Abstract: The zero-field splitting constant, D, and the gyromagnetic tensor of the off-center system KTaO3:Fe+ and SrCl2:Fe+ have been explored by means of calculations based on the density-functional theory at the C4v local equilibrium geometry. The calculated D values for KTaO3:Fe+ (9?cm?1) and SrCl2:Fe+ (53 cm?1) are found to be much higher than typical figures measured for insulating compounds containing common 3d Kramers ions with a spin S>1/2 in the ground state. This result together with the calculated g? and g? values concur with available experimental information. The high magnetic anisotropy derived for Fe+ in KTaO3 and SrCl2 is shown to be strongly related to the existence of a 4E excited state lying only at about 3000 and 600?cm?1, respectively, above the ground state. Implications of present findings in the search of new molecular magnets with high values of the magnetic anisotropy are discussed in some detail.

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