PRECISE ANALYTICAL CHARACTERIZATION OF INCOMMENSURATE SANDWICHED LAYERED COMPOUNDS [(PB,SN)S](1-2](M) ESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.28, M=1-3) - ROLE OF CATIONIC COUPLING ON THE PROPERTIES ANDTHE STRUCTURAL MODULATION(X)[(NB,TI)S)

Electron probe microanalysis of incommensurate sandwiched layered comp ounds of the general formula [(Pb,Sn)S](1+x)[(Nb,Ti)S-2](m) shows a sy stematic (Pb,Sn) depletion, interpreted as a partial (Ti,Nb) substitut ion into the pseudoquadratic (''Q'') layer (Pb,Sn)S. (Ti,Nb) substitut ing for (Pb,Sn)(2+) would have the +3 oxidation state, with an equal v is-a-vis transformation of(Ti,Nb)(4+) into (Ti,Nb)(3+) in the (Nb,Ti)S -2 adjacent layer. This will contribute to increase the global electro nic population of the Ti t(2g), or Nb d(z)(2) band, without electron d onation from the Q layer. This local heterovalent substitution induces a cationic coupling between the two kinds of layers, which permits th e understanding of the stabilization of these sandwiched structures, a s initially pointed out for the mineral cylindrite (Makovicky, 1974). According to this model, the substitution in the Q layer must be order ed at short distance (vernier rule, Makovicky and Hyde, 1981), and at the modulation scale (for 2n Q cations) the coefficient of substitutio n of(Ti,Nb) must be an integer. True incommensurability of these struc tures can be understood as long-distance fluctuations between two vern ier. ratios. The new chemical data prompt us to reexamine some of thei r physical properties, i.e., superconductivity. The presence of variou s local chemical disruptions would determine the existence of all laye red misfit chalcogenides.