ELECTRONIC-STRUCTURE AND MAGNETIC AND OPTICAL-PROPERTIES OF NIAS-TYPETRANSITION-METAL CHALCOGENIDES MX (M=FE, CO - X=S, SE)

Electronic band structure calculations are performed for MX (M = Fe, C o; X = S, Se) with the ideal NiAs-type structure by using a linearized augmented-plane-wave (LAPW) method. The denstity of states (DOS) at t he Fermi level in the non-magnetic state is significantly reduced with decreasing the lattice constant c. This result is consistent with the fact that FeS (or Fe7Se8) having a larger c value shows a magnetic or dering, while CoS (or Co7Se8) dose not. In the optical conductivity ca lculated by neglecting the transition matrix elements, a prominent pea k is found at 0.5 eV for FeSe, while a broad hump exists around 1.5 eV for CoSe. The results show good correspondence to experimental data o btained for Fe7Se8 and Co7Se8.