The origin of successive phase transitions observed in the layered perovskite α-Sr2CrO4 is studied by the density-functional-theory-based electronic structure calculation and mean-field analysis of the proposed low-energy effective model. We find that, despite the fact that the CrO6 octahedron is elongated along the c-axis of the crystal structure, the crystal-field level of nondegenerate 3dxy orbitals of the Cr ion is lower in energy than that of doubly degenerate 3dyz and 3dxz orbitals, giving rise to the orbital degrees of freedom in the system with a 3d2 electron configuration. We show that the higher (lower) temperature phase transition is caused by the ordering of the orbital (spin) degrees of freedom.
@article{arxiv.1511.06217,
title = {Reversed Crystal-Field Splitting and Spin-Orbital Ordering in $\alpha$-Sr$_2$CrO$_4$},
author = {T. Ishikawa and T. Toriyama and T. Konishi and H. Sakurai and Y. Ohta},
journal= {arXiv preprint arXiv:1511.06217},
year = {2017}
}