Maria Teresa Caldes

X-ray photoelectron spectroscopy and high resolution electron microscopy studies of Aurivillius compounds: Bi4−xLaxTi3O12(x=0, 0.5, 0.75, 1.0, 1.5, and 2.0)

In order to determine the fatigue-free origin of the ferroelectric Bi3.25La0.75Ti3O12, a series of x-ray photoelectron spectroscopy and high resolution electron microscopy studies were performed on the polycrystalline Bi4−xLaxTi3O12 (BLTx, x=0, 0.5, 0.75, 1.0, 1.5, and 2.0) powders. From the XPS study, the surfaces of all La-containing compounds are found to consist of one outermost Bi-rich region and followed by a La-rich region, instead of the chemical stoichiometry in the bulk. An HREM study on Bi3.25La0.75Ti3O12 further confirms that this surface configuration arises from some intergrowth defects with a thickness of 5 nm appearing on the crystal edge, which is also observed in compounds with x=1.0 and 2.0, but not in the poor fatigue-resistant Bi4Ti3O12 (x=0). This La-induced defect locally changes the chemical composition of the crystal surface, which probably possesses a different physical characteristic as compared to the bulk. Consequently, it could can be the physical nature of the interface, when in contact with the metal electrode, Pt, in ferroelectric nonvolatile memory. Since the fatigue phenomenon on a ferroelectric capacitor arises predominately from the pinning of domain walls on the metal-ferroelectric interface, the surface configurations of La-containing Bi4Ti3O12 compounds should be considered as one of the fatigue-free factors as well as the self-regulation of the Bi2O2 layer and the chemical stability of the perovskite slabs. In addition, the band gap of Bi3.25La0.75Ti3O12 is also estimated by UV absorption spectrum to be 3.9±0.1 eV.In order to determine the fatigue-free origin of the ferroelectric Bi3.25La0.75Ti3O12, a series of x-ray photoelectron spectroscopy and high resolution electron microscopy studies were performed on the polycrystalline Bi4−xLaxTi3O12 (BLTx, x=0, 0.5, 0.75, 1.0, 1.5, and 2.0) powders. From the XPS study, the surfaces of all La-containing compounds are found to consist of one outermost Bi-rich region and followed by a La-rich region, instead of the chemical stoichiometry in the bulk. An HREM study on Bi3.25La0.75Ti3O12 further confirms that this surface configuration arises from some intergrowth defects with a thickness of 5 nm appearing on the crystal edge, which is also observed in compounds with x=1.0 and 2.0, but not in the poor fatigue-resistant Bi4Ti3O12 (x=0). This La-induced defect locally changes the chemical composition of the crystal surface, which probably possesses a different physical characteristic as compared to the bulk. Consequently, it could can be the physical nature of the interface, whe...

High-temperature form of neodymium orthoborate, NdBO3

The framework of the high-temperature modification of NdBO3 is built up from NdO8 dodecahedra sharing edges and BO3 groups. The NdO8 polyhedra form corrugated sheets approximately parallel to the (111) plane and the borate groups are arranged in rows running parallel to the [011] direction. The Nd—O distances range from 2.315 (3) to 2.697 (4) A ° and the B—O distances from 1.346 (6) to 1.407 (6) A ° . The title structure is isotypic with triclinic EuBO3.

Lithium intercalation chemistry, microstructure and superconductivity in zirconium and hafnium nitride halides

Abstract Lithium intercalation in β-MNX (M=Zr, Hf; X=Cl, Br) leads to superconducting compounds with critical temperatures between 12 and 24 K. The lithium uptakes after treatment of the host materials with n-butyllithium/hexane solutions are ca. 0.2 atoms per formula for β-ZrNCl and β-ZrNBr, and between 0.07 and 0.67 for β-HfNCl. Electrochemical lithiation experiments agree with the results obtained with chemical methods, as samples with larger capacity on discharge are also those having larger lithium contents after chemical lithiation. Variations exist in the electrochemical profiles of different batches for the three compounds indicating differences in the intercalation reaction pathway. Both Zr and Hf compounds show poor electrochemical reversibility indicating that these materials are not suitable for electrochemical applications. High resolution electron microscopy images confirm the structural model previously reported, isotypic to SmSI. The major part of crystals from Zr compounds as well as from the β-HfNCI samples showing a high lithium intercalation degree show a regular stacking of the [X–M–N–N–M–X] layers, being almost free of defects. Hf samples exhibiting low lithium uptakes show a high proportion of crystals with a HfO 2 layer at their thin edges. This constitutes a physical barrier that obstruct the lithium diffusion through the van der Waals gap and hence the induction of superconductivity. In agreement with these results, magnetic measurements for Li x HfNCl show, in contrast to Li x ZrNX compounds, small superconducting fractions and very broad transitions indicating a distribution of critical temperatures and a heterogeneous nature of the samples.

Cationic and Anionic Disorder in CZTSSe Kesterite Compounds: A Chemical Crystallography Study

The cationic and anionic disorder in the Cu2ZnSnSe4-Cu2ZnSnS4 (CZTSe-CZTS) system has been investigated through a chemical crystallography approach including X-ray diffraction (in conventional and resonant setup), 119Sn and 77Se NMR spectroscopy, and high-resolution transmission electron microscopy (HRTEM) techniques. Single-crystal XRD analysis demonstrates that the studied compounds behave as a solid solution with the kesterite crystal structure in the whole S/(S + Se) composition range. As previously reported for pure sulfide and pure selenide compounds, the 119Sn NMR spectroscopy study gives clear evidence that the level of Cu/Zn disorder in mixed S/Se compounds depends on the thermal history of the samples (slow cooled or quenched). This conclusion is also supported by the investigation of the 77Se NMR spectra. The resonant single-crystal XRD technique shows that regardless of the duration of annealing step below the order-disorder critical temperature the ordering is not a long-range phenomenon. Finally, for the very first time, HREM images of pure selenide and mixed S/Se crystals clearly show that these compounds have different microstructures. Indeed, only the mixed S/Se compound exhibits a mosaic-type contrast which could be the sign of short-range anionic order. Calculated images corroborate that HRTEM contrast is highly dependent on the nature of the anion as well as on the local anionic order.

Tubulite, ~Ag2Pb22Sb20S53, a new Pb–Ag–Sb sulfosalt from Le Rivet quarry, Peyrebrune ore field (Tarn, France) and Biò, Borgofranco mines, Borgofranco d’Ivrea (Piedmont, Italy)

Tubulite, ~ Ag 2 Pb 22 Sb 20 S 53 , is a new Pb–Ag–Sb sulfosalt discovered at Le Rivet quarry, Peyrebrune ore field (Tarn, France) and Bio, Borgofranco mines, Borgofranco d’Ivrea (Piedmont, Italy). As indicated by the name, it forms very thin perfect micro-tubes, 100 to 600 µm in length, 40 to 100 μm in diameter, and only 1 to 2 μm in thickness; a hair-like variety is also present at Bio. At Le Rivet, it is associated with galena, pyrite, pyrrhotite, arsenopyrite, stibnite, and various Sb sulfosalts of Pb, Ag and Cu, in a gangue of quartz, baryte and carbonates. At Bio, it is associated with galena, sphalerite, chalcopyrite, pyrite, marcasite, and various Sb sulfosalts of Pb, Ag and Cu, within the same type of gangue. Tubulite is metallic black; optical properties could not be observed under the microscope, due to the crystal morphology. Electron microprobe analysis gave (wt.% – average of 8 anal.): Ag 2.7(2), Pb 46.6(9), Sb 26.1(8), S 17.8(5), Total 93.2(2.1). The low total is due to the thinness of the tube wall. According to crystallographic study (electron and X-ray diffraction), tubulite is monoclinic (space group Pc , P 2/ c or P 2 1 / c) with unit-cell parameters a 4.132(2), b 43.1(2), c 27.4(1) A, β 93.2°, V4872(40) A 3 , with Z = 2. Weak reflections in the [010] electron diffraction pattern indicate a 2a superstructure. Main diffraction lines are [d (A), ( I )]: 3.99 ( 35 ), 3.69 ( 60 ), 3.36 ( 100 ), 3.28 ( 55 ), 2.99 ( 55 ), 2.912 ( 55 ), 2.063 ( 75 ). The unit cell of tubulite is very close to those of sterryite and parasterryite; like these sulfosalts, tubulite is probably an expanded derivative of owyheeite. Its peculiar tubular morphology is discussed within the general framework of crystals whose habit presents a circular symmetry. It is proposed that micro-tubes were initiated by capillary forces acting on very thin lamellar crystallites around gas bubbles or liquid droplets.

Crystal Chemistry, Optical–Electronic Properties, and Electronic Structure of Cd1–xIn2+2x/3S4 Compounds (0 ≤ x ≤ 1), Potential Buffer in CIGS-Based Thin-Film Solar Cells

CdIn2S4 and In2S3 compounds were both previously studied as buffer layers in CIGS-based thin-film solar cells, each of them exhibiting advantages and disadvantages. Thus, we naturally embarked on the study of the CdIn2S4-In2S3 system, and a series of Cd1- xIn2+2 x/3S4 (0 ≤ x ≤ 1) materials were prepared and characterized. Our results show that two solid solutions exist. The aliovalent substitution of cadmium(II) by indium(III) induces a structural transition at x ≈ 0.7 from cubic spinel Fd3̅ m to tetragonal spinel I41/ amd that is related to an ordering of cadmium vacancies. Despite this transition, the variation of optical gap is continuous and decreases from 2.34 to 2.11 eV going from CdIn2S4 to In2S3 while all compounds retain an n-type behavior. In contrast with the Al xIn2-xS3 solid solution, no saturation of the gap is observed. Moreover, XPS analyses indicate a difference between surface and volume composition of the grains for Cd-poor compounds. The use of Cd1- xIn2+2 x/3S4 compounds could be a good alternative to CdIn2S4 and In2S3 to improve CIGS/buffer interfaces with a compromise between photovoltaic conversion efficiency and cadmium content.

Intercalation chemistry of superconducting zirconium and hafnium nitride halides

Abstract Lithium intercalation in MNX (M=Zr, Hf; X=Cl, Br) may lead to superconducting compounds with critical temperatures up to 24 K. Electrochemical lithiation experiments agree with the results obtained with chemical methods, as samples with larger capacity on discharge are also those having larger lithium contents after chemical lithiation. Variations exist in the electrochemical profiles of different batches for these compounds indicating differences in the intercalation reaction pathway. High-resolution electron microscopy images confirm the structural model isotypic to SmSI for β-MNX phases. The major part of crystals from ZrNX as well as from the HfNCl samples showing a high lithium intercalation degree exhibit a regular stacking of the layers, being almost free of defects. In contrast, samples with low lithium uptakes show a high proportion of crystals with a HfO 2 layer at their thin edges. In agreement with these results, magnetic measurements for Li x HfNCl indicate, in contrast to Li x ZrNX compounds, small superconducting fractions and very broad transitions consistent with a distribution of critical temperatures and a heterogeneous nature of the samples.