Predicted structure and stability ofA4B3O12δ-phase compositions

Abstract

A combination of atomistic simulation techniques has been employed to predict ordered structures for a series of A 4 B 3 O 12 ␦-phase compounds, where A is a 3+ cation ranging in size from Sc 3+ to Ho 3+ and B is a 4+ cation ranging from Ti 4+ to Zr 4+ . Experimentally, a fully ordered cation structure has yet to be resolved for any of these compounds. Monte Carlo energy-minimization calculations using short-range pair potentials identified three low-energy arrangements of A 3+ and B 4+ cations. The details of these three structures were analyzed with the layer motif method. To quantitatively determine the ␦-phase structure of each composition, the three configurations were reevaluated with density-functional theory. We also used special quasirandom structures to compare the ordered low-energy configurations to cation disorder. For all compositions considered, we find that at least one of the three ordered structures is lower in energy than the disordered structure, suggesting the thermodynamic stability of an ordered phase. Of the three ordered structures identified by this approach, one has not been identified previously in the literature for any composition. In addition, we discuss the stability of ␦-phase compounds with respect to other "ABO 4−x " fluorite-derivative compositions and predict the structure of compositions for which none has been reported.

Chemical ordering in substituted fluorite oxides: a computational investigation of Ho2Zr2O7 and RE2Th2O7 (RE=Ho, Y, Gd, Nd, La)

“…In this study, the diffraction patterns for the samples studied displayed only peaks characteristic of the fluorite structure, indicative of the stability of a disordered fluorite solid-solution phase, consistent with previous reports (see, e.g., Stanek et al 21. and references therein).…”

“…It is perhaps not surprising that the δ -Zr 3 Y 4 O 12 phase and lowest-energy A 2 B 2 O 7 configuration for Ho 2 Zr 2 O 7 have a similar oxygen coordination preference (i.e., reduced (increased) oxygen coordination for the tetravalent (trivalent) cation relative to the random phase), given that the Ho and Y cations have similar ionic radii (1.015 and 1.019 Å, respectively34). Given this observation, and previous work by Stanek et al 21. predicting stability of δ -phase structures for systems with cation size ratios bordering on those considered here, a more complete understanding of ordering tendencies in these systems would benefit from future work investigating the competition between δ -phase related ordering and the structures studied here for A 2 B 2 O 7 compositions.…”

Computational study of the energetics of charge and cation mixing in U1−xCexO2Hanken1, Stanek2, Grønbech‐Jensen3 et al. 2011Phys. Rev. BSelf Cite370251View full textAdd to dashboardCite

“…[17][18][19][20] for the O 2− , U 4+ , Ce 3+ and Ce 4+ ions, and are reproduced in Table II for reference. Also included in Table II are potential parameters for the U 5+ ion developed by one of us previously 39 . In applying these potentials to urania-ceria solid solutions with mixed charge states, an additional parameter is needed: uranium's fifth ionization energy minus cerium's fourth ionization energy.…”

Ion irradiation induced order-to-disorder transformations in δ-phase Sc4−xZr3+xO12+x/2

“…An easy means to visualize the d-phase structure is via a layered atom description of the crystal structure [17,18]. Initially, we will describe the idealized d-phase structure with the ideal, M 7 O 12 stoichiometry.…”