Nearly identical Pd nanocubes yield an x-ray powder diffraction pattern with interference fringes affording access to unprecedented structural details of nanocrystal size, shape, and complex atomic ...displacement for a billion-sized population. The excellent agreement between diffraction data and molecular dynamics (MD) provides strong experimental validation of MD simulations and the proposed data-interpretation paradigm. These results show that individual atomic displacements within the nanocubes are not only a function of disrupted bonds and the crystallographic plane of the adjacent surface, but are complex strain gradients extending across all surfaces of the particle strongly influenced by atomic displacements. This observation of nonuniform surface strain and the manner in which it is affected by different sizes, shapes, and locations within each facet could be the key to understanding many surface related properties of shaped nanocrystals including those associated with important catalysis applications.
The perovskite PbCrO3 is an antiferromagnetic insulator. However, the fundamental interactions leading to the insulating state in this single-valent perovskite are unclear. Moreover, the origin of ...the unprecedented volume drop observed at a modest pressure of P = 1.6 GPa remains an outstanding problem. We report a variety of in situ pressure measurements including electron transport properties, X-ray absorption spectrum, and crystal structure study by X-ray and neutron diffraction. These studies reveal key information leading to the elucidation of the physics behind the insulating state and the pressure-induced transition. We argue that a charge disproportionation 3Cr(4+) → 2Cr(3+) + Cr(6+) in association with the 6s-p hybridization on the Pb(2+) is responsible for the insulating ground state of PbCrO3 at ambient pressure and the charge disproportionation phase is suppressed under pressure to give rise to a metallic phase at high pressure. The model is well supported by density function theory plus the correlation energy U (DFT+U) calculations.
Water defines life on Earth from the cellular to the terrestrial level. Yet the molecular level arrangement in water is not well understood, posing problems in comprehending its very special ...chemical, physical and biological properties. Here we present high-resolution x-ray diffraction data for water clearly showing that its molecular arrangement exhibits specific correlations that are consistent with the presence of rings of H(2)O molecules linked together by hydrogen bonds into tetrahedral-like units from a continuous network. This level of molecular arrangement complexity is beyond what a simple 'two-state' model of water (Bernal and Fowler 1933 J. Chem. Phys.1 515-48) could explain. It may not be explained by the recently put forward 'chains-clusters of completely uncorrelated molecules' model (Wernet et al 2004 Science 304 995-9) either. Rather it indicates that water is homogeneous down to the molecular level where different water molecules form tetrahedral units of different perfection and/or participate in rings of different sizes, thus experiencing different local environments. The local diversity of this tetrahedral network coupled to the flexibility of the hydrogen bonds that hold it together may explain well the rich phase diagram of water and why it responds non-uniformly to external stimuli such as, for example, temperature and pressure.
In contrast to classical phase transitions driven by temperature,a quantum critical point (QCP) defines a transition at zero temperature that is driven by non-thermal parameters. In the known quantum ...critical d-electron systems, tuning the electronic bandwidth by means of changing the applied pressure or unit-cell dimensions, or tuning the d-state population, is used to drive the criticality. Here we describe how a novel chemical parameter, the breaking of bonds in Ge-Ge dimers that occurs within the intermetallic framework in SrCo2(Ge1−xPx)2, results in the appearance of a ferromagnetic (FM) QCP. Although both SrCo2P2 and SrCo2Ge2 are paramagnetic, weak itinerant ferromagnetism unexpectedly develops during the course of the dimer breaking, and a QCP is observed at the onset of the FM phase. The use of chemical bond breaking as a tuning parameter to induce QCP opens an avenue for designing and studying novel magnetic materials.
The densification of zirconia at very low temperatures (<400 °C) requires enhanced synergy between thermodynamics and kinetics. This work demonstrates an efficient single step approach combining ...amorphous hydrated zirconia and Spark Plasma Sintering at 350 °C and 600 MPa. The resulting zirconia ceramics exhibit a cohesive nanostructure with small average grain sizes (20 nm) and a predominantly monoclinic structural polymorph confirmed by both X-ray scattering analyses and High Resolution Transmission Electronic Microscopy. Remarkable Vickers hardness of 3.8 GPa for high level of porosity (30%) is explained by the density and homogeneous distribution of grains boundaries and meso/microporosities.
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The structures and thermoelectric properties of the double-filled (CaxCe1−x)Fe4Sb12 series (x=0, 0.25, 0.5, 0.75, and 1) have been studied using a combined experimental and computational methods. ...Compounds of (CaxCe1−x)Fe4Sb12 were obtained only for x=0, 0.5, and 1. Composition with x=0.25 was found to be a mixture of x=0 and 0.5 compounds, and composition with x=0.75 was found to be a mixture of x=1 and 0.5 compounds, respectively. Our conclusions on phase formation are supported by density functional theory (DFT) calculations. In Ca0.5Ce0.5Fe4Sb12, Ca substitution in the Ce site of CeFe4Sb12 leads to high hole concentrations, resulting in stronger semimetal transport as compared to CeFe4Sb12. Ca0.5Ce0.5Fe4Sb12 yields a slightly higher ZT value than that of CeFe4Sb12, which is attributed to its lower lattice thermal conductivity. Phonon mode calculations adopting a three-particle bending model suggest that thermal conductivity is reduced upon Ca substitution because of an additional vibration mode which involves both Ca and Ce atoms.
Detailed structural information to correlate with thermoelectric properties in a series of double-filled (CaxCe1−x)Fe4Sb12 skutterudite samples were obtained using synchrotron X-ray diffraction and first principle calculations. Display omitted
•Research focus on phase stability of CaCe double-filled p-type skutterudite.•Stable structure forms for an equiatomic occupancy of the voids by Ca and Ce.•Samples feature two-phase mixtures with variations of filling atoms ratio.•Phase stablility is investigated by DFT total energy calculations.•Sample with Equiatomic occupancy of Ca and Ce show higher ZT.
An yttria-alumina glass incorporating 4 wt % of SiO2 was prepared by the use of an aerodynamic levitator equipped with a laser heating device. Upon post synthesis annealing, the glass is observed to ...exhibit crystallization below the glass transition temperature. In the current study, this unusual behavior is examined using powder X-ray diffraction and infrared spectroscopy to follow crystallinity and optical absorbance as a function of annealing treatment. Double-resonance 27Al/29Si NMR spectroscopy and synchrotron diffraction experiments demonstrate that Si cations are incorporated in the formed Y3Al5O12 (YAG) crystals on the 4-fold coordination Al site during crystallization. This has important consequences on the resulting microstructure of the YAG glass-ceramics.
The valence matching principle is used to explain the loss of inversion symmetry in the noncentrosymmetric (NCS) polymorph of KNaNbOF5 in comparison to its centrosymmetric (CS) polymorph. The NbOF52– ...anion has five contacts to both potassium and sodium in the NCS polymorph, whereas in the CS polymorph there are only four contacts to potassium and six contacts to sodium. The lower average Lewis acidity of the cationic framework in the NCS polymorph relative to the CS polymorph reflects the loss of inversion symmetry. This lower average Lewis acidity is achieved during hydrothermal synthesis with a potassium-rich solution when the K:Na ratio in the reaction is greater than 1:1, as the Lewis acidity of potassium is lower than that of sodium. The contrasting coordination environments are manifested in secondary distortions that weaken the primary NbO interaction and lengthen the NbO bond in the NCS polymorph. An unusual heat-induced phase transition from the CS to the NCS polymorph was studied with in situ powder X-ray diffraction. The transition to the NCS polymorph upon cooling occurs through an intermediate phase(s).
High resolution synchrotron powder XRD, 89Y CPMG NMR, and 139La MAS NMR spectroscopy have been applied to eventually draw the phase diagram of the La2Si2O7–Y2Si2O7 system. The diagram presents a ...solid solubility region of G-(La,Y)2Si2O7, which extends to the La0.9Y1.1Si2O7 composition at any temperature of this study. Compositions richer in Y show two-phase domains, with G + α at T < 1450 °C and G + δ at T > 1450 °C. The Y-rich extreme is more complex, showing two solid solution regions of δ- and γ-(La,Y)2Si2O7 polymorphs which appear with increasing Y content, respectively. It is interesting to note that the La for Y substitution mechanism in the G-(La,Y)2Si2O7 polymorph is not homogeneous, but a preferential occupation of Y for the RE2 site is observed. Finally, the 89Y and 139La isotropic chemical shift values in G-(La,Y)2Si2O7 have been described here for the first time and assigned to the different RE crystallographic sites of the unit cell.
Several different approaches have traditionally been used for detection of X‐ray powder diffraction patterns, including area detectors, point detectors and position‐sensitive detectors. Each has ...advantages. This paper discusses use of a low‐cost CCD detector attached to a diffractometer arm, where line‐by‐line readout of the CCD is coupled to continuous motion of the arm. When this type of detector is used and where X‐ray optics are employed to focus the source image onto the detector plane both high‐resolution and rapid measurements can be performed, with data collection over a complete 2θ range. This is particularly advantageous for synchrotron applications but valuable also for Guinier diffractometer laboratory instruments. Peak resolutions are shown to be moderately better than what can be obtained with a position‐sensitive detector and significantly better than with an area detector. Many samples have intrinsically broadened peak shapes for which little improvement in data quality could be obtained with an analyzer‐crystal detector. With comparable numbers of modules, these CCD data collection speeds can be close to those with position‐sensitive detectors, but without the low‐angle asymmetry seen in the latter.
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