Structural, Electronic, and Magnetic Properties of KCrF₃: A Quantum-Mechanical Investigation

The structural, electronic, and magnetic properties of the KCrF₃ perovskite have been investigated using an all-electron Gaussian-type basis set and various functionals, including full-range and range-separated hybrids, as implemented in the crystal code. Structural optimizations were performed by imposing tetragonal (I4/mcm) and monoclinic (I112/m) space-group symmetries, corresponding to the experimentally observed phases above and below 250 K, respectively. Three AFM arrangements were considered and compared with the FM one: the AFMA phase (spin inversion between first-neighbor Cr ions along the c-axis) is more stable than the FM phase, in agreement with experimental evidence. The monoclinic I112/m structure is energetically favored over the tetragonal I4/mcm phase by 5.9 meV per transition-metal ion with the B3LYP functional, and by 3.4 meV when using PBE0 or HSE06. This small energy difference is associated with only minor changes in structural and electronic properties, including a slight volume contraction (-0.4%) and increase in the band gap (+2%). Mulliken population analysis indicates a Cr d-shell occupation of 3.907 |e|, which is very close to the formal d⁴ configuration. Nearly exactly three electrons populate the t_2g manifold (0.974 |e| each), while the remaining electron is distributed between the $d_{z^2}$ (0.360 |e|) and $d_{x^2-y^2}$ (0.624 |e|) orbitals. Spin-density maps clearly show the orbital ordering within the ab plane and provide insight into the stabilization mechanism of the AFMA phase, which is mediated by the polarization of the fluorine valence shell.