COVALENCY EFFECTS IN HYPERFINE INTERACTIONS IN ALKALI DITHIOFERRATES

Rksum6.- Les spectres Mossbauer des composes tktraedriques (sp3) de Fe(II1) de formule generale AFeS2 (A = K, Rb, Cs) ont Bte relevb entre 4,2 K et 300 K. RbFeS2 et CsFeS2 prksentent des structures hyperfines magnetiques a des temperatures plus basses que KFeSz, ce que nous avons attribue a des effets de liaison. L'hybridation tetraedrique dans ces composes facilite I'utilisation des calculs des gradients de champ electrique, des differences d'klectron6gativite et des constantes d'etalonnage du dkplacement isomkrique pour determiner la fraction de la densite des Blectrons 4s au site du noyau 57Fe. Des modeles simples de covalence permettent d'estimer I'ordre de grandeur des champs magnetiques internes et de relier les variations des parametres.hyperfins au caractere covalent des liaisons. Abstract.- Mossbauer spectra were taken of a series of compounds with tetrahedrally coordinat-ed (sp3) iron (111) having a general formula AFeS2, where A = K, Rb, Cs over a temperature range 4.2-300 K. RbFeS2 and CsFeSz exhibited magnetic hyperfine structures at lower temperatures than that observed for KFeS2, which we have correlated with bonding effects. The tetrahedral hybridiza-tion in these compounds facilitates the use of EFG calculations, electronegativity differences and isomer shift calibration constants, for determining the fractional 4s electron densities at the 57Fe nucleus. Simple covalency models are used to make order of magnitude estimates of the internal magnetic fields and for correlating the spread of hyperfine parameters with the covalent character of the bonds. 1. Introduction.- I