Band structure of polycrystalline ${\mathrm{Sb}}_4$${\mathrm{Mo}}_{20}$${\mathrm{O}}_{62}$ compounds

Transport-coefficient measurements (electrical conductivity, thermoelectric power, and Hall coefficients have been performed on a compact ${\mathrm{Sb}}_4$ ${\mathrm{Mo}}_{20}$ ${\mathrm{O}}_{62}$ polycrystalline compound in a wide temperature range (130-500 K). Experimental results are interpreted with the help of a $p$-type semiconductor model with two inverted deep levels near the mid-gap. Conduction mechanisms are governed by acoustical-phonon scattering of the carriers. EPR measurements confirm the theoretical model, which has been retained. Lastly, the model is discussed on the basis of an idealized projection of the ${\mathrm{Sb}}_4$ ${\mathrm{Mo}}_{20}$ ${\mathrm{O}}_{62}$ structure. The top of the valence band is assumed to be formed from the $d_{\mathrm{xy}}$ orbitals of some Mo atoms leading to narrow bonding $\pi$ bands while the donor and acceptor levels may be formed from the nonbonding $d_{\mathrm{xy}}$ orbitals of some Mo atoms of the distorted octahedron framework.