Zr)2O7 Pyrochlore. W.R.Panero, L.Stixrude, R.C.Ewing: Physical Review B, 2004, 70[5], 054110 (11pp)