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Abstract. -Mossbauer spectra were taken of a series of compounds with tetrahedrally coordinated (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 hybridization 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.

In this work we use ab initio Hartree-Fock effective core potentials to analyze N 2 dissociation on doped ͑bimetallic͒ Cr ͑110͒ surfaces described by Cr 4 MN 2 and Cr 3 M 2 N 2 (M ϭSc, Ti, V, Fe, Co, Ni, Zn, Mg, Ca) clusters in both perpendicular and inclined configurations. Our results indicate that the inclined state is energetically more favorable than the perpendicular configuration. In comparison with monometallic surfaces, some of the bimetallic systems, such as Cr 4 ScN 2 , Cr 4 TiN 2 , and Cr 3 V 2 N 2 , are found to have larger N-N distances, surface→N 2 charge transfers, higher Fermi energies, and smaller N-N stretching frequencies, indicating greater efficiency of these alloys for dissociation. The theoretical parameters obtained from these bimetallic catalytic systems are interpreted using a general mechanism proposed for dissociation of diatomic molecules on transition metal surfaces. ͓S0163-1829͑99͒02639-9͔