Chalkogenides of the transition elements. X. X-ray, neutron, Mössbauer, and magnetic studies of pentlandite and the π phases π(Fe, Co, Ni, S), Co 8MS 8, and Fe 4Ni 4MS 8 ( M = Ru, Rh, Pd)
The metal atoms in the cubic π phases (Fe, Co, Ni, M) 9S 8 occupy 4-coordinated majority and 6-coordinated minority sites in the ratio 8:1. X-ray and neutron powder diffraction of Co 8MS 8 and Fe 4Ni 4MS 8 ( M = Fe, Co, Ni, Ru, Rh, Pd) has shown that the structures are ordered, i.e., with M segregated in the minority sites, when M is a 4 d element, and at best partially ordered when M is a 3 d element. Magnetic susceptibility measurements and Mössbauer 57Fe spectra show that Co 9S 8 and the natural π(Fe, Ni, S) phase, pentlandite, remain Pauli-paramagnetic down to 4.2°K, with no resultant magnetic moments on the metal atoms, and hence are broad-band metals. Linear dependence of the isomer shifts at the two types of sites in a variety of (Fe, Co, Ni) 9S 8 compositions indicates the existence of a composite s- d conduction band. Analysis of the variation of the quadrupole splitting at the majority site with composition leads to the conclusion that only a few levels at the bottom of the conduction band are occupied. The number of electrons in the band is approximately proportional to the total d-electron content of the unit cell. A tentative band scheme based on these results and on other available evidence is proposed. The effect of composition on the interatomic distances in the π phases is discussed.