Fe(Se,Te) Thin Films Deposited through Pulsed Laser Ablation from Spark Plasma Sintered Targets.

Iron-based superconductors are under study for their potential for high-field applications due to their excellent superconducting properties such as low structural anisotropy, large upper critical fields and low field dependence of the critical current density. Between them, Fe(Se,Te) is simple to be synthesized and can be fabricated as a coated conductor through laser ablation on simple metallic templates. In order to make all the steps simple and fast, we have applied the spark plasma sintering technique to synthesize bulk Fe(Se,Te) to obtain quite dense polycrystals in a very short time. The resulting polycrystals are very well connected and show excellent superconducting properties, with a critical temperature onset of about 16 K. In addition, when used as targets for pulsed laser ablation, good thin films are obtained with a critical current density above 105 A cm-2 up to 16 T.

Comment on "Structural transition and superconductivity in hydrothermally synthesized FeX (X = S, Se)" by U. Pachmayr, N. Fehn and D. Johrendt, Chem. Commun., 2016, 52, 194.

The occurrence of triclinic structural modification of ß-FeSe was reported in the literature; in particular this polymorph was claimed to be observed at low temperature in samples prepared by hydrothermal synthesis. The establishment of triclinic symmetry was argued based on peculiar features characterizing some selected X-ray powder diffraction lines. Using high-resolution synchrotron X-ray powder diffraction, the same features were observed for an aged ß-FeSe sample prepared by a solid state synthesis technique. Moreover, by refining the anisotropic microstrain parameters it was demonstrated that the diffraction pattern can be fairly fitted by applying the well-recognized orthorhombic structural model adopted for the low temperature phase of ß-FeSe. This result indicates that the occurrence of the triclinic polymorph for ß-FeSe can be ruled out.

Structural properties of defective (CH3NH3)2Cu(Cl1-xBrx)4 compounds.

The crystal structures of (CH3NH3)2Cu(Cl1-xBrx)4 compounds have been investigated by means of synchrotron powder X-ray diffraction and pair distribution function analysis at room temperature. As a result, new insights are gained about the structural properties of these compounds, suggesting a monoclinic symmetry (space group No. 14: P21/c - C_{2h}^{5}) induced by the co-operative orbital ordering produced by the Jahn-Teller distortion characterizing the 3d9 Cu2+ ion. In contrast to previous studies, a significant amount of vacancies is found at halogen positions, a feature that can be likely ascribed to the synthesis technique adopted in the present study. Br atoms preferentially occupy axial positions, likely on account of reduced steric hindrance at these sites.

Cardiopulmonary Support During Catheter Ablation of Ventricular Arrhythmias With Hemodynamic Instability: The Role of Inducibility.

Background: Catheter ablation is a treatment option for sustained ventricular tachycardias (VTs) that are refractory to pharmacological treatment; however, patients with fast VT and electrical storm (ES) are at risk for cardiogenic shock. We report our experience using cardiopulmonary support with extracorporeal membrane oxygenation (ECMO) during catheter ablation of VT. Methods: Sixty-two patients (mean age 68 ± 9 years; 94% male) were referred to our center for catheter ablation of repeated episodes of hemodynamically unstable ventricular arrhythmias. ES was defined as the occurrence of three or more VT/ventricular fibrillation episodes requiring electrical cardioversion or defibrillation in a 24-h period. All patients had hemodynamically unstable VTs. Results: Thirty-one patients (group 1) performed catheter ablation without ECMO support and 31 patients (group 2) with ECMO support. At the end of the procedure, ventricular inducibility was not performed in 16 patients of group 1 (52%) due to significant hemodynamic instability. Ventricular inducibility was performed in the other 15 patients (48%); polymorphic VTs were inducible in eight patients. In group 2, VTs were not inducible in 29 patients (93%); polymorphic VTs were inducible in two patients. The median follow-up duration was 24 months. Four patients of group 1 (13%) and five patients of group 2 (16%) died due to refractory heart failure. An implantable cardioverter-defibrillator intervention (shock or antitachycardia pacing) was documented in 13 patients of group 1 (42%) and six patients of group 2 (19%). Conclusions: Extracorporeal membrane oxygenation support during catheter ablation for hemodynamically unstable VTs is a useful tool to prevent acute procedural heart failure and to reduce arrhythmic burden.

Cyan Emission in Two-Dimensional Colloidal Cs2CdCl4:Sb3+ Ruddlesden-Popper Phase Nanoplatelets.

Metal halide perovskites are one of the most investigated materials in optoelectronics, with their lead-based counterparts being renowned for their enhanced optoelectronic performance. The 3D CsPbX3 structure has set the standard with many studies currently attempting to substitute lead with other metals while retaining the properties of this material. This effort has led to the fabrication of metal halides with lower dimensionality, wherein particular 2D layered perovskite structures have captured attention as inspiration for the next generation of colloidal semiconductors. Here we report the synthesis of the Ruddlesden-Popper Cs2CdCl4:Sb3+ phase as colloidal nanoplatelets (NPs) using a facile hot injection approach under atmospheric conditions. Through strict adjustment of the synthesis parameters with emphasis on the ligand ratio, we obtained NPs with a relatively uniform size and good morphological control. The particles were characterized through transmission electron microscopy, synchrotron X-ray diffraction, and pair distribution function analysis. The spectroscopic characterization revealed most strikingly an intense cyan emission under UV excitation with a measured PLQY of ∼20%. The emission was attributed to the Sb3+-doping within the structure.

Systematic Study on TiO2 Crystallization via Hydrothermal Synthesis in the Presence of Different Ferrite Nanoparticles as Nucleation Seeds.

In the present work, the crystallization of anatase TiO2 nanoparticles (NPs), using different ferrite nanoparticles with different chemical composition, dimensions and shape as nucleation seeds, was investigated. In particular, CoFe2O4, NiFe2O4 and Fe3O4 NPs with a volume ratio equal to 1:1000 with respect of TiO2 amount, were used in order to investigate the synthesis of nanocrystalline tetragonal anatase TiO2 by a hydrothermal synthesis. In addition, Lu2O3 nanoparticles were also used to detect the effect of a non-magnetic nanoparticle on the synthesis and nanocrystallization of titania. For each sample, a deep physical characterization was performed by XRD (with a Rietveld refinement of the structural data), FE-SEM, STEM, HRTEM, DSC analysis and BET surface area measurement. Furthermore, for some samples, the photocatalytic activity was investigated by degradation of methylene blue in aqueous medium, in the framework of a standard ISO 10678:2010 protocol. The hydrothermal synthesis was performed with a 3 hours' thermal treatment, at a pressure of approximatively 9 bar and a temperature significantly lower (Tmax═150 °C) than the usual temperature necessary to obtain crystalline anatase TiO2 (Tcryst═350 °C). The results give evidence that the mere presence of a nucleation seeds in the hydrothermal reactor, without any particular need for the composition and morphology, leads to crystalline anatase TiO2 nanoparticles with high photocatalytic performances.

The puzzling structure of Cu5FeS4 (bornite) at low temperature.

The crystal structure of Cu5FeS4 (bornite) has been investigated using synchrotron X-ray powder diffraction at temperatures between 10 and 275 K. Diffraction data confirm that bornite crystallizes in the orthorhombic space group Pbca at 275 K. The unit-cell volume decreases continuously on cooling, but undergoes an abrupt contraction below ∼65 K, where a first-order Pbca→Pca21 structural transition takes place. The primary active mode yielding the observed ordered structure corresponds to the irreducible representation Γ2-, with wavevector (0,0,0). Pair distribution function analysis shows strong discrepancies between the local and the average structure. The average Fe-S bond length obtained through the EXAFS local probe is consistent with the values independently provided by X-ray powder diffraction data, strongly supporting the preferred location of Fe.

Atomic-scale distortions and temperature-dependent large pseudogap in thin films of the parent iron-chalcogenide superconductor Fe1+y Te.

We investigate with scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT) calculations the surface structures and the electronic properties of Fe1+y Te thin films grown by pulsed laser deposition. Contrary to the regular arrangement of antiferromagnetic nanostripes previously reported on cleaved single-crystal samples, the surface of Fe1+y Te thin films displays a peculiar distribution of spatially inhomogeneous nanostripes. Both STM and DFT calculations show the bias-dependent nature of such features and support the interpretation of spin-polarized tunneling between the FeTe surface and an unintentionally magnetized tip. In addition, the spatial inhomogeneity is interpreted as a purely electronic effect related to changes in hybridization and Fe-Fe bond length driven by local variations in the concentration of excess interstitial Fe cations. Unexpectedly, the surface density of states measured by STS strongly evolves with temperature in close proximity to the antiferromagnetic-paramagnetic first-order transition, and reveals a large pseudogap of 180-250 meV at about 50-65 K. We believe that in this temperature range a phase transition takes place, and the system orders and locks into particular combinations of orbitals and spins because of the interplay between excess interstitial magnetic Fe and strongly correlated d-electrons.