Ab initiostudy of antiferromagnetic rutile-typeFeF2

The periodic unrestricted Hartree-Fock method has been used to calculate the ground-state spin-polarized wave function and total energy of tetragonal ${\mathrm{FeF}}_2$. Contracted Gaussian-type functions represent the atomic orbitals of Fe and F atoms. The nature of the antiferromagnetic insulator, with a d-d type band gap, is reproduced correctly by the calculations. On the basis of the density of electron states, Mulliken population data, and charge- and spin-density maps, the ${\mathit{d}}_{\mathit{x}\mathit{y}}^2$/${\mathit{d}}_{\mathit{x}\mathit{z}}^1$ ${\mathit{d}}_{\mathit{y}\mathit{z}}^1$ splitting of ${\mathit{t}}_{2\mathit{g}}^4$ states is proved for the ${\mathrm{Fe}}^{2+}$ ion, consistent with the Jahn-Teller distortion of the ${\mathrm{FeF}}_6$ octahedron. A limited charge back transfer from ${\mathrm{F}}^{\mathrm{-}}$ to ${\mathrm{Fe}}^{2+}$ is observed, with ${\mathit{z}}_{\mathrm{Fe}}$=+1.84‖e‖. The equilibrium structural configuration and its evolution with pressure up to 12 GPa have been calculated, including internal relaxation. The athermal equation of state, structural, and elastic parameters show a good agreement with experimental data.