Lanthanum zirconate (La2Zr2O7, LZ) and various doped forms of LZ have been shown to be effective catalysts. Doped forms of LZ can catalyze fuel and methane reforming processes, while both LZ and doped forms of LZ are able to catalyze the combustion of methane. In addition, doped forms of LZ have been tested as catalysts for the synthesis of higher alcohols by CO hydrogenation and to promote oxygen reduction in solid oxide fuel cells. To explain the ability of LZ and doped forms of LZ to function as catalysts, an understanding of LZ surface structures is needed. Accordingly, certain LZ surfaces that are expected to be prevalent in prepared samples of LZ, namely, the low-index (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 LZ crystal, termed ideal surface terminations, are considered, as well as several defective surface terminations of a given surface. Surface free energies are computed, and surface phase diagrams (SPDs) of the (001), (011), and (111) surfaces are obtained. From these SPDs, the thermodynamically preferred surface termination of the (001), (011), or (111) surface is concluded to be defective, versus ideal, over a large range of temperature T and oxygen gas partial pressure pO2 conditions. This finding is unchanged if the SPDs are determined more accurately by taking into account certain vibrational contributions obtained from computed phonon dispersions. Thus, evidence is provided, in the form of SPDs, that the (001), (011), and (111) surfaces are nonideal over large sets of T and pO2 conditions. In addition, a way to use results to characterize the surfaces of synthesized LZ crystallites is discussed, but the shapes or dimensions of the crystallites first need to be obtained.

Density-functional study of the La2Zr2O7 low-index surfaces

255th National Meeting (2018)

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