Orbital ordering in the two-dimensional ferromagnetic semiconductor Rb₂ CrCl₄

We present the results of electronic structure calculations for the two-dimensional ferromagnet Rb₂ CrCl₄. They are obtained by the augmented spherical wave (ASW) method as based on density functional theory and the local density approximation. In agreement with experimental data, Rb₂ CrCl₄ is found to be semiconducting and displays long-range ferromagnetic order of the localized Cr 3d moments. The magnetic properties are almost independent of the structural modifications arising from the Jahn-Teller instability, which leads from the parent body-centered tetragonal K₂ NiF₄ structure to a side-centered orthorhombic lattice. In contrast, the insulating gap is observed only for the latter structure due to the reduced symmetry. Our results thus suggest to interpret the orbital ordering as a necessary prerequisite for the opening of the gap rather than for the ferromagnetic order.