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1.
J Phys Chem Lett ; 10(23): 7398-7405, 2019 Dec 05.
Artigo em Inglês | MEDLINE | ID: mdl-31721591

RESUMO

Here we report on the first structural and optical high-pressure investigation of MASnBr3 (MA = [CH3NH3]+) and CsSnBr3 halide perovskites. A massive red shift of 0.4 eV for MASnBr3 and 0.2 eV for CsSnBr3 is observed within 1.3 to 1.5 GPa from absorption spectroscopy, followed by a huge blue shift of 0.3 and 0.5 eV, respectively. Synchrotron powder diffraction allowed us to correlate the upturn in the optical properties trend (onset of blue shift) with structural phase transitions from cubic to orthorhombic in MASnBr3 and from tetragonal to monoclinic for CsSnBr3. Density functional theory calculations indicate a different underlying mechanism affecting the band gap evolution with pressure, a key role of metal-halide bond lengths for CsSnBr3 and cation orientation for MASnBr3, thus showing the impact of a different A-cation on the pressure response. Finally, the investigated phases, differently from the analogous Pb-based counterparts, are robust against amorphization showing defined diffraction up to the maximum pressure used in the experiments.

2.
Materials (Basel) ; 12(15)2019 Jul 25.
Artigo em Inglês | MEDLINE | ID: mdl-31349538

RESUMO

A study was performed to investigate the hydrogen embrittlement behavior of 18-Ni 300 maraging steel produced by selective laser melting and subjected to different heat treatment strategies. Hydrogen was pre-charged into the tensile samples by an electro-chemical method at the constant current density of 1 A m-2 and 50 A m-2 for 48 h at room temperature. Charged and uncharged specimens were subjected to tensile tests and the hydrogen concentration was eventually analysed using quadrupole mass spectroscopy. After tensile tests, uncharged maraging samples showed fracture surfaces with dimples. Conversely, in H-charged alloys, quasi-cleavage mode fractures occurred. A lower concentration of trapped hydrogen atoms and higher elongation at fracture were measured in the H-charged samples that were subjected to solution treatment prior to hydrogen charging, compared to the as-built counterparts. Isothermal aging treatment performed at 460 °C for 8 h before hydrogen charging increased the concentration of trapped hydrogen, giving rise to higher hydrogen embrittlement susceptibility.

3.
Front Chem ; 6: 526, 2018.
Artigo em Inglês | MEDLINE | ID: mdl-30430105

RESUMO

The need for high efficiency energy production, conversion, storage and transport is serving as a robust guide for the development of new materials. Materials with physical-chemical properties matching specific functions in devices are produced by suitably tuning the crystallographic- defect- and micro-structure of the involved phases. In this review, we discuss the case of Rare Earth doped Ceria. Due to their high oxygen diffusion coefficient at temperatures higher than ~500°C, they are very promising materials for several applications such as electrolytes for Solid Oxide Fuel and Electrolytic Cells (SOFC and SOEC, respectively). Defects are integral part of the conduction process, hence of the final application. As the fluorite structure of ceria is capable of accommodating a high concentration of lattice defects, the characterization and comprehension of such complex and highly defective materials involve expertise spanning from computational chemistry, physical chemistry, catalysis, electrochemistry, microscopy, spectroscopy, and crystallography. Results coming from different experimental and computational techniques will be reviewed, showing that structure determination (at different scale length) plays a pivotal role bridging theoretical calculation and physical properties of these complex materials.

4.
Inorg Chem ; 57(22): 14347-14352, 2018 Nov 19.
Artigo em Inglês | MEDLINE | ID: mdl-30382704

RESUMO

A ferrite in the Sr2Tl2O5-type MFe3O5 family with M = Co has been synthesized at 12 GPa pressure. Neutron diffraction shows the sample to be Co deficient with composition Co0.6Fe3.4O5. The Co/Fe cation distribution is found to be profoundly different from those of MFe3O5 analogs and lies between normal and inverse limits, as Co2+ substitutes across trigonal prismatic and one of the two octahedral sites. CoFe3O5 shows complex magnetic behavior with weak ferromagnetism below TC1 ≈ 300 K and a second transition to ferrimagnetic order at TC2 ≈100 K. Spin scattering of carriers leads a substantial increase in the hopping activation energy below TC1, and a small negative magnetoresistance is observed at low temperatures.

5.
Dalton Trans ; 47(37): 12951-12963, 2018 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-30151526

RESUMO

Phase relationship and structural behaviour in the substitutional series LaNi13-xGax and CeNi13-xGax have been studied by a combination of X-ray powder diffraction measurements, differential scanning calorimetry, electron diffraction tomography and metallographic analyses. The sequence of morphotropic phase transformations has been found in the series LaNi13-xGax resulting in five varieties of the NaZn13 structure: the cubic phase with aristotype structure at x = 2 (space group Fm3[combining macron]c, Pearson symbol cF112), two tetragonal phases at x = 2.5-4.25 (space group I4/mcm, Pearson symbol tI56-I) and 7-7.5 (space group I4/mcm, Pearson symbol tI56-II), both with an atomic arrangement of the CeNi8.5Si4.5 type and two orthorhombic phases at x = 4.5-5.75 (LaNi7In6 structure type, space group Ibam, Pearson symbol oI56) and x = 6.37-6.87 (a new derivative of the NaZn13, prototype structure, space group Fmmm, Pearson symbol oF112). The related series CeNi13-xGax shows similar behaviour. The corresponding tI56-I ↔oI56 ↔oF112 ↔tI56-II phases are formed at x = 4-4.25, 4.5-6, 6.37-6.87 and 7-7.37, respectively. In contrast to the lanthanum analogues, the phase with cubic symmetry was not found for this system. Complex twinned and multiple twinned (twinning of twins) domain structures which are revealed for the tetragonal and both orthorhombic phases clearly indicate temperature-induced polymorphic phase transitions during the formation of these phases. LaNi13-xGax samples show paramagnetic behavior, whereas the CeNi13-xGax series exhibits Curie-Weiss paramagnetism.

6.
Nanomaterials (Basel) ; 8(9)2018 Aug 22.
Artigo em Inglês | MEDLINE | ID: mdl-30135359

RESUMO

This work highlights the importance of the hydrophilicity of a catalyst's active sites on an oxygen reduction reaction (ORR) through an electrochemical and physico-chemical study on catalysts based on nitrogen-modified carbon doped with different metals (Fe, Cu, and a mixture of them). BET, X-ray Powder Diffraction (XRPD), micro-Raman, X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), Scanning Transmission Electron Microscopy (STEM), and hydrophilicity measurements were performed. All synthesized catalysts are characterized not only by a porous structure, with the porosity distribution centered in the mesoporosity range, but also by the presence of carbon nanostructures. In iron-doped materials, these nanostructures are bamboo-like structures typical of nitrogen carbon nanotubes, which are better organized, in a larger amount, and longer than those in the copper-doped material. Electrochemical ORR results highlight that the presence of iron and nitrogen carbon nanotubes is beneficial to the electroactivity of these materials, but also that the hydrophilicity of the active site is an important parameter affecting electrocatalytic properties. The most active material contains a mixture of Fe and Cu.

7.
Inorg Chem ; 57(2): 879-891, 2018 Jan 16.
Artigo em Inglês | MEDLINE | ID: mdl-29280608

RESUMO

The structure evolution in the CeO2-Sm2O3 system is revisited by combining high resolution synchrotron powder diffraction with pair distribution function (PDF) to inquire about local, mesoscopic, and average structure. The CeO2 fluorite structure undergoes two phase transformations by Sm doping, first to a cubic (C-type) and then to a monoclinic (B-type) phase. Whereas the C to B-phase separation occurs completely and on a long-range scale, no miscibility gap is detected between fluorite and C-type phases. The transformation rather occurs by growth of C-type nanodomains embedded in the fluorite matrix, without any long-range phase separation. A side effect of this mechanism is the ordering of the oxygen vacancies, which is detrimental for the application of doped ceria as an electrolyte in fuel cells. The results are discussed in the framework of other Y and Gd dopants, and the relationship between nanostructuring and the above equilibria is also investigated.

8.
Inorg Chem ; 56(19): 11513-11523, 2017 Oct 02.
Artigo em Inglês | MEDLINE | ID: mdl-28901753

RESUMO

Metal-rich, mixed copper-rhodium sulfide Cu3-δRh34S30 that represents a new Cu-filled variant of the Rh17S15 structure has been synthesized and structurally characterized. Copper content in the [CuRh8] cubic cluster was found to vary notably dependent on the chosen synthetic route. Full site occupancy was achieved only in nanoscaled Cu3Rh34S30 obtained by a rapid, microwave-assisted reaction of CuCl, Rh2(CH3CO2)4 and thiosemicarbazide at 300 °C in just 30 min; whereas merely Cu-deficient Cu3-δRh34S30 (2.0 ≥ δ ≥ 0.9) compositions were realized via conventional high-temperature ceramic synthesis from the elements at 950 °C. Although Cu3-δRh34S30 is metallic just like Rh17S15, the slightly enhanced metal content has a dramatic effect on the electronic properties. Whereas the Rh17S15 host undergoes a superconducting transition at 5.4 K, no signs of the latter were found for the Cu-derivatives at least down to 1.8 K. This finding is corroborated by the strongly reduced density of states at the Fermi level of the ternary sulfide and the disruption of long-range Rh-Rh interactions in favor of Cu-Rh interactions as revealed by quantum-chemical calculations.

9.
Inorg Chem ; 56(15): 9343-9352, 2017 Aug 07.
Artigo em Inglês | MEDLINE | ID: mdl-28718656

RESUMO

The title compound was synthesized by a reaction of the elemental educts in a corundum crucible at 1200 °C under an Ar atmosphere. The excess of Ga used in the initial mixture served as a flux for the subsequent crystal growth at 600 °C. The crystal structure of Yb4Ga24Pt9 was determined from single-crystal X-ray diffraction data: new prototype of crystal structure, space group C2/m, Pearson symbol mS74, a = 7.4809(1) Å, b = 12.9546(2) Å, c = 13.2479(2) Å, ß = 100.879(1)°, V = 1260.82(6) Å3, RF = 0.039 for 1781 observed reflections and 107 variable parameters. The structure is described as an ABABB stacking of two slabs with trigonal symmetry and compositions Yb4Ga6 (A) and Ga12Pt6 (B). The hard X-ray photoelectron spectrum (HAXPES) of Yb4Ga24Pt9 shows both Yb2+ and Yb3+ contributions as evidence of an intermediate valence state of ytterbium. The evaluated Yb valence of ∼2.5 is in good agreement with the results obtained from the magnetic susceptibility measurements. The compound is a bad metallic conductor.

10.
ACS Appl Mater Interfaces ; 8(15): 9745-54, 2016 Apr 20.
Artigo em Inglês | MEDLINE | ID: mdl-27019104

RESUMO

Crystal shape control on a series of anatase photocatalysts was achieved by varying the amount of HF employed as a capping agent in their hydrothermal synthesis. A systematic comparison between their physicochemical properties, determined by several complementary surface and bulk techniques before and after thermal treatment at 500 °C, allowed one to discern the influence of the relative amount of exposed {001} crystal facets among a series of effects simultaneously affecting their oxidative photocatalytic activity. The results of both formic acid and terephthalic acid photo-oxidation test reactions point to the primary role played by calcination in making {001} facets effectively photoactive. Annealing not only removes most of the residual fluorine capping agent from the photocatalyst surface, thus favoring substrate adsorption, but also produces morphological modifications to a crystal packing that makes accessible a larger portion of surface {001} facets due to the unpiling of platelike crystals. The photocatalyst bearing the highest amount of exposed {001} facets (60%) shows the highest photoactivity in both the direct and the (•)OH-radical-mediated photocatalytic test reaction.

11.
IUCrJ ; 2(Pt 5): 511-22, 2015 Sep 01.
Artigo em Inglês | MEDLINE | ID: mdl-26306193

RESUMO

A new hierarchical approach is presented for elucidating the structural disorder in Ce1-x Gd x O2-x/2 solid solutions on different scale lengths. The primary goal of this investigation is to shed light on the relations between the short-range and the average structure of these materials via an analysis of disorder on the mesocopic scale. Real-space (pair distribution function) and reciprocal-space (Rietveld refinement and microstructure probing) analysis of X-ray powder diffraction data and electron spin resonance (ESR) investigations were carried out following this approach. On the local scale, Gd- and Ce-rich droplets (i.e. small regions a few ångströms wide) form, exhibiting either a distorted fluorite (CeO2) or a C-type (Gd2O3) structure in the whole compositional range. These droplets can then form C-type nanodomains which, for Gd concentrations x Gd ≤ 0.25, are embedded in the fluorite matrix. At the site percolation threshold p C for a cubic lattice (x Gd = p C ≃ 0.311), C-type nanodomains percolate inside each crystallite and a structural phase transformation is observed. When this occurs, the peak-to-peak ESR line width ΔH pp shows a step-like behaviour, which can be associated with the increase in Gd-Gd dipolar interactions. A general crystallographic rationale is presented to explain the fluorite-to-C-type phase transformation. The approach shown here could be adopted more generally in the analysis of disorder in other highly doped materials.

12.
Phys Chem Chem Phys ; 15(22): 8495-505, 2013 Jun 14.
Artigo em Inglês | MEDLINE | ID: mdl-23403753

RESUMO

Doped ceria materials are widely studied for their application in solid oxide fuel cell devices. In this work we report on the average and local structure evolution of La-doped ceria (Ce(1-x)La(x)O(2-x/2), x = 0.25) under fuel cells' operating conditions. The effect of doping on the average structure is investigated using conventional Rietveld analysis of neutron powder diffraction data. The extent of disorder as well as the local structure evolution at high temperature are investigated by employing very hard X-rays under both air and reducing atmosphere.

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