Pseudotetragonal Layers Research Articles

Vibrational Properties of a Naturally Occurring Semiconducting van der Waals Heterostructure

We present vibrational properties of Franckeite, which is a naturally occurring van der Waals heterostructure consisting of two different semiconducting layers. Franckeite is a complex layered crystal composed of alternating SnS$_2$ like pseudohexagonal and PbS-like pseudotetragonal layers stacked on top of each other, providing a unique platform to study vibrational properties and thermal transport across layers with mass density and phonon mismatches. By using micro-Raman spectroscopy and first-principles Raman simulations, we found that the PbS-like pseudotetragonal structure is mostly composed of Pb$_3$SbS$_4$. We also discovered several low-frequency Raman modes that originate from the intralayer vibrations of the pseudotetragonal layer. Using density functional theory, we determined all vibrational patterns of Franckeite, whose signatures are observed in the Raman spectrum. By studying temperature dependent Raman spectroscopy (300 K - 500 K), we have found different temperature coefficients for both pseudotetragonal and pseudohexagonal layers. We believe that our study will help understand the vibration modes of its complex heterostructure and the thermal properties at the nanoscale.

Molecular Structure of Hydrazoic Acid from 55 K to Close to the Melting Point Determined with Synchrotron Radiation.

Hydrazoic acid (HN3) is the simplest covalent azide, potentially explosive, and strongly toxic with both a low boiling and a low melting point (309 and 193 K, respectively). The monoclinic structure, recently solved by X-ray single-crystal diffraction at 100(2) K, is built up by tetramers (HN3)4 in unique pseudotetragonal layers with N-H···N hydrogen bonds, but with only weak van der Waals bonds between them. As also observed in 2H-graphite, nearly planar layers are stacked parallel to (001) with the sequence A, B, ..., A, B. We report here on a polycrystalline sample of HN3 that retains the monoclinic structure between 55(5) and 180(5) K with nearly linear increase of the lattice parameters a and b, but with steeper, partly nonlinear increase for the lattice parameter c. Near the melting point additional reflections are observed in the diffractograms which may indicate structural stress in the planar layers.

Luxembourgite, AgCuPbBi<sub>4</sub>Se<sub>8</sub>, a new mineral species from Bivels, Grand Duchy of Luxembourg

Abstract. Luxembourgite, ideally AgCuPbBi4Se8, is a new selenide discovered at Bivels, Grand Duchy of Luxembourg. The mineral forms tiny fibres reaching 200 µm in length and 5 µm in diameter, which are deposited on dolomite crystals. Luxembourgite is grey, with a metallic lustre and without cleavage planes; its Mohs hardness is 3 and its calculated density is 8.00 g cm−3. Electron-microprobe analyses indicate an empirical formula Ag1.00(Cu0.82Ag0.20Fe0.01)Σ1.03Pb1.13Bi4.11(Se7.72S0.01)Σ7.73, calculated on the basis of 15 atoms per formula unit. A single-crystal structure refinement was performed to R1=0.0476, in the P21∕m space group, with a=13.002(1), b=4.1543(3), c=15.312(2) Å, β=108.92(1)∘, V=782.4(2) Å3, Z=2. The crystal structure is similar to that of litochlebite and watkinsonite and can be described as an alternation of two types of anionic layers: a pseudotetragonal layer four atoms thick and a pseudohexagonal layer that is one atom thick. In the pseudotetragonal layers the Bi1, Bi2 ,Bi3, Pb, and Ag1 atoms are localised, while the Cu2 and Bi4 atoms occur between the pseudotetragonal and the pseudohexagonal layers. Bi1, Bi2, and Bi3 atoms occur in weakly distorted octahedral sites, whereas Bi4 occurs in a distorted 7-coordinated site. Ag1 occupies a fairly regular octahedral site, Cu2 a tetrahedral position, and Pb occurs on a very distorted 8-coordinated site.

STRUCTURES AND CLASSIFICATION OF INDIUM SULFOSALTS, AND THE CRYSTAL STRUCTURE OF Sn12In19(Se,S)41

The new synthetic phase, ~Sn12In19(Se,S)41, synthesized in the condensed system Fe–Sn–Sb–In–S–Se at 600°C, is monoclinic with a 56.29(4), b 3.924(3), c 15.916(10) A, β 102.56(1)°, space group C 2 /m , and Z = 2. It occurs as needle-like crystals together with chemically analyzed Fe9S10, FeIn2(S,Se)4, ~SbSn9(Se,S)10 and ~Sn4Sb3In2(S,Se)11. The crystal structure has been refined to a conventional R factor R 1 of 6.29% for 2339 unique reflections with F o > 4σ (F o ) . There are 16 unique sites of Sn and In and 21 unique sites of (Se,S) in the asymmetric unit. It is a composite-layer structure with two kinds of layers in regular alternation; together, they undergo a step-like modulation. The pseudotetragonal layers are three atomic planes thick and modified by steps into oval-shaped fragments containing octahedra of indium and trigonal coordination prisms of tin. Single-octahedron pseudohexagonal layers are of two regularly alternating kinds: those with an overlap of two octahedra in the step region and those with a gap of one octahedron in those regions. The title compound, ~Sn12In19(Se,S)41, is a member of a M15+NS20+N series of closely related composite-layer structures with alternating, periodically sheared pseudohexagonal and pseudotetragonal layers (a “sliding series” according to its main structural principle of slabs with fixed step-like configurations sliding past one another and thus creating chemical and structural differences on the planes of their contact). A parallel series with less frequently sheared layers is known only from the Pb6In13.34S26 – Sn5In14S26 pair. Two cannizzarite-related structures with layers unsheared, one potential cylindrite-type structure, two families of rod-based structures, one known complex sulfide with In–In bonds, and the structure of PbIn2S4 are further principal structure-types of complex indium sulfides in addition to members of the pavonite homologous series and several individual sulfosalt structures that are in need of revision. Alternation of pseudotetragonal and pseudohexagonal layers and formation of rod-based structures are the two basic principles in the construction of complex sulfides and selenides of indium.

THE CRYSTAL STRUCTURE OF 7H : 12Q CANNIZZARITE FROM VULCANO, ITALY

The crystal structure of cannizzarite, ideally Pb 48 Bi 56 (S 124 Se 8 ) 132 , a fumarole sublimate from the La Fossa crater, on the island of Vulcano, Italy, with a nominal composition Pb 47.9 Bi 55.8 (S 121.8 Se 10.5 ) ∑132.3 and unit-cell parameters a 38.86(3), b 4.090(3), c 39.83(3) A, β 102.30(3)°, Z = 1, has been solved and refined in space group P 2 1 / m from X-ray single-crystal diffraction data, to an R 1 of 10.7% on the basis of 3612 unique reflections with [ F o > 4σ( F o )]; the data were collected on a Bruker P3 diffractometer with a CCD detector and Mo K α radiation. The cannizzarite crystal studied has 104 cations and 132 anions in a unit cell. Refinement shows that about eight anions per cell contents are selenium, the bulk being sulfur. The crystal structure is composed of pseudotetragonal layers (Q layers), two cation–anion planes thick, of the (100) PbS type that alternate with double-octahedron pseudohexagonal layers (H layers) of the (111) PbS type. Both types of layer contain Pb and Bi; their distribution was studied by means of bond-valence calculations, polyhedron characteristics and bond-length ratios of the opposing Me –anion bonds. The unit-cell contents are Me 48 X 48 for the Q layers, and Me 56 X 84 for the H layers. Selenium was found only in H layers. The structure determined represents the shorter of the two modules, Pb 12 Bi 14 S 33 and Pb 17 Bi 20 S 47 , found in the structure of synthetic cannizzarite proposed earlier, and proves their independent existence.

First (3 + 2)-dimensional superspace approach to the structure of levyclaudite-(Sb), a member of the cylindrite-type minerals

The structure of synthetic levyclaudite-(Sb), approximately (Pb(1 - y)Sb(y)S)(1.357)[Sn(1 - x)(Cu(2))(x)S(2)], has been determined by single-crystal X-ray diffraction on the basis of the (3 + 2)-dimensional superspace approach. This misfit-layer compound, of the cylindrite type, results from the combination of two heavily modulated triclinic Q and H subsystems with a common q wavevector and only one shared reciprocal axis (stacking direction). The Q pseudo-tetragonal layer, approximately (Pb(0.70)Sb(0.30)S), derived from the NaCl archetype, is positively charged; the H pseudo-hexagonal layer, approximately (Sn(0.85)Cu(0.30)S(2)), derived from the CdI(2) archetype, is negatively charged, owing to the replacement of Sn(4+) in an octahedral coordination by Cu(+) pairs in an opposite triangular coordination. The analysis shows a strong transverse displacive modulation of the two layers, referred to as a 'mondulation', correlated to a maximal Sb site occupation factor in the concavity of the Q layer undulation. The wavelength control of the ;mondulation' obeys the vernier principle (14c(Q) congruent with 13c(H)), which would correspond to an energy minimization through a charge transfer density modulation wave, common to all two-dimensional misfit-layer inorganic compounds.

Chiral Molecular Magnets: Synthesis, Structure, and Magnetic Behavior of the Series [M(L‐tart)] (M = MnII, FeII, CoII, NiII; L‐tart = (2R,3R)‐(+)‐tartrate)

A new series of layered magnets with the formula [M(L-tartrate)] (M = Mn(II), Co(II), Fe(II), Ni(II); L-tartrate = (2R,3R)-(+)-tartrate) has been prepared. All of these compounds are isostructural and crystallize in the chiral orthorhombic space group I222, as found by X-ray structure analysis. Their structure consists of a three-dimensional polymeric network in which each metal shows distorted octahedral coordination bound to four L-tartrate ligands, two of which chelate through an alcohol and a carboxylate group and the other two bind terminally through a monodentate carboxylate group. The chirality of the ligand imposes a Delta conformation on all metal centers. Magnetically, the paramagnetic metal centers form pseudotetragonal layers in which each metal is surrounded by four other metals, with syn,anti carboxylate bridges. These salts show intralayer antiferromagnetic or ferromagnetic interactions, depending on the electronic configuration of the metal, and weak interlayer antiferromagnetic interaction. In all cases the magnetic properties are strongly affected by the anisotropy of the system, and the presence of magnetic canting has been found. The Mn derivative behaves as a weak ferromagnet with a critical temperature of 3.3 K. The Ni derivative shows very unusual magnetic behavior in that it exhibits antiferromagnetic ordering below 6 K, the onset of spontaneous magnetization arising from spin reorientation into a canted phase below 4.5 K, and a field-induced ferromagnetic state above 0.3 T at 2 K, behavior typical of metamagnets. The Fe and Co derivatives show antiferromagnetic interactions between spin carriers, but do not order above 2 K.

Atomic-scale observations of franckeite surface morphology

Franckeite, approximately Pb4.6Ag0.2Sn2.5Fe0.8Sb2S12.6, consists of alternating pseudohexagonal (H) and pseudotetragonal (Q) layers. Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) of freshly cleaved franckeite, from San José, Bolivia, revealed the atomic structure of the pseudohexagonal component. On AFM images, the expected pattern with b = 3.2 Å was observed. STM revealed a √3 × √3 superstructure, with b' = 6.3 Å, interpreted to be caused by tunneling effects. The pseudotetragonal layer was not identified in any images.

Electron microscopy study of tubular crystals (BiS)1+δ(NbS2)n

Big crystals (mm-sized) with anomalous tubular morphology were synthesised in the Bi–Nb–S system. Their structural study by electron microscopy and related techniques revealed that they can be described as misfit layer structures, a type of composite modulated structures. The misfit monolayer ≈BiNbS3 and bilayer ≈BiNb2S5 phases appear as well in the preparation but with lamellar morphology as it is typical for these kind of compounds. They consist of periodical intergrowth of pseudotetragonal layers BiS (Q) with pseudohexagonal layers NbS2 (H). ≈BiNbS3 has a stacking sequence …Q,H,Q,H,… and ≈BiNb2S5 has a stacking sequence …Q,H,H,Q,H,H,…Backscattered electron imaging, wavelength dispersive X-ray spectroscopy and transmission electron microscopy of transversal cross-sections of single tubular crystals showed that these crystals present a strong compositional and inter-laminar stacking disorder along the tube radius. This disorder is suggested to be the cause for the original wrapping of the layers that gives rise to the tubes. Besides the disordered areas, some ordered slabs have been found with stacking sequences corresponding to binary 3R–NbS2 (≈6Å), ≈BiNb2S5 (≈17.4Å) (which dominate in the crystal) and to a new related phase ≈BiNb4S9 with a stacking sequence …Q,H,H,H,H,Q,H,H,H,H,… and a periodicity of ≈29.2Å.

Crystal Structures of Complex Lanthanide Sulfides With Built-in Non-Commensurability

Non-commensurability on the boundaries of adjacent structural layers (rods, blocks) represents an important structure-building principle for several structural families of complex lanthanide sulfides. In the layer-misfit sulfides 'ABS3', pseudotetragonal layers with lanthanide ions alternate with octahedral layers or trigonal prismatic layers. Among more complex, fragment-recombination structures based on similar layer strips, (A) a family of derivatives of the structure of Yb3S4, (B) structures that represent anti-(out-of-)phase modulated non-commensurate layer sequences, and (c) lanthanide sulfosalts (with bismuth and antimony) are described in some detail. Layer-match modes, their relationships to the ratios of cation radii, and relations to simpler archetypal structures are analysed.

Electron microscopy of cylindrite and franckeite

Previous descriptions of the crystallography and crystal chemistry of the sulphosalts cylindrite, incaite, franckeite and potosiite are reviewed and assessed. Our new electron diffraction and electron microscopy results are then described and interpreted; first for cylindrites and then for franckeites. Images with [001] and [010] zone axes are available for the first time (from appropriate sections produced by cutting, polishing and ion-beam thinning across the strong, natural (100) cleavage). These are crucial for analysing the nature of the two layer types in these minerals, and the way(s) in which they are stacked. Cylindrite is confirmed as having a 2-atom thick pseudo-tetragonal layer, while in the remaining species this layer is 4 atoms thick. It seems likely that in the latter these layers are SnS-type rather than PbS-type. Details of the corrugation of the layers are also revealed.