Structures of misfit layer compounds (MS) nTS 2 (M Sn, Pb, Bi, rare earth metals; TNb, Ta, Ti, V, Cr; 1.08< n

Abstract

Misfit layer compounds (MS) nTS 2 (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 TS 2, with atoms TNb, Ta in slightly distorted trigonal prisms of sulphur atoms (as in the parent compounds 2H-NbS 2 and 2H-NbS 2 and 2H-TaS 2) and atoms TTi, V, Cr in trigonal antiprisms of sulphur atoms ( e.g. as in IT-TiS 2). The MS and TS 2 subsystems have a different geometry in the a- b plane, the stacking direction being c. The MS lattice is approximately quadratic ( a 1≈ b 1 in the range 5.4−6.1 ≅ A ̊ , γ 1 = 90°) while the TS 2 lattice is rectangular ( b 2 a 2 ≈3 1 2 , a 2 in the range 3.3−3.5 Å, γ 2 = 90°). The subsystems are always centred, either C centred ( c ≈ 11−12 Å) or F centred ( c≈22−24 Å). Combinations CC, CF, FC and FF (first MS, second TS 2) were found. Compounds with TNb, Ta have orthorhombic or monoclinic subsystems, the monoclinic angle being β ( β 1 generally not equal to β 2). Compounds with TTi, V, Cr have monoclinic subsystems, the monoclinic angle being α, or triclinic subsystems for which the c axes diverge. Stacking variants ( CC, CF) exist also in this case. Compounds (PbS) 1.14(NbS 2) 2 and (PbS) 1.18(TiS 2) 2 with stacking sequence MSTS 2TS 2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 TS 2, the M atoms being in between rows of sulphur of TS 2 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 TS 2 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.