Structure, microstructure, and high-temperature transport properties of La_1-xCa_xMnO_3-δ thin films and polycrystalline bulk materials

Single-phase samples of La_1-xCa_xMnO_3-δ (LCMO), x≈0.3, prepared by pulsed reactive crossed beam laser ablation on SrTiO₃ (STO) substrates, and soft chemistry synthesized powders were studied by various methods. The precise study of the crystal structure and microstructures by a combination of electron diffraction and high-resolution electron microscopy revealed a monoclinic distortion of the GdFeO₃-type structure, P2₁/c space group, in both types of materials, i.e., the thin films and powder compound. The analysis of the LCMO/STO interface showed nonhomogeneous stress states and a composition that results in a different superstructure from the usual detected structure. The temperature-dependent thermoelectric power in the case of thin films presented an anomalous behavior compared to those from the powder compound. A structural transition at high temperature (T≈750 K) influences the thermopower behavior as well as the thermal conductivity values.