Bulk and surface simulation studies of La1-xCaxMnO3

The structural properties of La1-xCaMnO3 are investigated by employing bulk and surface simulation techniques. The potential parameters reproduce the crystal structures of both end compositions, giving good agreement with experimental data. The calculated variation of the lattice parameters with Ca concentration also agrees with experimental values. Our calculations predict the formation of a solid solution and that, at very low temperatures and near x = 1/2, the cations preferentially order on both A and B sites. The driving mechanism for cation ordering arises from both the coulomb and ion-size terms, with the relative proportions depending on composition. However, at temperatures where high-temperature synthesis normally occurs, the ordering of the cations is random. Surface energies are calculated for (010) and (110) surfaces for the end compositions: in LaMnO3, Mn-terminated layers are more stable, whereas Ca-terminated layers are found to be more stable for the CaMnO3 system. It is proposed that the surface structure of intermediate compositions is controlled by a subtle interplay between the different cation ionic strengths and their respective concentrations.