Self-organization of atomic order and electronic structure in LaSrMnO films

A linear relationship between the critical temperatures T_max and T_min in the temperature dependences of the resistance of La_0.6Sr_0.2Mn_1.2O₃ single-crystal films that have a mesoscopic irregularity (metallic clusters in an insulating matrix) is found. A correlation between the atomic order and electronic structure of the films is studied by taking X-ray diffraction patterns and optical absorption spectra. It is shown that a rise in T_max and a simultaneous decrease in T_min cause correlated local changes in cluster areas of the structure. Namely, the volume occupied by a family of Mn-O planes with large interplanar spacings (d=2.04–2.08 Å) shrinks, while the volume occupied by a family of closer spaced (d=1.90–1.99 Å) planes grows. In the electronic subsystem, the density of states at ?ω=1.5 and 2.4 eV, which are due to Mn³⁺ and Mn⁴⁺ ions, increases, and the contribution from Mn²⁺ states at ?ω=0.9 eV decreases. As the charge states associated with Mn³⁺ and Mn⁴⁺ ions become dominant, the Mn-O binding energy grows. As a result, the contribution of the structural states with smaller d increases, thereby raising the density of states in the electronic subsystem at energies between 0.5 and 2.7 eV. The effect of self-organization in the multicomponent LaSrMnO system shows up in the transition from the heavily distorted rhombohedral to the less distorted orthorhombic structure.