Entanglement of charge transfer, hole doping, exchange interaction and   octahedron tilting angle and their influence on the conductivity of   La1-xSrxFe0.75Ni0.25O3-{\delta}: A combination of x-ray spectroscopy and   diffraction
  Substitution of La by Sr in the 25% Ni doped charge transfer insulator LaFeO3 creates structural changes that inflect the electrical conductivity caused by small polaron hopping via exchange interactions and charge transfer. The substitution forms electron holes and a structural crossover from orthorhombic to rhombohedral symmetry, and then to cubic symmetry. The structural crossover is accompanied by a crossover from Fe3+-O2--Fe3+ superexchange interaction to Fe3+-O2--Fe4+ double exchange interaction, as evidenced by a considerable increase of conductivity. These interactions and charge transfer mechanism depend on superexchange angle, which approaches 180{\deg} upon increasing Sr concentration, leading an increased overlap between the O (2p) and Fe/Ni (3d) orbitals. 
Selma Erat, Artur Braun, Cinthia Piamonteze, Zhi Liu, Alejandro   Ovalle, Hansj\"urgen Schindler, Thomas Graule, Ludwig J. G...
cond-mat.str-el cond-mat.mtrl-sci