Increase In Local Disorder Research Articles

Effect of Ce4+-substitution at A and B sites of Nd2Zr2O7: A study for plutonium incorporation in pyrochlores

Zirconate pyrochlores have been envisaged as promising candidates for inert matrix fuel applications and immobilization of nuclear waste. Therefore, Ce4+-substituted Nd2Zr2O7 samples with general formula [Nd1-xCex]2Zr2O7+δ (0.0 ≤ x ≤ 0.5) and Nd2[Zr1-xCex]2O7 (0.0 ≤ x ≤ 1.0) were synthesized by solid state route and thoroughly characterized by X-ray diffraction (XRD) in order to mimic plutonium incorporation at A- and B-sites of Nd2Zr2O7 pyrochlore which are crystallographically distinct. The local structure has also been probed by Raman spectroscopy and X-ray absorption near edge structure (XANES) spectroscopy. Interestingly, the substitution of Ce4+ at different sites yields distinct phase relations. Ce4+-substitution at A- (Nd3+) site retains pyrochlore-type phase up to nominal composition with x = 0.4 and F-type phase was observed for composition with x = 0.5. However, Ce4+-substitution at B- (Zr4+) site demonstrated transformation of pyrochlore-type phase to F-type phase through a biphasic phase field consisting of these two phases. Raman spectroscopic studies on Nd2[Zr1-xCex]2O7 (0.0 ≤ x ≤ 1.0) system clearly indicated existence of C-type ordering in largely F-type phases for compositions with x = 0.6 and beyond. XANES studies exhibited a change in Zr local coordination upon successive Ce4+-substitution at ‘Nd’ site and pointed towards increase in local disorder across the pyrochlore phase regime. Ce LIII edge XANES also revealed significant changes in ‘Ce’ local environment in Ce-substituted samples.

Thiol Treatment Creates Selective Palladium Catalysts for Semihydrogenation of Internal Alkynes

Summary Surface and interfacial engineering of heterogeneous metal catalysts is effective and critical for optimizing selective hydrogenation for fine chemicals. By using thiol-treated ultrathin Pd nanosheets as a model catalyst, we demonstrate the development of stable, efficient, and selective Pd catalysts for semihydrogenation of internal alkynes. In the hydrogenation of 1-phenyl-1-propyne, the thiol-treated Pd nanosheets exhibited excellent catalytic selectivity (>97%) toward the semihydrogenation product (1-phenyl-1-propene). The catalyst was highly stable and showed no obvious decay in either activity or selectivity for over ten cycles. Systematic studies demonstrated that a unique Pd-sulfide/thiolate interface created by the thiol treatment was crucial to the semihydrogenation. The high catalytic selectivity and activity benefited from the combined steric and electronic effects that inhibited the deeper hydrogenation of C=C bonds. More importantly, this thiol treatment strategy is applicable to creating highly active and selective practical catalysts from commercial Pd/C catalysts for semihydrogenation of internal alkynes.

Local Disorder and the Success of Firms in Residential Neighbourhoods

According to economic geography literature, the success of firms is affected by the local context, in particular when firms are socio-spatially embedded. We expect this effect to be stronger when firms face an increase in local disorder. We analysed data on 344 firms (active in retail, eating and drinking establishments, personal services and private education, business services, cultural activities, manufacturing and building) in 108 Dutch residential neighbourhoods, and data on the changes in social and physical disorder of those neighbourhoods, to examine firm success determinants. We find that it is not the degree of disorder that matters to local firms turnover, but rather recent changes in local disorder. More in particular, we find that local firm turnover is negatively affected by an increase in local disorder, but only when a firm depends on daily visits from predominantly local customers. Our results suggest that physical and social local interventions to create safe and clean public spaces will indirectly positively influence local firms and subsequently, the neighbourhood economy. This spill-over effect is promising for both residents, who benefit from local amenities and local ‘buzz’, and local entrepreneurs, whose firm success is stimulated.

Spectroscopic ellipsometry characterization of ZnO:Sn thin films with various Sn composition deposited by remote-plasma reactive sputtering

ZnO:Sn thin films were deposited onto thermally oxidized silicon substrates using a remote plasma reactive sputtering. Their optical constants (refractive index n and extinction coefficient k) were determined from ellipsometric data recorded over a wide spectral range (0.05–6eV). Parametrization of ZnO:Sn complex dielectric permittivity consists of a parameterized semiconductor oscillator function describing the short wavelength absorption edge, a Drude oscillator describing free carrier absorption in near-infrared part of spectra and a Lorentz oscillator describing the long wavelength absorption edge and intra-band absorption in the ultra-violet part of the spectra. Using a Mott-Davis model, the increase in local disorder with increasing Sn doping is quantified from the short wavelength absorption edge onset. Using the Wemple-DiDomenico single oscillator model for the transparent part of the optical constants spectra, an increase in the centroid distance of the valence and conduction bands with increasing Sn doping is shown and only slight increase in intensity of the inter-band optical transition due to Sn doping occurs. The Drude model applied in the near-infrared part of the spectra revealed the free carrier concentration and mobility of ZnO:Sn. Results show that the range of transparency of prepared ZnO:Sn layers is not dramatically affected by Sn doping whereas electrical conductivity could be controlled by Sn doping. Refractive index in the transparent part is comparable with amorphous Indium Gallium Zinc Oxide allowing utilization of prepared ZnO:Sn layers as an indium-free alternative.

Vacancy-mediated three-center four-electron bonds in GeTe-Sb2Te3phase-change memory alloys

Although GeTe-${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ (GST) alloys are widely used in data storage, many fundamental issues are still under debate. Here, we demonstrate that the presence of vacancies in the crystalline phase has far-reaching consequences, namely, a triad of twofold coordinated Te atoms with lone-pair electrons generated around the vacancy enables the formation of soft three-center four-electron bonds, whose properties provide an explanation for the unusual characteristics of GST, in particular, the increase in local disorder upon crystallization, the co-existence of a very fast switching rate with a large property contrast, the possibility of a solid-solid amorphization process that excludes conventional melting, and the drastic difference in crystallization behavior between GST and the ideal binary GeTe. Anisotropy of the three-center bonds may serve as an additional degree of freedom for information recording and provide a unified explanation for a variety of unique effects observed in lone-pair semiconductors.

Does Local Disorder Occur in the Pyrochlore Zirconates?

The zirconates Ln(2)Zr(2)O(7) (Ln = lanthanoid) have been studied using a combination of Zr L-edge X-ray absorption near edge structure (XANES) and synchrotron X-ray and neutron powder diffraction methods. These studies demonstrate that as the size of the lanthanoid cation decreases, the local structure evolves smoothly from the ideal pyrochlore toward the defect fluorite rather than undergoing an abrupt transformation. The Zr L-edge spectrum is found to be extremely sensitive to changes in the local coordination environment and demonstrates an increase in local disorder across the pyrochlore oxides. The sensitivity of the XANES measurements enables us to identify the progressive nature of the transition that could not be detected using bulk diffraction techniques.

Frustrated spin correlations in diluted spin iceHo2−xLaxTi2O7

We have studied the evolution of the structural properties as well as the static and dynamic spin correlationsof spin ice Ho2Ti2O7, where Ho was partially replaced by non-magnetic La. The crystal structure of diluted samplesHo2−xLaxTi2O7 was characterized by x-ray and neutron diffraction and by HoLIII-edgeand Ti K-edge extended x-ray absorption fine structure (EXAFS) measurements.It is found that the pyrochlore structure remains intact until aboutx = 0.3, but a systematic increase in local disorder with increasing Laconcentration is observed in the EXAFS data, especially from the TiK edge.Quasi-elastic neutron scattering and ac susceptibility measurements show that, inx≤0.4 samples at temperatures above macroscopic freezing, the spin–spin correlationsare short ranged and dynamic in nature. The main difference with pure spinice in the dynamics is the appearance of a second, faster, relaxation process.

A structural study of haematite samples prepared from sulfated goethite precursors: the generation of axial mesoporous voids

A study of the reflection width anisotropy evident in powder X-ray diffraction patterns of haematite prepared from sulfated goethite precursors calcined at low temperatures has been made. HRTEM has shown that the sulfation procedure is responsible for the generation of mesoporous voidage regions which run axially along acicular α-Fe2O3 crystals, producing the effects observed in X-ray diffraction. Energy dispersive X-ray elemental mapping has demonstrated the association of such voidage regions with sulfur. Sulfation also inhibits sintering of the haematite crystallites and is associated with an increase in local disorder as evidenced by Mossbauer spectroscopic studies. At higher calcination temperatures the reflection width anisotropy, acicular morphology and porosity is lost. This study indicates that sulfation of goethite may prove a successful and simple route to the synthesis of nanoporous haematite.