AN X-RAY PHOTOEMISSION SPECTROSCOPY STUDY OF INTERLAYER CHARGE-TRANSFER IN SOME MISFIT LAYER COMPOUNDS

The misfit layer compounds with a general formula (MX)1+deltaTX2 (with M = Sn, Pb, Bi or rare earth metal; X = S, Se; T = Ti, V, Cr, Nb, Ta and 0.05 < delta < 0.25) have a stacking of MX double layers with a NaCl-type structure alternated by TX2 sandwiches. The unit cell axes in the plane of the layers of both subsystems are equal in the b direction but are incommensurate in the a direction. To understand the stability of this remarkable and ordered stacking of the two different layers, charge transfer from the MX subsystem to the TX2 subsystem has been suggested. In this study misfit layer compounds with Sn, Pb and Bi as the M element have been investigated with x-ray photoelectron spectroscopy. The information obtained from the core levels shows that for the Sn and Pb containing compounds the Pb and Sn atoms are divalent, and no significant charge transfer takes place between the layers. The spectra of the Bi misfit layer compound indicate a valency close to three for Bi. The stability of the misfit layer compounds is attributed to the presence of covalent interlayer bonds.