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Synthetic FeS2 and RuS2 single crystals were prepared by the flux method using PbCl2, bismut, and tellurium as different solvents. FeS2 and some RuS2 samples were doped with elementary vanadium, other RuS2 crystals with manganese and nickel. The EPR spectra revealed Mn2+ in the low-spin state and Ni2+. The lineshape of the Mn sextet turned out to be asymmetrical. In some RuS2 samples we detected an isotropic structureless rather narrow signal with a linewidth down to 0.05 mT and attributed it to the conductivity. Its lineshape becomes symmetrical and Lorentzian by grinding the samples to a polycrystalline powder. The EPR parameters of appropriate spinHamiltonians were determined. The intensities of the allowed and forbidden lines depended very differently on the microwave power due to different degrees of saturation.

Synthetic pyrite crystals doped with halogen (CI, Br) or manganese (Mn) were investigated by ESR at 78 K with an X-band spectrometer and a TE102 cavity with 100 kc field modulation. The crystals were preferably measured with the directions 〈100〉, 〈110〉, and 〈111〉 parallel to the static magnetic field HO. The ESR spectrum of Mn-doped crystals showed a sextet which can be explained by a spin-Hamiltonian for an electron spin S = 1/2 in interaction with the nuclear spin I = 5/2 of the Mn nucleus for axial symmetry. Due to the observed spin and due to the orientation of the paramagnetic centers the spectra were assigned to Mn2+ in the low spin state which resides on cation site in the FeS2, lattice. In halogen-doped crystals four pairs of lines can be interpreted by the interaction of the two isotopes of I = 3/2, 79Br, 81Br and 35Cl, 37Cl, respectively, with an electron spin S = 1/2. According to the observed symmetry of the ESR signals the paramagnetic centers can be explained as (S-X)2- radicals (X = CI, Br) which substitute for the (S2)2- dumb-bells of pyrite. All crystals evoked extremely narrow ESR lines, especially the halogen-doped crystals with linewidths down to 0.03 mT. Excellent agreement between measured and calculated spectra up to the finest details has been obtained.

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