A. Gómez-Herrero

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 approximately BiNbS3 and bilayer approximately 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). approximately BiNbS3 has a stacking sequence ...Q,H,Q,H,... and approximately 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 (approximately 6 A), approximately BiNb2S5 (approximately 17.4 A) (which dominate in the crystal) and to a new related phase approximately BiNb4S9 with a stacking sequence ...Q,H,H,H,H,Q,H,H,H,H,... and a periodicity of approximately 29.2 A.

New ecological pigments in the Ca–Yb–S system

Abstract The microstructure (via SEM, TEM and X-ray powder data), magnetic and optical colour parameters of the solid solution Ca 1− x Yb 2/3 x □ 1/3 x S (with 0≤ x ≤1) were investigated. Several samples have been prepared by heating mixtures of CaS and Yb 2 S 3 (white and yellow colour, respectively) in sealed silica tubes and by sulphurating the amorphous precursors of both metals in a H 2 S+Ar steam in CS 2 . Scanning electron micrographs show small particle size, up to 5 μm, in the second preparation samples. All the samples are bluish-green coloured and suitable for pigment applications. For x ≈0.11, the cation vacancies (produced during the process 3Ca 2+ →2Yb 3+ +?) are randomly distributed and a decrease in the NaCl-type unit cell volume of 1% was observed accompanied by cluster formation in the image. For x =0.33, we observed a cubic superstructure of NaCl-type, with a =2 a 0 =11.280(3) A; a 0 =5.50 A for pure CaS. Higher x values give mixtures of two phases: (a) rhombohedral symmetry, a r =7.987(6) A and α =58.6 (°), and (b), orthorhombic Yb 3 S 4 -type , with a =12.641(8) A, b =3.844(4) A and c =13.01(1) A.

Electron microscopy study of incommensurate modulated structures in misfit ternary chalcogenides

The study of misfit structures by means of transmission electron microscopy and associated techniques is reviewed. It is complementary to X-ray crystallography and provides a local and direct view of the beautiful complexity of these systems. Three different types are presented in this work. Misfit columnar structures can be contemplated as a case of one-dimensional misfit structures while misfit layer structures are considered as two-dimensional misfit structures. Some extra dimensionality is added when these misfit layer structures wrap to give rise to tubular crystals. Electron microscopy/diffraction shows clearly in many examples the presence of structural modulations as weak satellite reflections that are very difficult to detect by X-ray diffraction methods. Besides, high-resolution images show the presence of stacking defects in some of these misfit layer structures.

A TEM and XRD study of the SbNbSe3 misfit layer structures

Two new misfit layer structures have been synthesized within the Sb-Nb-Se system. Powder X-ray diffraction and electron microscopy techniques (electron diffraction, HREM, XEDS) have been used to determine the nature of their structure. According to TEM and XEDS data (for more than 15 crystals studied) both phases are monolayer type, i.e. (SbSe)1+delta (NbSe2). Electron microscopy reveals a composite modulated structure that consists of the periodical intergrowth of a pseudotetragonal SbSe layer, denominated as Q, and a pseudohexagonal layer NbSe2, denominated as H. Both layers fit along b, stack along c and do not fit along a (misfit) giving rise to an incommensurate modulation along this direction. The two phases differ in the symmetry of the Q layers being in one case orthorhombic (for delta = 0.17) and monoclinic in the other (for delta = 0.19). After the characterization of the sample by electron microscopy the unit cells of the basic layers could be refined for both phases by powder X-ray diffraction: aQ = 5.824(2) A, bQ = 5.962(5) A, cQ = 23.927(6) A, alpha = 90 degrees, beta = 90 degrees and gamma = 90 degrees and aH = 3.415(5) A, bH = 5.962(6) A,, cH = 11.962(1) A, alpha = 90 degrees, beta = 90 degrees and gamma = 90 degrees for the orthorhombic phase; aQ = 5.844(2) A, bQ = 5.981(1) A, cQ = 23.919(5) A, alpha = 90 degrees, beta = 90 degrees and gamma = 96.00(3)degrees and aH = 3.439(1) A, bH = 5.994(2) A, cH = 11.956(3) A, alpha = 90 degrees, beta = 90 degrees and gamma = 90 degrees for the monoclinic phase. The phase with the monoclinic Q-sublattice often appears as twinned crystals. The more abundant crystals are disordered intergrowths of both monolayer phases.

Transmission electron microscopy study of Y1−x□xCr2S4, x∼1/3 phase

Abstract A preliminary transmission electron microscopy study of the YCr3S6 phase has been performed. The compound crystallises with an orthorhombic average structure, space group Pmnb, a=3.485 (2) A, b=10.847 (3) A, c=12.701 (3) A. The average crystal structure can be related to the CaFe2O4-type. In addition, the electron diffraction patterns show a three-fold superstructure along the a axis due to an ordered occupancy of the trigonal prismatic environment position by the yttrium atoms. In some cases it has been found that the modulation is slightly incommensurate. The presence of disorder giving rise to layers of diffuse intensity perpendicular to a* is related to the electron beam interaction as well as changes in the phase of the structure modulation.

Solid solutions Ln10S14+xO1−x (Ln:Ce, Nd) as possible pigments

Abstract The microstructure (via SEM, TEM), and color parameters of the solid solution (Ce 1− y Nd y ) 10 S 14+ x O 1− x (0≤ y ≤0.70) (0≤ x ≤1) were investigated. The samples structure corresponds to the Pr 10 S 14 O type. The scanning electron micrographs (SEM) show small particle size (around 100 nm). The samples have been characterized by transmission electron microscopy (TEM) and associated techniques. The observed variation in the powder X-ray diffraction refined unit cell parameters is due to the variable oxygen content.

Carbon nanostructures produced by chlorination of Cr3C2 and Cr(acac)3

The preparation of nanocarbons and the study of their promising applications have attracted considerable attention in the last years [1,2]. Carbide-derived carbons (CDC) are a well-known group of nanostructured carbon materials, prepared by direct chlorination of the metal carbides according to the following reaction: MCx(s) + y/2 Cl2(g) → x C(s) + MCly(g). Previously, we have reported the preparation of CDC from NbC [3] as well as the results of the direct chlorination of metallocenes: ferrocene [4] and cobaltocene [5]. In this work we are presenting the characterization of nanostructured carbon samples obtained from Cr3C2 and from Cr(acac)3 - acac: acetylacetonate, (C5H7O2)− -, mainly by means of TEM and associated techniques (EDX, EELS), employing a CM 200FEG microscope.