Defect-free and Defective Surfaces of the Pyrochlore Oxide La 2 Zr 2 O 7 : A Theoretical Study
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Defect-free and Defective Surfaces of the Pyrochlore Oxide La2Zr2O7: A Theoretical Study Yves A. Mantz U.S. Department of Energy, National Energy Technology Laboratory, 3610 Collins Ferry Road and PO Box 880, Morgantown, WV 26507, U.S.A. ABSTRACT In this work, the low index faces of lanthanum zirconate (La2Zr2O7, LZ) are studied at the level of density-functional theory, representing the first theoretical attempt to characterize the surfaces of a pyrochlore oxide. All possible surface terminations formed by cleaving a perfect crystal are considered, as well as selected defective surfaces. After deriving the expression for the free energy of an LZ surface, surface free energies are computed. The most stable surface terminations are identified, their geometric and electronic structures discussed, and a motivation provided for calculating ratios of certain surface free energies more accurately for comparison to experimental results that will be obtained. INTRODUCTION The pyrochlore oxides are a class of compounds with general formula A2B2O7, where A and B are metallic cations that can be either trivalent and tetravalent or divalent and pentavalent, respectively. Because many different pairs of ions are suitable for the pyrochlore structure, resulting in a spectrum of material properties, they are being considered for use in a variety of applications, e.g., as host matrices for radioactive wastes and surplus actinides, as oxygen-ion conductors for sensors and fuel cells, and as oxidation and reduction catalysts.[1] Catalytic applications are of particular interest. For example, it was recently shown at our laboratory that LZ, when doped with Rh or Sr and Rh metals, is an effective catalyst for the partial oxidation of fuel into syngas (CnHm + O2 → H2 + COx).[2] In this context, an understanding of LZ surface properties would be useful. However, despite extensive experimental studies of LZ thin films[3] and crystallites,[2, 4, 5] our atomistic level understanding of the surfaces is limited. Thus, a motivation is provided for the present computational study, the first of a pyrochlore oxide surface. A key obstacle is that the defect-free surfaces of LZ are metastable (or, more specifically, polar). Thus, the goal is to characterize the surfaces as a baseline for future work. THEORY Computations are performed at the level of density-functional theory (DFT) and the pseudopotential approximation, as implemented in the Vienna ab initio simulation package (VASP).[6] Computational details are: the generalized gradient approximation (GGA) (PW91 functional), the projector augmented wave (PAW) pseudopotential method, spin-restricted DFT, a plane-wave basis set (250 eV cutoff), and a Monkhorst-Pack k-point sampling scheme (4×4×4 for bulk, 4×4×1 for surfaces). Results reported are converged with respect to the treatment of electron spin, plane-wave cutoff, and k-point mesh density. The surfaces are modeled using the periodic slab technique, in which a slab model (82-142 atoms) plus a 16 Å vacuum region is periodically replicated in 3
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