Thermodynamics of Oxygen Chemistry on PbTiO 3 and LaMnO 3 (001) Surfaces
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Thermodynamics of Oxygen Chemistry on PbTiO3 and LaMnO3 (001) Surfaces Ghanshyam Pilania and R. Ramprasad Chemical, Materials, and Biomolecular Engineering, Institute of Materials Science University of Connecticut, Storrs, CT 06269 ABSTRACT We present a first principles thermodynamic study of O ad-atom and vacancy formation on the AO- and BO2-terminated (001) surfaces of the PbTiO3 (PTO) and LaMnO3 (LMO) cubic perovskites. Our results show that, owing to the highly energetically unfavorable nature of O vacancy formation on these surfaces, O vacancies appear only at high temperatures and practically irrelevant low pressures on the (T, p) surface phase diagram. In contrast, effortless formation of O ad-atoms on the surfaces is encountered at practically achievable pressures and temperatures. Above room temperature and close to atmospheric pressures, we predict clean PbO and TiO2-terminated (001) PTO surfaces as the stable surface phases while partially or fully O ad-atom covered surfaces are found to be more stable for LMO. These results are consistent with the observation that LMO is far more active towards oxidation catalysis than PTO. INTRODUCTION For perovskites, our knowledge of surface processes occurring during catalytic oxidation processes is still limited. The catalytic activity of the perovskite oxides has generally been associated with oxygen vacancies, surface adsorbed oxygen and the presence of transition metal ions with mixed valance states [1, 2]. In the present contribution, DFT total energy calculations combined with the first principles thermodynamics (FPT) approach have been used to compute surface phase diagrams of PbTiO3 (PTO) and LaMnO3 (LMO) (001) surfaces. It is assumed that the gaseous O2 is the principal source of surface oxygen and only the lean limit, in which oxygen is the (nearly) exclusive surface species, is considered. Both AO- (A=Pb, La) and BO2- (B=Ti, Mn) terminated (001) perovskite surfaces in equilibrium with O2 at some temperature and pressure (defined by an oxygen chemical potential μO) are modeled. THEORY DFT calculations were performed with the periodic supercell plane-wave basis approach as implemented in the Vienna Ab initio Simulation Package (VASP) [3]. The interactions between the valence and the frozen core electrons were simulated with the projector augmented wave (PAW) method [4], and the electron exchange and correlation interactions were treated within the local density approximation (LDA) [5]. Other technical details are as described in Ref. 6. Within these approximations the calculated PTO and LMO bulk lattice constants in the cubic phase are 3.887 Å and 3.802 Å, underestimated with respect to the corresponding experimental values of 3.97 Å and 3.947 Å, respectively [7, 8]. These calculated bulk lattice constants were then further used to construct surface models in slab geometries. Depending on the value of the μO, a perovskite surface in equilibrium with gaseous O2 may exist with varying levels of O vacancies/ad-atoms or alternatively, it may remain as a clean
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