High-resolution X-ray Powder Diffraction Research Articles

Ammonium fluoride mediated mechano chemical synthesis of A2PdF6 (A = K, Rb) along with their catalytic role in environmental remediation

To highlight the power of mechanochemistry to oxidize metal ions leading to the discovery of newer phases, ammonium fluoride mediated synthesis of A2PdF6 (A=Rb,K) has been carried out. Grinding reactions of ammonium fluoride with palladium (II) acetate and further cation exchange with AF (A=Rb,K) were closely monitored ex-situ characterizations such as high resolution powder X-ray diffraction (PXRD), fourier transform infrared (FT-IR) and Raman spectroscopy measurements. For the reaction involving RbF, Rb2PdF6 possessing K2PtCl6 type structure was confirmed from the Rietveld refinement of PXRD pattern, high resolution transmission emission microscopy (HR-TEM) and selected area electron diffraction (SAED) measurements. Elemental composition and oxidation state of palladium in Rb2PdF6 were examined using X-ray photoelectron spectroscopy (XPS) analysis. Ligand to metal charge transfer and d-d transitions of Pd (IV) were present in the UV–vis diffuse reflectance spectrum of Rb2PdF6 with a band gap of 2.67eV estimated using the Kubelka–Munk function. Its role as a catalyst to degrade the aqueous dye solutions (Methylene Blue, Malachite Green, Rhodamine-6G and Rhodamine-B) under UV–vis radiation has been evaluated along with their kinetics. The catalyst was active after four cycles of use. The reduction of Pd4+ to Pd0 by NaBH4 has been favourably used for the efficient and rapid conversion of highly carcinogenic, toxic, mutagenic Cr6+ to less toxic Cr3+. K2PdF6 possessing cubic K2PtCl6-type structure has also been synthesized for the first time by this method which presented yet another example for the alteration of reaction landscapes by mechanical forces.

Effect of the synthesis conditions on the properties of La0.15Sm0.35Sr0.08Ba0.42FeO3 − δ cathode material for SOFCs

The perovskite La0.15Sm0.35Sr0.08Ba0.42FeO3−δ has been prepared by the glycine nitrate (GNC) route, varying the fuel/oxidizer ratio (glycine/nitrate, G/N=1 and 2) and cooling rate (slow cooling and air-quenched), in order to study the influence of sample preparation on the materials' properties, in the context of their application as a cathode material for SOFCs. For this, the performance of the prepared mixed ion and electron conducting perovskite oxides is dictated by their structure, oxygen stoichiometry (3−δ), chemical composition and thermal expansion properties. High-resolution Synchrotron X-ray powder diffraction patterns were collected at room temperature and at 700 and 800°C. It was found that the materials had a cubic crystal structure at these temperatures. As expected, 3−δ decreased as temperature increased, and was slightly smaller for the quenched sample. Higher electrical conductivity values were obtained for the sample with G/N=1 (air-quenched) in the cooling rate. At 700 and 800°C the cathode synthesized with G/N=1 and air-quenched showed the smallest polarization resistance values in impedance spectroscopy studies. Therefore, the physicochemical and electrochemical characterization clearly demonstrated the influence of the synthetic conditions on the cathode performance.

Detailed Investigation of Anionic Redox Chemistry in Li-Rich Li2Ir0.75Sn0.5O3 Electrode Materials for Li-Ion Batteries

Li-rich layered oxides, xLi2MnO3∙(1-x)LiMO2 (M = Ni, Co, Mn) have attracted significant attention as cathode materials for lithium ion batteries in recent years. Although they exhibit promising capacities, up to 300mAh/g, due to transition metal redox reactions and unconventional oxygen redox reaction, they have several critical problems: (1) large irreversible capacity loss in the first cycle due to the release of oxygen and lithium from the lattice at the end of first charge, (2) insufficient cycling performance, (3) side reactions with electrolyte at high cut-off voltage and, (4) phase transformation that leads to voltage decay during cycling [1]. In order to improve the structural stability and investigate single redox cation, Li2M1-xSnxO3 (M = Ru, Ir) type Li-rich materials have been recently designed by taking advantage of 4d, 5d transition metals that can increase the M-O covalency. These materials are structurally related to the 3d transition metal-based traditional Li-rich cathode materials. Increased M-O bond covalency results in lower voltage plateau compared to 3d transition metal based Li-rich cathode materials and thus enables the investigation of intrinsic redox chemistry by avoiding the side reactions with electrolytes at higher voltage. In these cathode materials electrochemical process is described as ‘a cumulative and reversible cationic (Mn+ → M(n+1)+) and anionic (O2- → O2 2-) redox process, due to the d-sp hybridization associated with a reductive coupling mechanism’. This mechanism is derived by using the X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) techniques [2,3]. However, the structural changes in Li-rich layered materials during anionic redox chemistry are very complex and still remain a controversial issue that needs further investigation. In order to understand the origin of high capacity of Li-rich cathode materials, systematic investigation is required to understand the structural changes in the bulk materials during electrochemical cycling. In this study, we synthesized Li2Ir0.75Sn0.25O3 electrode material that delivered first cycle charge and discharge capacity of 214mAh/g (corresponds to removal of ~2 moles of Li+) and 188mAh/g, respectively and after 100 cycles, it was still able to deliver 128mAh/g of discharge capacity with 71% capacity retention. We performed synchrotron radiation-based high-resolution X-ray powder diffraction (HRPD) and hard & soft X-ray absorption spectroscopy (XAS) along with several electrochemical characterization techniques, to investigate the redox reaction in Li2Ir0.75Sn0.25O3 electrode in terms of local and bulk crystal structural changes, variations in the oxidation states of transition metal and oxygen ions in Li2Ir0.75Sn0.25O3, respectively. Sn K-edge XANES spectra do not show any edge shift implying that Sn-ions are not participating in the electrochemical reaction. However, we found that there is minimal contribution from Ir-ions in the charge compensation, as inferred by small changes in the Ir L3-edge XANES edge position. So the charge is mostly compensated by anionic redox chemistry mainly involving oxygen. O K-edge soft XAS spectra were measured to quantify the charge compensation by oxygen and metals. However, anionic redox chemistry is very complex as it involves significant structural changes. Understanding these structural changes during lithium extraction and insertion is a key to investigate the charge compensation mechanism of 4d and 5d metals-based Li-rich cathode materials. Rietveld refinement and profile refinement by FAULTS [4] were performed on the measured HRPD pattern to understand the detailed crystal structure variations and formation of stacking faults during charge and discharge. Detailed findings will be presented at the time of the meeting. [1] J. Wang et al. Advanced Energy Materials6 (2016) 1600906 [2] M. Sathiya et al. Nature Materials12 (2013) 827–835 [3] E. McCalla et al. Science350 (2015) 1516–1521 [4] M. Casas-Cabanas et al. Journal of Applied Crystallography49 (2016) 2259-2269

Structural and functional characterisation of KNNS–BNKZ lead-free piezoceramics

ABSTRACTPromising piezoelectric properties have been reported recently for lead-free 0.96(K0.48Na0.52Nb0.95Sb0.05)–0.04Bi0.5(Na0.82K0.18)0.5ZrO3 (KNNS–BNKZ) ceramics. The presence of coexisting rhombohedral and tetragonal phases is thought to play a key role in their functional properties, but a thorough understanding is currently lacking. In this experiment, (1−x)KNNS–(x)BNKZ ceramics with x = 0–0.05 were prepared by the mixed-oxide method. High-resolution synchrotron X-ray powder diffraction (SXPD) measurements reveal that the addition of BNKZ into KNNS ceramics leads to an increase in the rhombohedral–orthorhombic phase transition temperature (TR−O) and a decrease in the orthorhombic–tetragonal phase transition temperature (TO−T) leading to orthorhombic–tetragonal and rhombohedral–tetragonal phase coexistence at room temperature for compositions with x = 0.02 and 0.04, respectively. By combining the SXPD results with microstructure, evidence is also found for the occurrence of chemical heterogeneity, which could provide an additional means to control the functional properties. The structural observations are correlated with changes in the dielectric and ferroelectric properties.

Reaction Heterogeneity in LiNi0.8Co0.15Al0.05O2 Induced by Surface Layer

Through operando synchrotron powder X-ray diffraction (XRD) analysis of layered transition metal oxide electrodes of composition LiNi0.8Co0.15Al0.05O2 (NCA), we decouple the intrinsic bulk reaction mechanism from surface-induced effects. For identically prepared and cycled electrodes stored in different environments, we demonstrate that the intrinsic bulk reaction for pristine NCA follows solid-solution mechanism, not a two-phase as suggested previously. By combining high resolution powder X-ray diffraction, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and surface sensitive X-ray photoelectron spectroscopy (XPS), we demonstrate that adventitious Li2CO3 forms on the electrode particle surface during exposure to air through reaction with atmospheric CO2. This surface impedes ionic and electronic transport to the underlying electrode, with progressive erosion of this layer during cycling giving rise to different reaction states in particles with an intact versus an eroded Li2CO3 surf...

Phase identification and structure investigation of novel quaternary rare-earth substituted titanates

Novel quaternary lanthanide-substituted oxides of stoichiometry LnxY2−xTi2O7 (where Ln is lanthanum, neodymium, samarium, gadolinium, or ytterbium) were prepared by traditional high-temperature, solid-state techniques and characterized by X-ray powder diffraction. Samples with nominal values of x up to 1.0 were attempted. The well-studied ternary cubic pyrochlore compound yttrium titanium oxide (Y2Ti2O7, space group Fd-3m, Z = 8), served as a parent structural framework in which Ln3+ cations were substituted on the Y3+ site. Laboratory-grade X-ray powder diffraction data revealed pure quaternary pyrochlore phases for LnxY2−xTi2O7 with x ≤ 0.2. Pyrochlore phase purity was verified by Rietveld analysis using high-resolution synchrotron X-ray powder diffraction data when x ≤ 0.2, however, for La3+ substitution specifically, pure quaternary pyrochlore formed at x<0.1. Band gap energies on selected samples were determined using optical diffuse reflectance spectroscopy and showed that these materials can be classified as electrical insulators with indirect band gap energies around 3.7eV.

Growth Oscillatory Zoning in Erythrite, Ideally Co3(AsO4)2·8H2O: Structural Variations in Vivianite-Group Minerals

The crystal structure of an oscillatory zoned erythrite sample from Aghbar mine, Bou Azzer, Morocco, was refined using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data, Rietveld refinement, space group C2/m, and Z = 2. The crystal contains two sets of oscillatory zones that appear to have developed during epitaxial growth. The unit-cell parameters obtained are a = 10.24799(3) Å, b = 13.42490(7) Å, c = 4.755885(8) Å, β = 105.1116(3)°, and V = 631.680(4) Å3. The empirical formula for erythrite, obtained with electron-probe micro-analysis (EPMA), is [Co2.78Zn0.11Ni0.07Fe0.04]∑3.00(AsO4)2·8H2O. Erythrite belongs to the vivianite-type structure that contains M1O2(H2O)4 octahedra and M22O6(H2O)4 octahedral dimers that are linked by TO4 (T5+ = As or P) tetrahedra to form complex layers parallel to the (010) plane. These layers are connected by hydrogen bonds. The average &lt;M1–O&gt;[6] = 2.122(1) Å and average &lt;M2–O&gt;[6] = 2.088(1) Å. With space group C2/m, there are two solid solutions: M3(AsO4)2·8H2O and M3(PO4)2·8H2O where M2+ = Mg, Fe, Co, Ni, or Zn. In these As- and P-series, using data from this study and from the literature, we find that their structural parameters evolve linearly with V and in a nearly parallel manner despite of the large difference in size between P5+ (0.170 Å) and As5+ (0.355 Å) cations. Average &lt;T–O&gt;[4], &lt;M1–O&gt;[6], and &lt;M2–O&gt;[6] distances increase linearly with V. The average &lt;As–O&gt; distance is affected by M atoms, whereas the average &lt;P–O&gt; distance is unaffected because it contains shorter and stronger P–O bonds. Although As- and P-series occur naturally, there is no structural reason why similar V-series vivianite-group minerals do not occur naturally or cannot be synthesized.

Catalyst Shelf Life: Its Effect on Nitrogen-Doped Carbon Nanotubes

In this work is presented a study of the shelf life of an iron-impregnated MgO (Fe/MgO) sample and an explanation of the effect of its aging on morphology and electroactivity of nitrogen-doped carbon nanostructures prepared by using this catalyst at different ages. Characterizations of Fe/MgO by thermogravimetric analysis (TGA), temperature-programmed desorption mass spectroscopy (TPD-MS), high-resolution X-ray powder diffraction (HR-XRPD), and X-ray photoelectron spectroscopy (XPS) have been performed at different aging times. The catalyst is formed by crystalline MgO, Mg(OH)2, and FeO phases. During aging, CO2 adsorption induces the formation of amorphous carbonate, resulting in a decrease of specific surface area, a redistribution of porosity, a reduction of the iron percentage at the surface, and an increase of iron oxide particle size. Different surface and bulk properties are used to explain different morphologies and electroactivities of the nitrogen-doped carbon nanotubes synthesized at different ...

Elaborating the Crystal Structures of MgAgSb Thermoelectric Compound: Polymorphs and Atomic Disorders

Gaining insight into crystal structure is essential for understanding thermoelectric transport mechanisms and predicting thermoelectric properties. The main challenge in studying thermoelectric mechanisms is often imprecise or wrong models of the crystal structure. This work examines the structure modifications observed in MgAgSb thermoelectric materials by multitemperature high-resolution synchrotron radiation powder X-ray diffraction (SR-PXRD). Rietveld refinement reveals large atomic displacement parameters (ADPs) of the Ag1 atoms at the 4a position indicating possible atomic disorder, which may contribute to the low thermal conductivity observed in α-MgAgSb. The temperature dependence of anisotropic structural parameters indicates a tendency of increasing structural symmetry in α-MgAgSb with increasing temperature, largely contributing to the temperature evolution of the thermoelectric properties. Two MgAgSb polymorphs (β-MgAgSb and γ-MgAgSb) coexist at 700 K, and only the γ-MgAgSb crystalline phase i...

Possibility to realize spin-orbit-induced correlated physics in iridium fluorides

Recent theoretical predictions of "unprecedented proximity" of the electronic ground state of iridium fluorides to the SU(2) symmetric $j_{\mathrm{eff}}=1/2$ limit, relevant for superconductivity in iridates, motivated us to investigate their crystal and electronic structure. To this aim, we performed high-resolution x-ray powder diffraction, Ir L$_3$-edge resonant inelastic x-ray scattering, and quantum chemical calculations on Rb$_2$[IrF$_6$] and other iridium fluorides. Our results are consistent with the Mott insulating scenario predicted by Birol and Haule [Phys. Rev. Lett. 114, 096403 (2015)], but we observe a sizable deviation of the $j_{\mathrm{eff}}=1/2$ state from the SU(2) symmetric limit. Interactions beyond the first coordination shell of iridium are negligible, hence the iridium fluorides do not show any magnetic ordering down to at least 20 K. A larger spin-orbit coupling in iridium fluorides compared to oxides is ascribed to a reduction of the degree of covalency, with consequences on the possibility to realize spin-orbit-induced strongly correlated physics in iridium fluorides.

Development of greener methodology for the synthesis of CdSe quantum dots and characterization of their thin films

Cadmium selenide (CdSe) quantum dots (QDs), being a well established highly luminescent semiconducting optical material, has diverse applications in the recent fields of optoelectronics, light emitting diodes and photo voltaic solar cell, attributed to its tunable band gap as a consequence of their tunable size and shape. Besides having tremendous utility, most of the synthesis routes of CdSe QDs reported, seems to be hazardous. Therefore, in the present study, we are reporting a considerably, greener method to synthesize CdSe QDs by using sodium borohydride, as the reducing agent. The highly stabilized QDs, over a period of one year, were characterized using high resolution transmission electron microscope and powder X-ray diffraction technique. Nanocrystallites with hexagonal and cubic polycrystallinity, exhibiting 3–12 nm size were successfully observed, without using any capping reagent. The UV–visible absorption is also revealing good agreement by showing a blue shift at the range of 450–470 nm. The QDs are further subjected to the fabrication of homogeneous thin films upon a glass template using spin coater. The thin films were further characterized using atomic force microscopy, which showed an average area (root mean square) roughness of 5.7898 nm at 2.53 µm area.

Correlating the Influence of Two Magnetic Ions at the A-Site with the Electronic, Magnetic, and Catalytic Properties in Gd1-x Dy x CrO3.

Considering the absence of reports dealing with the perovskite-structured orthochromites containing two A-site magnetic rare-earth ions, GdCrO3 and progressively Dy3+-substituted samples of the series Gd1–xDyxCrO3 have been synthesized employing the epoxide-mediated sol–gel procedure. The samples were characterized extensively using high-resolution powder X-ray diffraction, thermal analysis, Fourier transform infrared, Raman, and UV–visible spectroscopies, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) measurements. Monophasic samples possessing an orthorhombic perovskite structure emerged by calcining the xerogels formed by the reaction of rare-earth nitrates, chromium(III)chloride, and propylene oxide at 800 °C for 2 h. Uniform presence of wormlike morphology was observed in both the field emission SEM (FE-SEM) and TEM images of the samples. Zero-field and field-cooled magnetic measurements using a SQUID magnetometer down to 4 K showed that the Neel temperature of Gd0.5Dy0.5CrO3 was 155 K, more or less midway between the values observed for GdCrO3(169 K) and DyCrO3 (146 K). For the Gd0.5Dy0.5CrO3 sample, a spin reorientation was observed at ∼38 K when measured under an applied field. Because the optical band gap, determined by Kubelka–Munk function, of these chromites was around 3 eV, their application as a catalyst for the photodegradation of the aqueous rhodamine-6G dye solution was demonstrated, in which the percentage of the total dye that was degraded varied with the average ionic radius of A-site ions. A similar systematic trend was observed even for the catalytic oxidation of the XO dye in the presence of H2O2, with DyCrO3 influencing the reaction to a greater extent followed by Gd0.5Dy0.5CrO3 and GdCrO3. Both the photocatalytic and catalytic reactions followed pseudo-first-order kinetics.

Anionic silicate organic frameworks constructed from hexacoordinate silicon centres

Crystalline frameworks composed of hexacoordinate silicon species have thus far only been observed in a few high pressure silicate phases. By implementing reversible Si-O chemistry for the crystallization of covalent organic frameworks, we demonstrate the simple one-pot synthesis of silicate organic frameworks based on octahedral dianionic SiO6 building units. Clear evidence of the hexacoordinate environment around the silicon atoms is given by 29Si nuclear magnetic resonance analysis. Characterization by high-resolution powder X-ray diffraction, density functional theory calculation and analysis of the pair-distribution function showed that those anionic frameworks-M2[Si(C16H10O4)1.5], where M = Li, Na, K and C16H10O4 is 9,10-dimethylanthracene-2,3,6,7-tetraolate-crystallize as two-dimensional hexagonal layers stabilized in a fully eclipsed stacking arrangement with pronounced disorder in the stacking direction. Permanent microporosity with high surface area (up to 1,276 m2 g-1) was evidenced by gas-sorption measurements. The negatively charged backbone balanced with extra-framework cations and the permanent microporosity are characteristics that are shared with zeolites.

High-Temperature Crystal Structure and Chemical Bonding in Thermoelectric Germanium Selenide (GeSe).

The discovery of the ultra-high thermoelectric figure of merit of 2.6 in SnSe has drawn attention to other lead-free IV-VI orthorhombic semiconductors. GeSe has been predicted to possess thermoelectric performances comparable to SnSe. Here, a complete structural study is reported of GeSe with temperature by means of high-resolution synchrotron powder X-ray diffraction. In the orthorhombic phase, the evolution of the bond distances with temperature is shown to deviate significantly with respect to SnSe. Analysis of the chemical bonding within the Quantum Theory of Atoms in Molecules shows that GeSe is ionic with van der Waals interlayer interactions. The signature of the N shell lone pair of Ge is also evident from both the electron density Laplacian and the ELF topologies.

Highly ordered polyaniline as an efficient dye remover

Polyaniline was synthesized by the chemical oxidative polymerization procedure at room temperature employing hydrogen peroxide (H2O2) as oxidant and ferrous chloride (FeCl2·2H2O) and vanadyl sulphate (VOSO4·H2O) as co-catalysts, respectively. The obtained polymers were characterized by high resolution powder X-ray diffraction, Fourier transform infrared spectroscopy, Raman, UV–Visible, photoluminescence spectroscopy, thermogravimetric Field Emission Scanning Electron Microscopy (FESEM) and Transmission Electron Microscopy (TEM) techniques. Ordered arrangement indicative of semi-crystalline nature of polyaniline was evidenced from the presence of intense reflection at d = 13.72 Å in the powder X-ray diffraction pattern followed by two lesser intense peaks at 4.61 and 3.47 Å. Fourier transform infrared spectroscopy and Raman spectroscopic results indicated the polyaniline to be emeraldine salt form. Fibrous morphology was observed in scanning electron microscope images. Nearly 93% of Methyl Orange dye was adsorbed in 30 min by the ordered polyaniline at room temperature. No significant difference in the crystallinity and/or ordering was noticed in the powder X-ray diffraction pattern after dye adsorption. The correlation between the ordered structure of polyaniline and its higher adsorption property derived in the current study has the potential to fabricate devices consisting polyaniline to detect dye molecules.

Structural characterization of caffeine–oxalic acid co-crystals from the powder diffraction pattern at the SPring-8 BL02B2 beamline

In this work, we developed an X-ray powder diffractometer system equipped with six solid-state detectors and used it to perform ab initio structure determination from the powder diffraction pattern data obtained for a caffeine–oxalic acid co-crystal. The crystal structure obtained from the powder diffraction data was consistent with the previously solved single-crystal structure (Trask reference), although slightly larger (by about 2%). The co-crystallization of pharmaceutically active molecules can modulate their physical properties such as solubility, stability, and bioavailability. For the investigation of pharmaceutical complexes, the ability to visualize molecular interactions such as hydrogen bonding would be very helpful toward understanding their physical properties. Given the rate at which the high-throughput screening of pharmaceutical complexes has grown, an analogous high-volume, high-resolution X-ray powder diffraction technique with high-throughput data collection ability would be useful. We also solved the crystal structures of an inorganic complex and metal organic framework, zinc acetate dihydrate and CPL-1, in order to demonstrate the performance of our new diffractometer system.

Soft, Wet-Chemical Synthesis of Metastable Superparamagnetic Hexagonal Close-Packed Nickel Nanoparticles in Different Ionic Liquids.

The microwave-induced decomposition of bis{N,N'-diisopropylacetamidinate}nickel(II) [Ni{MeC(NiPr)2 }2 ] or bis(1,5-cyclooctadiene)nickel(0) [Ni(COD)2 ] in imidazolium-, pyridinium-, or thiophenium-based ionic liquids (ILs) with different anions (tetrafluoroborate, [BF4 ]- , hexafluorophosphate, [PF6 ]- , and bis(trifluoromethylsulfonyl)imide, [NTf2 ]- ) yields small, uniform nickel nanoparticles (Ni NPs), which are stable in the absence of capping ligands (surfactants) for more than eight weeks. The soft, wet-chemical synthesis yields the metastable Ni hexagonal close-packed (hcp) and not the stable Ni face-centered cubic (fcc) phase. The size of the nickel nanoparticles increases with the molecular volume of the used anions from about 5 nm for [BF4 ]- to ≈10 nm for [NTf2 ]- (with 1-alkyl-3-methyl-imidazolium cations). The n-butyl-pyridinium, [BPy]+ , cation ILs reproducibly yield very small nickel nanoparticles of 2(±1) nm average diameter. The Ni NPs were characterized by high-resolution transmission electron microscopy (HR-TEM) and powder X-ray diffraction. An X-ray photoelectron spectroscopic (XPS) analysis shows an increase of the binding energy (EB ) of the electron from the Ni 2p3/2 orbital of the very small 2(±1) nm diameter Ni particles by about 0.3 eV to EB =853.2 eV compared with bulk Ni0 , which is traced to the small cluster size. The Ni nanoparticles show superparamagnetic behavior from 150 K up to room temperature. The saturation magnetization of a Ni (2±1 nm) sample from [BPy][NTf2 ] is 2.08 A m2 kg-1 and of a Ni (10±4 nm) sample from [LMIm][NTf2 ] it is 0.99 A m2 kg-1 , ([LMIm]=1-lauryl-3-methyl- imidazolium). The Ni NPs were active catalysts in IL dispersions for 1-hexene or benzene hydrogenation. Over 90 % conversion was reached under 5 bar H2 in 1 h at 100 °C for 1-hexene and a turnover frequency (TOF) up to 1330 molhexane (molNi )-1 h-1 or in 60 h at 100 °C for benzene hydrogenation and TOF=23 molcyclohexane (molNi )-1 h-1 .

Supercritical Flow Synthesis of Pt1–xRux Nanoparticles: Comparative Phase Diagram Study of Nanostructure versus Bulk

The thermodynamic stability of nanocrystals is different from that of bulk systems, and nanoscale phase diagrams are to a large degree unknown. Here we present a systematic investigation of the Pt1–xRux phase diagram through supercritical flow synthesis of Pt1–xRux nanoparticles across the entire compositional range. The synthesis was done in stoichiometric steps of 0.1 using an ethanol–toluene mixture as solvent at 450 °C and 200 bar. The products were characterized by high-resolution synchrotron powder X-ray diffraction, transmission electron microscopy, and elemental mapping of individual particles using energy-dispersive X-ray spectroscopy. The diffraction data revealed a single-phase face-centered cubic (fcc) alloy for x ≤ 0.2, while an additional hexagonal close-packed (hcp) phase emerges as x approaches 1. This behavior deviates significantly from the bulk phase diagram, where a biphasic region is only observed for 0.62 < x < 0.8. Thus, compositional design of Pt–Ru alloys is more flexible on the n...

Triclinic crystal structure distortion of multiferroic BiMn7O12.

The quadruple perovskite BiMn7O12 obtained via high-pressure synthesis was investigated by high-resolution synchrotron X-ray powder diffraction over a temperature range of 10 to 295 K. Careful Rietveld analysis reveals triclinic lattice distortion of BiMn7O12 at 295 K, which increases upon cooling to 10 K. Also hkl-dependent anisotropic Bragg reflection shape was introduced to give a precise description of the diffracted intensities. Importantly BiMn7O12 crystal structure was described in the non-centrosymmetric I1 triclinic space group. We also demonstrate the use of irreducible representations analysis (ISODISTORT program) for crystal structure distortion from Im to I1 space group. The irreducible representation which describes crystal structure distortion points towards possible ferroelectricity. Finally anisotropic thermal lattice expansion was observed.

Thermal expansion of the heavy-fermion superconductor PuCoGa5

We have performed high-resolution powder x-ray diffraction measurements on a sample of $^{242}$PuCoGa$_{5}$, the heavy-fermion superconductor with the highest critical temperature $T_{c}$ = 18.7 K. The results show that the tetragonal symmetry of its crystallographic lattice is preserved down to 2 K. Marginal evidence is obtained for an anomalous behaviour below $T_{c}$ of the $a$ and $c$ lattice parameters. The observed thermal expansion is isotropic down to 150 K, and becomes anisotropic for lower temperatures. This gives a $c/a$ ratio that decreases with increasing temperature to become almost constant above $\sim$150 K. The volume thermal expansion coefficient $\alpha_{V}$ has a jump at $T_{c}$, a factor $\sim$20 larger than the change predicted by the Ehrenfest relation for a second order phase transition. The volume expansion deviates from the curve expected for the conventional anharmonic behaviour described by a simple Gr\"{u}neisen-Einstein model. The observed differences are about ten times larger than the statistical error bars but are too small to be taken as an indication for the proximity of the system to a valence instability that is avoided by the superconducting state.