Structure of multilayer ZrO2/SrTiO3

Multilayered oxide heteroepitaxial systems, including that of a 1-nm-thick Y2O3-stabilised ZrO2 (YSZ) sandwiched between layers of SrTiO3 (STO) [1], have been a subject of much interest lately due to their significantly enhanced ionic conductivities as compared to the bulk materials. We aim to provide the foundation for understanding this increase in conductivity by considering the atomic configurations at the interfaces of such systems, specifically a ZrO2/STO multilayer system. Possible stable lattice structures of pure ZrO2 in the system are explored using a genetic algorithm in which the interatomic interactions are modelled by simple pair potentials. The energies of several of the more stable of these structures are then evaluated more accurately within density functional theory (DFT). We find that the fluorite ZrO2 phase is unstable as a coherently strained epitaxial layer in the multilayer system. Instead, anatase-, columbite-, rutile-, and pyrite-like ZrO2 epitaxies are found to be more stable, with the anatase-like epitaxy being the most stable structure over a wide range of chemical potential of the components. We also find a high energy metastable structure resembling the tetragonal fluorite structure which is predicted by DFT to be stabilised by SrO-terminated STO but not by TiO2-terminated STO. Introduction The increasing fascination with materials in the nanoscale stems from the fact that nanomaterials boast vastly different properties from their bulk counterparts, which may be tunable for novel technological applications. In the field of ionic conductivity, for instance, heteroepitaxial nanothin films promise significantly enhanced ionic conductivity as compared to the corresponding materials in the bulk, due to the density of interfaces present in the system [2–6]. Specifically, multilayered heterosystems consisting of alternating layers of oxide ion conductors and insulators have demonstrated an improvement of up to two orders of magnitude in ionic conductivity [7]—that is, until eight orders of magnitude enhancement was reported in a STO/YSZ/STO trilayer system just above room temperature by Garcia-Barriocanal et al. [1]. This finding would have the potential to promote rapid growth of the intermediate temperature solid oxide fuel cell (SOFC) industry, as presently one of the limitations of the technology is the low ionic conductivity of the electrolyte material at low temperatures. However, although the authors claim that the conductivity measured in their YSZ/ STO multilayers is purely ionic, the nature of the conductivity has been repeatedly questioned since the publication as oxygen ions have not been shown to possess such high mobility at low temperatures [3, 4, 8]. In SOFC, electronic conductivity in the electrolyte would be detrimental to the operating efficiency of the cell. Cavallaro et al. [9] attempted to reproduce the heterosystem using pulsed laser deposition but found that the YSZ films were discontinuous, in contrast to the original system in which the films fabricated with RF sputtering were continuous and epitaxial. Nevertheless, the total conductivity reported in [9] was nearly as high as in [1] and undoubtedly not ionic. W. L. Cheah (&) M. W. Finnis Department of Materials, Imperial College London, Exhibition Road, London SW7 2AZ, UK e-mail: [email protected] 123 J Mater Sci (2012) 47:1631–1640 DOI 10.1007/s10853-011-5985-7