Reversed Crystal-Field Splitting and Spin-Orbital Ordering in $\alpha$-Sr$_2$CrO$_4$

TL;DR

This study reveals that in $ ext{Sr}_2 ext{CrO}_4$, the crystal-field splitting is reversed, leading to unique spin and orbital orderings that explain its phase transitions, based on density functional theory and mean-field analysis.

Contribution

It demonstrates the reversed crystal-field splitting in $ ext{Sr}_2 ext{CrO}_4$ and links it to the material's phase transitions through theoretical modeling.

Findings

Reversal of crystal-field level ordering in $ ext{Sr}_2 ext{CrO}_4$

Orbital ordering causes high-temperature phase transition

Spin ordering causes low-temperature phase transition

Abstract

The origin of successive phase transitions observed in the layered perovskite $\alpha$-Sr$_2$CrO$_4$ 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 CrO$_6$ octahedron is elongated along the $c$-axis of the crystal structure, the crystal-field level of nondegenerate $3d_{xy}$ orbitals of the Cr ion is lower in energy than that of doubly degenerate $3d_{yz}$ and $3d_{xz}$ orbitals, giving rise to the orbital degrees of freedom in the system with a $3d^2$ electron configuration. We show that the higher (lower) temperature phase transition is caused by the ordering of the orbital (spin) degrees of freedom.