ROLE OF VACANCIES IN MISFIT LAYERED COMPOUNDS - THE CASE OF THE GADOLINIUM CHROMIUM SULFIDE COMPOUND

The misfit layer compound (GdS)1.27CrS2 has been found to be nonstoichiometric. The nonstoichiometry is caused by gadolinium vacancies which were detected by a careful microprobe chemical analysis. This layered composite phase is built by alternate stacking of [GdS] and [CrS2] layers along the c axis. The different sublattices ([GdS] and [CrS2]) both have an orthorhombic symmetry. The b parameters (b1 and b2) and c parameters (c1 and c2) of both subsystems are identical whereas the two a parameters have different values (a1 = 5.454(1) angstrom and a2 = 3.451(1) angstrom). The a1/a2 ratio is irrational and determines the incommensurate character of this compound. The complete X-ray structure determination has already been published. Therefore, only the studies of the [GdS] sublattice (a1 = 5.454(1) angstrom, b1 = 5.8098(6) angstrom, c1 = 21.461(4) angstrom; space group Cmca; Z = 8; R = 0.054) and of the projection of the complete structure onto the (b,c) plane (space group: Cmca; R = 0.058) were redone in context with the new results. The results of the chemical analysis were confirmed by this study. The exact chemical formula of the compound is therefore (Gd1.16square30.11S1.27)CrS2. Including the vacancies in the composition leads to an almost exact charge balance between Gd3+, Cr3+, and S2- (1.16 X Gd3+ + Cr3+ = 6.48; 3.27 X S2- = 6.54). The electronic transfer from the [GdS] to the [CrS2] layer is such that no excess electrons exist. Magnetic and electrical transport measurements on single crystals are discussed, and for the first time, the semiconducting behavior of this class of compounds can be explained. The present results can also help to understand the discrepancies observed in the (LnS)xVS2 series.