First-principles Study of Perovskite Ultrathin Films: Stability and Confinement Effects

In this first-principles study we investigate the atomic, electronic, and vibrational structure of BaZrO₃(001) ultrathin films and surfaces, using a hybrid functional and a local Gaussian-like basis set. The low-index nonpolar (001) surface is known to be the most stable. We considered both possible kinds of nonpolar terminations (BaO and ZrO₂) for the (001) surface. The systems were studied using a slab model. Ultrathin films were modeled using slabs with the number of atomic planes ranging from three to seven, whereas surfaces were modeled with much thicker slabs composed of 15 atomic planes. In order to estimate the Gibbs free energy at finite temperatures, lattice vibrational frequencies were also calculated. We found that phonons noticeably affect the relative thermodynamic stability of the two termination layers: while at room temperature the BaO termination has the lowest energy, at intermediate temperatures (500 K) both terminations can coexist, and at higher temperatures (900 K) the ZrO₂-terminated surface becomes the most stable. We considered the effect of two-dimensional confinement on the structural, electronic, and vibrational properties of these ultrathin films. We found these confinement effects to be short ranged, with the properties of three-plane films to be the only ones that noticeably differ from the bulk material. Finally, we briefly consider confinement effects in such ultrathin films containing neutral and fully charged oxygen vacancies (charge states 0 and +2). We show, in particular, how lattice vibrations affect the Gibbs formation energy of a neutral oxygen vacancy making it completely independent of the film thickness at high temperatures (1000 K), due to cancellation of enthalpy and entropy contributions.