Abstract
Using muffin-tin orbitals and the atomic-sphere approximation, we have studied the band structures of Chevrel-phase molybdenum chalcogenides, . Generally, these compounds exist for a broad variety of elements, and . may be between 0 and 2, depending on the element . We present level schemes, computed for a range of Mo and potentials, for three clusters appropriate for the crystal structures of , , and Pb, respectively. Self-consistent Mo and potentials have been estimated. The cluster levels give the positions of the -like bands, while the widths and dispersions are estimated analytically in the tight-binding approximation taking the covalent mixing with the states into account. The 30 bands are grouped into narrow subbands derived from the levels for an isolated octahedron. The Fermi level falls in a doubly degenerate band with Mo wave functions of character and the bandwidths vary between 65 and 35 mRy in the compounds considered. The band is probably crossed by a five times wider, singly degenerate band of predominantly character. The and bands are the only ones crossing the Fermi level in the ternaries but, in the binaries, the octahedra are elongated and a 50-35 mRy wide band, split off from a triply degenerate band, furthermore overlaps the band. The susceptibilities measured for Sn and Pb are in good agreement with our estimates, states/(spin Mo-atom Ry) and mRy, of the band density of states and the effective exchange-interaction parameter. From the measured electronic-specific-heat coefficients we deduce the value for the electron-phonon enhancement. In accord with experimental phonon spectra we estimate frequencies of 10 and 15 meV for a rocking mode of and units, respectively. For the average electron-phonon matrix element in the Gaspari-Gyorffy and atomic-sphere approximations we find (Ry/bohr . The magnitude and extreme sensitivity to local environment effects of the spin-orbit coupling in the band offer an explanation for the high critical magnetic fields measured in the ternaries.
- Received 28 June 1976
DOI:https://doi.org/10.1103/PhysRevB.17.1209
©1978 American Physical Society