Relationships between crystal structure and magneto-transport properties in C.M.R. thin film of lacunar Lanthanum manganite before and after annealing

La_0.8MnO_3-δ,) epitaxial thin films have been deposited by Injection-MOCVD on MgO. As-deposited film has a semiconducting-metal and para-ferromagnetic transition at about 220 K ; after annealing under air for 3 h at 700°C, this transition temperature is about 310K. The crystal structure of the film has been studied by XRD and TEM, before and after annealing. XR data collections have been recorded from an 1 mm ² thin film piece, using a conventional 4-circlesdiffractometer designed for single crystals. In both cases the apparent symmetry is cubic with a cell parameter twice that of the classical perovskite. In fact, the true crystal structure is monoclinic with microdomain twinning. Atomic positions of all the atoms have been determined. Improvement of the magneto-transport properties after annealing is due to strengthening of the magnetic exchange interactions (Mn-O-Mn angles change from about 165° to 1.76°) and to increasing of the Mn⁴⁺/Mn³⁺ ratio.