Atomic, electronic and thermodynamic properties of cubic and orthorhombic LaMnO3 surfaces

We studied in detail the atomic and electronic structure of the LaMnO3 surfaces, in both cubic and orthorhombic phases, combining GGA-plane wave approach, as implemented into the VASP-4.6.19 computer code, with a slab model. These studies are complemented by a thermodynamic analysis of the surface stability at different gas pressures and temperatures. The obtained results are compared with similar studies for other ABO3-perovskites. 2008 Elsevier B.V. All rights reserved. The ABO3-type perovskite manganites and cobaltates (A = La, Sr, Ca; B = Mn, Co) are important functional materials with numerous high-tech applications [1]. Some of them require understanding and control surface properties. An important example is LaMnO3 (LMO) in its function as a cathode for solid oxide fuel cells (SOFC) [2–4]. LMO has a cubic structure above 750 K. Below this temperature the structure is orthorhombic, with four formula units in a primitive cell. Due to the high spin state of Mn 3+ ion, LMO reveals several magnetic configurations. Below TN = 140 K the A-type antiferromagnetic (AF) phase is the ground state. This corresponds to the ferromagnetic coupling in the basal ab (xy) plane combined with an antiferromagnetic coupling in the c (z) direction (in the Pbnm setting). There exist also other magnetic states: the ferromagnetic (FM) one, and the two antiferromagnetic GAF and CAF states. In the former all the spins are antiferromagnetically coupled to their nearest neighbors, whereas in the latter state the spins are antiferromagnetically coupled in the basal plane and ferromagnetically between the planes (along the c axis). Understanding of the pure LMO-type surface properties is a necessary first step for the spintronic low-temperature applications and for establishing the mechanism of oxygen reduction on these surfaces which is a key issue in improving the performance of SOFC cathodes at high temperatures [1]. Several first-principle studies of the LMO surfaces were undertaken recently ([5] and the references therein). However, these studies were limited to the cubic structure, often for unrelaxed surfaces, and zero temperature. In this paper, we compare properties of cubic and orthorhombic LMO surfaces at high temperatures and realistic gas pressures. We discuss here three surfaces – (0 0 1), (1 1 0) and (1 1 1) – with different terminations and in different magnetic states described above for the bulk LMO. The main calculated properties include the surface energies, atomic displacements, effective charges, and the Gibbs free energies under different external conditions.