AB-INITIO STUDY OF ANTIFERROMAGNETIC RUTILE-TYPE FEF2

The periodic unrestricted Hartree-Fock method has been used to calculate the ground-state spin-polarized wave function and total energy of tetragonal FeF2. 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 d(xy)(2)/d(xz)(1)d(yz)(1) splitting of t(2g)(4) states is proved for the Fe2+ ion, consistent with the Jahn-Teller distortion of the FeF6 octahedron. A limited charge back transfer from F- to Fe2+ is observed, with z(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.