Oxygen $1s$ near-edge x-ray absorption spectroscopy and x-ray diffraction investigations on ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_{1+x}\mathrm{Mn}{\mathrm{O}}_4$ $(0⩽x⩽0.5)$ single crystals evidence a doping- and temperature-dependent redistribution of $e_g$ electrons between $d_{3z^2-r^2}$, $d_{x^2-y^2}$, and $d_{3x^2-r^2}∕d_{3y^2-r^2}$ orbitals: For undoped $\mathrm{La}\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_4$ the $e_g$ electrons predominantly reside on $d_{3z^2-r^2}$ orbitals, nevertheless a small amount of charge carriers is found on in-plane $d_{x^2-y^2}$ orbitals. These in-plane electrons are gradually transferred to the $d_{3z^2-r^2}$ states upon cooling. Sr doping, however, does not only provide holes to the system but also induces a continuous transfer of electrons from $d_{3z^2-r^2}$ to $d_{3x^2-r^2}∕d_{3y^2-r^2}$ orbitals and, finally long-range charge-exchange-type orbital ordering develops for ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{1.5}\mathrm{Mn}{\mathrm{O}}_4$ below $T⩽225\mathrm{K}$. The data indicate a short-range orbital ordering for the intermediate doping range.

• Received 31 May 2006

DOI:https://doi.org/10.1103/PhysRevB.74.184414