A. Lafond

X-Ray structure determination and superconductivity of a new layered misfit compound with a franckeite-like stacking, [(Pb,Sb)S]2.28NbS2

Abstract The crystal structure of [(Pb,Sb)S]2.28NbS2 has been determined. This synthetic compound belongs to the layer misfit family with the structure type of franckeite. Its structure is built up from two [(Pb,Sb)S] layers (noted “Q”) alternating with one [NbS2] layer (noted “H”) along the stacking direction ( c axis). The cell parameters of this composite structure are: b = 5.8285(9) A , c = 17.649(6) A , α = 86.41(2)°, β = 86.55(3)°, γ = 89.97(2)° : Q sublattice a = 5.864(2) A : H sublattice a = 3.33 A . Antimony atoms are specifically located at the interface within the two “Q” layers as in natural franckeite. Stereochemical activity of Sb3+ (with its lone electron pair) stabilizes the double “Q” entity which has the SnS structure, giving the following structural formulation: [PbS (Pb0.6Sb0.4)S]1.14NbS2. A superconducting transition is observed at Tc = 1.05 K, a temperature lower than that observed for the corresponding 1Q/1H misfit derivative. This feature may be related to a decrease of the oxidation state for Nb, from +4 in the stoichiometric binary 2H-NbS2 compound (Tc = 6.2 K) to partially +3 in this compound. Crystal chemical constraints which govern the stabilization of franckeite-type compounds are discussed.

Misfit layered compounds: polytypism, multilayer stages, non-stoichiometry and electronic structure, self-misfit compounds

Abstract The field of misfit layered compounds is evolving in several directions. Current questions are (i) polytypism in the TX 2 layers and its influence on the physical properties, (ii) non-stoichiometry of the LnX layers and (iii) compounds which are also layered but commensurate along the a direction. All these fields are treated and looked upon especially towards their influence on the electronic structure.

Préparation, détermination structurale et propriétés magnétiques d'un nouveau composé lamellaire incommensurable: Le sulfure de gadolinium et de chrome

(GdS)1.27CrS2 is a new misfit layer compound in which two types of slabs, |GdS| and |CrS2|, alternate along the stacking direction c→. The crystal structure of this compound has been determined by a composite approach which leads to average structures and their relative arrangement. The two sublattices are orthorhombic with the same b and c parameter. The misfit character stems from the different geometry in the 2-D lattices, square for GdS and hexagonal for CrS2. The ratio of GdS to CrS2 (1.27) is determined by the 2×aCrS2aGdS one. Linear suceptibility behavior is given and compared to those of similar compounds (LaS)1.19CrS2 and (NdS)1.23CrS2.

A new misfit bilayer compound in the samarium titanium sulfur system

Abstract The composite bilayer misfit compound (SmS) 1.25 (TiS 2 ) 2 was prepared and its structure determined via the composite approach by single crystal X-ray diffraction. This compound crystallizes with monoclinic unit cells (SmS subcell: a 1 = 5.473(4) A, b 1 = 5.854(2) A, c 1 = 33.53(1) A, α 1 = 93.36(5) °, β 1 = 90 °, γ 1 =90 °; TiS 2 subcell a 2 = 3.425(3) A, b 2 = 5.854(2) A, c 2 = 33.53(1) A, α 2 = 93.36(5) °, β 2 = 90 °, γ 2 = 90 °. The mismatch of periodicities occurs along the a-direction. The structure consists of an alternated stacking of SmS (NaCl structure type) and (TiS 2 ) 2 slabs along the c-direction. Presence of interstitial Ti atoms within the van der Waals gap between adjacent TiS 2 layers is revealed. Thus, the stacking mode within the (TiS 2 ) 2 block is markedly different from that observed with misfit bilayer homologues such as (PbS) 1.18 (TiS 2 ) 2 or (SbS) 1.15 (TiS 2 ) 2 where no extra Ti atoms were found. Electrical transport measurements show a metallic behaviour down to 4.2 K. Magnetic study shows a behaviour mainly due to the Sm 3+ van Vleck paramagnetic ion.

Magnetic behavior in the incommensurate phase ∼LaCrS3:: effect of the partial substitution of Cr by Ti on the magnetic properties

Abstract Dilution of the magnetic interactions between Cr 3+ ions by Ti 3+ ions was observed in the CrS 2 layer of the misfit-layer compound ∼LaCrS 3 . Pure ∼LaCrS 3 has complex magnetic properties which are reminiscent of spin glass behavior. This magnetic behavior comes from both the modulated character of the structure and the magnetic frustration of the planar-antiferromagnetic-triangular network of Cr 3+ ions. Thus, there is a large hysteresis between the zero field cooled and the field cooled magnetic susceptibility curves below the transition temperature (≈75 K). Formation of a solid solution ∼LaCr 1− x Ti x S 3 by the addition of Ti 3+ ions results in the decrease of transition temperature up to a doping level of x ≈0.5, where the transition is no longer observed. The magnetic behavior of the phase with x ≈0.5 is similar to that of several random exchange antiferromagnetic compounds.