Structural phase transitions and stress accommodation in „La0.67Ca0.33MnO3...1Àx :„MgO...x composite films

Composite (La0.67Ca0.33MnO3)12x :(MgO)x films were prepared by metalorganic aerosol deposition on a ~100!MgO substrate for different concentrations of the ~MgO! phase (0<x<0.8). At x'0.3 a percolation threshold in conductivity is reached, at which an infinite insulating MgO cluster forms around the La0.67Ca0.33MnO3 grains. This yields a drastic increase of the electrical resistance for films with x.0.3. The film structure is characterized by x-ray diffraction and transmission electron microscopy. The local structure of the La0.67Ca0.33MnO3 within the film depends on the MgO concentration which grows epitaxially along the domain boundaries. A different structural phase transition from the orthorhombic Pnma structure to an unusual rhombohedral R 3̄c structure at the percolation threshold x'0.3 is found for La0.67Ca0.33MnO3. A threedimensional stress accommodation in thick films through a phase transition is suggested.