Compounds based on lanthanum zirconate (La2Zr2O7) have been shown at the National Energy Technology Laboratory and elsewhere to be effective catalysts in energy applications such as fuel reforming. In addition, La2Zr2O7-based materials have been tested as catalysts to promote oxygen reduction in solid oxide fuel cells. To explain the effectiveness of La2Zr2O7-based catalysts, an understanding of the structural properties of La2Zr2O7 surfaces is needed. Accordingly, certain La2Zr2O7 surfaces that are expected to be the most prevalent in prepared samples of La2Zr2O7, namely, the low-index or (001), (011), and (111) surfaces, are examined at the level of density-functional theory. All possible surface terminations of these surfaces formed by cleaving a perfect crystal, termed ideal surface terminations, are considered, as well as several defective surface terminations of a given surface. These different surface terminations are characterized by computing their surface free energies. A defective (001), (011), or (111) surface termination is shown to be thermodynamically preferred to any ideal (001), (011), or (111) surface termination, respectively, over a large range of temperature and oxygen gas partial pressure conditions. This finding is unchanged upon taking into account error in the determination of a set of conditions, a procedure that includes adding vibrational contributions to surface free energies and other quantities, with contributions obtained from computed phonon dispersions and characteristic atomic frequencies. By making this comparison of ideal and defective surface terminations, evidence is provided that all three low-index surfaces are defective or otherwise not ideal, for example, reconstructed, under large sets of conditions. After this conclusion is drawn, a possible use of results is proposed to interpret experimental data that still needs to be obtained.

Density-functional study of the La2Zr2O7 low-index faces

253rd National Meeting (2017)

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