STRUCTURES OF MISFIT LAYER COMPOUNDS (MS)N TS2 (M = SN, PB, BI, RARE-EARTH-METALS, T = NB, TA, TI, V, CR, 1.08-LESS-THAN-N-LESS-THAN-1.23)

Misfit layer compounds (MS)(n)TS2 (M = Sn, Pb, Bi, rare earth metals; T = Nb, Ta, Ti, V, Cr; 1.08 < n < 1.23) are built of alternate double layers of MS, actually a distorted {001} slice of f.c.c. MS (for some compounds hypothetical), and sandwiches of TS2, with atoms T = Nb, Ta in slightly distorted trigonal prisms of sulphur atoms (as in the parent compounds 2H-NbS2 and 2H-NbS2 and 2H-TaS2) and atoms T = Ti, V, Cr in trigonal antiprisms of sulphur atoms (e.g. as in 1T-TiS2). The MS and TS2 subsystems have a different geometry in the a-b plane, the stacking direction being c. The MS lattice is approximately quadratic (a1 almost-equal-to b1 in the range 5.4-6.1 congruent-to angstrom, gamma-1 = 90-degrees) while the TS2 lattice is rectangular (b2/a2 almost-equal-to 3(1/2), a2 in the range 3.3-3.5 angstrom, gamma-2 = 90-degrees). The subsystems are always centred, either C centred (c almost-equal-to 11-12 angstrom) or F centred (c almost-equal-to 22-24 angstrom). Combinations CC, CF, FC and FF (first MS, second TS2) were found. Compounds with T = Nb, Ta have orthorhombic or monoclinic subsystems, the monoclinic angle being beta (beta-1 generally not equal to beta-2). Compounds with T = Ti, V, Cr have monoclinic subsystems, the monoclinic angle being alpha, or triclinic subsystems for which the c axes diverge. Stacking variants (CC, CF) exist also in this case. Compounds (PbS)1.14(NbS2)2 and (PbS)1.18(TiS2)2 with stacking sequence MS-TS2-TS2-MS etc. have been found recently. For all compounds investigated it was found that the subsystems have a common (b*, c*) reciprocal plane. This stems from the condition that layers cannot intersect and the interaction between M and sulphur of TS2, the M atoms being in between rows of sulphur of TS2 parallel to [100]. For the lanthanide (Ln) compounds the distances Ln-S of the LnS double layer and the distances of Ln to sulphur of TS2 are about the same and in agreement with Ln(III). The interaction between the subsystems is strong. For M = Sn, Pb the M-S distances of the MS double layer indicate M(II); the interaction between subsystems is weaker than for M = Ln.