Ab initiostudy ofMF2(M=Mn,Fe,Co,Ni)rutile-type compounds using the periodic unrestricted Hartree-Fock approach

Abstract

The ab initio periodic unrestricted Hartree-Fock method has been applied in the investigation of the groundstate structural, electronic, and magnetic properties of the rutile-type compounds MF2 ( M5Mn, Fe, Co, and Ni!. All electron Gaussian basis sets have been used. The systems turn out to be large band-gap antiferromagnetic insulators; the optimized geometrical parameters are in good agreement with experiment. The calculated most stable electronic state shows an antiferromagnetic order in agreement with that resulting from neutron scattering experiments. The magnetic coupling constants between nearest-neighbor magnetic ions along the @001#, @111#, and @100 #~ or @010#! directions have been calculated using several supercells. The resulting ab initio magnetic coupling constants are reasonably satisfactory when compared with available experimental data. The importance of the Jahn-Teller effect in FeF2 and CoF2 is also discussed. Several transition-metal ~TM! oxides and fluorides crystallize in the rutile-type structure, which is the simplest and most common MX 2 structure, where each M atom is octahedrally coordinated by the X ligands. Slight distortions of the MF6 structural units are present due to electrostatic and/or Jahn-Teller effects. The physical properties of the TM compounds with rutile-type structure vary considerably: from ionic or covalent insulators to metals, from diamagnetic to strong ferromagnetic or antiferromagnetic semimetallic or insulating systems. This feature is a challenge to modern theoretical methods. In this context a series of compounds whose physical properties exhibits a smooth variation from one member to the other provide a suitable test of theory. This is precisely the case of the MF2 ( M5Mn, Fe, Co, and Ni! family of ionic compounds in which the interplay between structural parameters, chemical bonding, and magnetic coupling may be expected to display such a smooth variation. Two different ab initio periodic approaches are commonly used in solid-state physics to study TM compounds: the Hartree-Fock, 1,2 and the density-functional ~in its local or gradient corrected variants 3 ! schemes. In the latter both the exchange and correlation parts of the electron-electron interaction are taken into account in an approximate way. In the former the electron exchange part is treated exactly while electron correlation is neglected. When the spin-unrestricted form of the Hartree-Fock approach ~UHF! is used in the study of insulating magnetic systems, the correct sign and a reasonably good description of the magnitude of the magnetic coupling constants usually results. The ab initio UHF