Structural analysis of diamond mosaic crystals for neutron monochromators using synchrotron radiation

The beams extracted from thermal neutron sources such as nuclear reactors are monochromatised by Bragg diffraction using imperfect single crystals with an angular mosaic spread of typically 0.2–0.8°. For neutron wavelengths below 1.5 Å, the highest reflectivity of all crystalline materials is expected for diamond. Nowadays diamond single crystals with an appropriate mosaic spread exceeding a thickness of 1 mm can be grown by heteroepitaxy on an Ir/yttria-stabilised zirconia bilayer deposited on a Si(001) single crystal.To explain the observed neutron reflectivity being below the theoretically expected value, we have studied the spatial distribution of the mosaic structure of two crystals by high resolution X-ray diffraction using a laboratory X-ray source and synchrotron radiation. The first sample (A) showed a uniform mosaic spread of 0.18° ± 0.02° across the 1 cm wide sample. The peak shift of the X-ray rocking curves of 0.08° indicated a weak curvature of the crystal lattice. The measured absolute neutron peak reflectivity of 34% corresponded to 90% of the value predicted by theory. The peak width of the neutron rocking curve for the second sample (B) was twice as big, but here the peak reflectivity of 13% corresponded to only half of the theoretical value.This unfavourable behaviour could be assigned to a substantial spatial variation of the mosaic spread deduced from the synchrotron X-ray studies. X-ray diffraction with high spatial resolution indicated a mosaic block size below 50 μm for sample A. This was consistent with chemical etching experiments on the surface of a comparable sample which showed both randomly distributed dislocations and others that are arranged in boundaries of several 10 μm large domains.     

Synthesis of large single crystal diamond plates by high rate homoepitaxial growth using microwave plasma CVD and lift-off process

A process of making a large, thick single crystal CVD diamond plates has been developed. This process consists of high rate homoepitaxial growth of CVD diamond and subsequent lift-off process using ion implantation. By using this process, single crystal CVD diamond plates with the size of about 10 × 10 × 0.2–0.45 mm³ have been successfully fabricated. The crystallinity of the CVD diamond plates has been evaluated by X-ray topography, polarized light microscopy and high resolution X-ray diffraction. The results indicate the pretreatment of the seed substrate has strong effect on the crystallinity of the CVD diamond plates.     

Synthesis and structural properties of GaN particles from GaO2H powders

Gallium nitride(GaN) powders have been synthesized by nitriding gallium oxyhydroxide (GaO₂H) powders in the flow of NH₃ gas at a nitridation temperature of 950 °C for 35 min. X-ray powder diffraction (XRD) patterns and Fourier transform infrared (FTIR) spectra reveal that simple heat treatment of GaO₂H in the flow of NH₃ leads to the formation of hexagonl GaN with lattice constants a = 3.191 Å, and c = 5.192 Å at 950 °C through intermediate conversion of β-Ga₂O₃. X-ray photo-electron spectroscopy (XPS) confirms the formation of bonding between Ga and N, and yields that the surface stoichiometry of Ga : N approximates 1 : 1. Transmission electron microscopy (TEM) image indicates that GaN particle is a single crystal, and its morphology is ruleless.     

The equilibrium phase boundary between hexagonal and cubic boron nitride

The equilibrium phase boundary between hexagonal and cubic boron nitride was determined over the pressure range 3–6 GPa and the temperature range 1200–2200°C. Absolute pressure and temperature were estimated based on the minimum P–T point of diamond formation in the conventional metal catalyst system using the Kennedy–Kennedy equilibrium line between graphite and diamond. Above 3.8 GPa, we determined the phase boundary by detecting the formation of cubic BN in the catalyst-containing system and by the reverse transformation from cubic BN to hexagonal BN. Below 3.8 GPa, we determined the boundary by careful observation of the transformation behavior of cubic BN powder. The rate of phase transformation from cubic BN to hexagonal BN was evaluated from the ratio of X-ray diffraction peak intensities of both phases. We found that the amount of cubic BN phase clearly changed at the phase boundary. Two sets of the results could be plotted with a uniquely determined boundary line expressed by the equation P(GPa)=T(°C)/465+0.79, which is located about 300°C higher at 5 GPa than the line determined by Bundy and Wentorf in 1963.     

Pulsed electric field induced diamond synthesis from carbon nanotubes with solvent catalysts

Spark plasma sintering (SPS) was used to synthesize diamond from multiwalled carbon nanotubes (MWCNTs) with Fe35Ni powders as solvent catalysts. The MWCNTs/Fe35Ni mixtures were spark plasma sintered at various conditions. The microstructures and diamond phase were analyzed by using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscope techniques. Experimental results showed that the diamond crystals can be synthesized from MWCNTs/Fe35Ni by using the SPS at lower temperature of 1200 °C under very low pressure of 70 MPa. Well-crystallized cubic diamonds consisted of mono-crystals and poly-crystals exhibiting particle sizes ranged from 10 to 40 μm. The Fe35Ni catalysts achieved an effective enhancement for diamond conversion from MWCNTs during the SPS. A model was also proposed to describe the diamond growth and revealed as a layer-by-layer growth mechanism.     

Experimental determination of bulk modulus of 14 Å tobermorite using high pressure synchrotron X-ray diffraction

Using a diamond anvil cell, 14 Å tobermorite, a structural analogue of calcium silicate hydrates (C–S–H), was examined by high-pressure synchrotron X-ray diffraction up to 4.8 GPa under hydrostatic conditions. The bulk modulus of 14 Å tobermorite was calculated, K_o = 47 GPa. Comparison of the current results with previous high pressure studies on C–S–H(I) indicates that: (1) the compression behavior of the lattice parameters a and b of 14 Å tobermorite and C–S–H(I) are very similar, implying that both materials may have very similar Ca–O layers, and also implying that an introduction of structural defects into the Ca–O layers may not substantially change in-plane incompressibility of the ab plane of 14 Å tobermorite; and (2) the bulk modulus values of 14 Å tobermorite and C–S–H(I) are dominated by the incompressibility of the lattice parameter c, which is directly related to the interlayer spacing composed of dreierketten silicate chains, interlayer Ca, and water molecules.     

Diamond crystal optics for self-seeding of hard X-rays in X-ray free-electron lasers

We report design, fabrication, and results of X-ray topography characterization of thin synthetic type IIa diamond crystal plates and crystal-holder assemblies developed for the Hard X-ray Self-Seeding project at the Linac Coherent Light Source. The goal of the project was to achieve generation of fully coherent hard X-rays using the self-seeding concept and the single-crystal diamond wake monochromator [Geloni et al., J. Mod. Opt. 58, 1391 (2011)]. High crystal quality, crystal thickness of ≈ 0.1–0.2 mm and strain-free crystal mount were the main requirements. Nearly defect-free diamond plates of (001) orientation, with thicknesses of 0.1 mm and 0.15 mm, and of a trapezoidal shape were fabricated and preliminarily evaluated. The plates were further characterized using X-ray topography. These tests helped to minimize strain in crystals induced by mounting in crystal holders and to determine defect-free crystal regions. Self-seeding experiments were conducted at the Linac Coherent Light Source using the diamond plates and crystal-holder assemblies selected by our studies. Fully coherent 8.33-keV X-rays with 5 × 10^− 5 relative bandwidth were produced [Amann et al., Nat. Photonics 6, 693 (2012)].     

Novel carbon diamond-like phases LA5, LA7 and LA8

In this work, the structure and properties of seven diamond-like carbon phases obtained by linking the graphene layers were calculated using DFT and PM3 methods. The LA5 (Cmca), LA7 (Cmcm), and LA8 (I4₁/amd) diamond-like phases were predicted and studied in this work for the first time. Values of the unit cell parameters of the predicted phases are: a = 4.337 Å, b = 5.024 Å, and c = 4.349 Å for LA5 phase; a = 4.942 Å, b = 4.808 Å, and c = 4.390 Å for LA7 phase; and a = 4.906 Å and c = 4.960 Å for LA8 phase. For these LA5, LA7, and LA8 phases, various structure characteristics, densities, cohesive energies, bulk moduli, electronic densities of states and X-ray patterns were calculated. The comparative analysis showed that the diamond-like phase properties depend on the extent of their structure deformation relative to the cubic diamond structure.     

Epitaxial lateral overgrowth (ELO) of homoepitaxial diamond through an iridium mesh

In the present work iridium layers forming a mesh on diamond have been studied as potential candidates for buried electrodes or stopping layers in an ELO process for heteroepitaxial diamond. Thin iridium layers (∼ 15 nm) were deposited by e-beam evaporation at ∼ 700 °C on the facets of individual (001)-oriented CVD diamond crystallites and macroscopic Ib HPHT substrates with off-axis angles of several degrees. The heteroepitaxial iridium films formed a mesh with 10–200 nm large holes. These were penetrated by homoepitaxial diamond in a microwave plasma chemical vapour deposition process (MWPCVD) burying the iridium layer completely after 15 min of diamond growth. High resolution X-ray diffraction including reciprocal space mapping and Raman spectroscopy was used to characterize the structural properties of the diamond overlayer on the Ib HPHT substrate. It was monocrystalline with an FWHM of 0.03–0.05° of the X-ray rocking curve. Its lattice planes were tilted by ∼ 0.01° with respect to the substrate and showed a macroscopic strain of − 10^− 4 perpendicular to the surface. Besides the smaller lattice constant due to the lack of nitrogen the strain is mostly attributed to a tensile in-plane stress state. Strain and tilt can be attributed to the lateral overgrowth and the off-axis angle of the substrate.     

Incorporating platinum precursors into a NaA-zeolite synthesis mixture promoting the formation of nanosized zeolite

Sodium A-zeolite with different platinum contents were prepared by directly incorporating platinum precursor (Pt(NH₃)₄Cl₂) into the zeolite during synthesis. Pt/KA-zeolite was then obtained by ion-exchange with KCl solution. The effect of platinum concentration on the crystal morphology and platinum dispersion was investigated by hydrogen chemisorption, scanning electron microscopy (SEM), X-ray diffraction (XRD), and time-of-flight secondary ion mass spectrometer (TOF-SIMS). SEM results revealed a bimodal crystal size distribution for NaA-zeolite with low platinum concentration (<1 wt%). Chamfered edges were found for all the larger cubic crystals (～4 μm). Two types of cubic crystals, smooth surface cubes with chamfered edges and rough surface cubes, were observed for the smaller crystals (～400 nm). The proportion of the rough surface nanosized cubes increased as the platinum content increased. At about 4 wt% platinum, only nanosized rough surface cubic crystals were obtained, which were transformed into nanocrystals of KF-zeolite after ion-exchange with KCl, as indicated by X-ray diffraction results. TOF-SIMS data taken before and after sputtering the surface layers revealed that platinum was distributed homogeneously inside of the zeolite, which was supported by hydrogen chemisorption results, indicating that platinum particles were confined in the zeolitic cages for both the microsized and nanosized cubic crystals. A mechanism was proposed to elucidate the role of platinum precursor on the nucleation and growth of nanosized zeolite, which is consistent with all of the characterization results.     

Low temperature synthesis of polycrystalline explosion phase boron nitride submicron-powders

High purity quasi-octahedral polycrystalline explosion phase boron nitride (e-BN) submicron-powders were synthesized in an autoclave at 600 °C for 6 h using Mg, FeB_1.3 and NH₄Cl as the raw materials. Samples were characterized by SEM, FSEM, EDX, TEM, HRTEM, SAED, X-ray powder diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. The as-synthesized e-BN powders demonstrated a face-centered cubic structure with lattice parameter of 8.300 Å. Besides the e-BN samples presented a quasi-octahedral morphology with an average diameter of 0.8 μm. The effects of time, temperature, raw materials and possible catalytic reaction mechanism of the growth of quasi-octahedral polycrystalline e-BN were also discussed.     

Characterization of crystallinity of a large self-standing homoepitaxial diamond film

We have investigated the crystallinity of a self-standing homoepitaxial diamond film grown at a high rate using a polarized optical microscope, X-ray rocking curves and X-ray topographs. The self-standing film was made by removing a synthetic type-Ib(001) substrate after the film growth using a “lift-off process”. The cross-nicol optical microscope image demonstrated that there exists strain spreading at a relatively large area in the film and the strain inherits over that in the substrate. From cross-sectional X-ray topographs, the dominant defects, observed as dotted patterns in plan-view topographs and micrographs, are dislocations extending along the film growth direction 〈001〉 and generating at the interface between the film and the substrate. The density of such defects counted from these measurements was ~ 1 × 10³ cm^? 2. The dotted patterns observed in X-ray topographs of (220), (? 220) and (400) diffraction planes are almost the same with each other, suggesting that dominant line defects along 〈001〉 are mixed dislocations with a burgers vector directing between 〈001〉 and in-plane of the sample. The high resolution X-ray rocking curve showed that the peak is very sharp with full width at half maximum of 7.56″, which indicates that imperfection of our single-crystal diamond plate is almost the same as that of high-quality high-pressure high-temperature, synthetic type-Ib diamond.     

Synchrotron X-ray diffraction study on a single nanowire of PX-phase lead titanate

Synchrotron X-ray diffraction (XRD) measurements of Pb–Ti–O fine acicular crystals (nanowires) of the so-called PX-phase have been performed. The θ–2θ patterns of the nanowires dispersed on substrates with two different geometries indicated that the PX-phase Pb–Ti–O nanowire has [1 1 0] along its short axis and [0 0 1] along its longitudinal axis. Using a focused micro-X-ray beam, we first demonstrated the XRD analysis of a single nanowire of PX-phase Pb–Ti–O. Bragg reflections from the same single nanowire with scattering vectors at two different angles suggested its tetragonal unit cell having the rather large a-axis lattice constant.     

Soft chemistry routes for the preparation of Ag-CoFe2O4 nanocomposites

Silver-cobalt ferrite nanocomposites (Ag-CoFe₂O₄) were synthesized through wet ferritization process and self-propagating combustion method. The structure, morphology, surface chemistry and magnetic properties of the nanocomposites were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and vibrating sample magnetometer (VSM). X-ray diffraction patterns confirmed the formation of CoFe₂O₄ and Ag nanoparticles with cubic symmetry. The average crystallite size of CoFe₂O₄ by wet ferritization ranged between 60 Å and 87 Å; for those obtained by self-propagating combustion was in the range 232–290 Å. SEM micrographs revealed different morphological features of nanocomposites. Ag-CoFe₂O₄ obtained by wet ferritization exhibited typical superparamagnetic behaviour. The antimicrobial and anti-biofilm properties of all silver-cobalt ferrites were evaluated. The results revealed that the Ag-CoFe₂O₄ nanocomposites exhibited good microbicidal and anti-biofilm features.     

New data on the thermal behavior of 14 Å tobermorite

The thermal behavior of two specimens of 14 Å tobermorite (plombièrite) was studied in situ at the GILDA beamline (ESRF, Grenoble, France). During dehydration, plombièrite shortens its basal spacing from 14 to 11 Å, through a progressive approaching of the complex structural modules characterizing its crystal structure. Upon heating, the 11 Å phase progressively contracts its c periodicity, with its d₀₀₂ varying from 11.7 to 11.3 Å. At ca. 300 °C, a 9.6 Å phase appears; it is stable up to ca. 700 °C. Above this temperature, it expands its basal spacing up to 10.2 Å, before transforming into wollastonite.Moreover, one specimen was heated at 150 °C for 4 h and the heated product, identified as 11 Å tobermorite through X-ray powder diffraction, was used to collect micro-Raman spectra. The heated product shows single chains, in contrast with the 11 Å natural specimens studied up to now in which double wollastonite-like chains occur.     

Synthesis,structure and characterization of a new nonlinear optical calcium borate [Ca2B5O9]·[H(OH)2]

A new nonlinear optical crystal [Ca₂B₅O₉]·[H(OH)₂] (1) has been synthesized under hydrothermal condition. Compound 1 crystallizes in the monoclinic space group C2 with lattice constants a = 10.111(2) Å, b = 7.754(11) Å, c = 6.198(14) Å, β = 127.04(3)° and Z = 2. The fundamental building block (FBB) of 1 is a [B₅O₁₂] unit with two BO₃ triangles and three BO₄ tetrahedra. 1 was characterized by single-crystal X-ray diffraction, IR and UV–Vis spectroscopy, thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD) respectively. Also, powder second harmonic generation (SHG) measurements indicate that 1 is phase matchable and displays a SHG response of about 3.5 times that of KDP (KH₂PO₄).     

Synthesis,structure and fluorescence properties of a uranyl-2,5-pyridinedicarboxylic acid coordination polymer: The missing member of the UO22+-2,n-pyridinedicarboxylic series

A 3D uranium-pyridinedicarboxylate, compound (1) [UO₂(C₇H₃NO₄)], represents the final member of a series of materials incorporating a series of 2,n-pyridinedicarboxylic (pydc) acids (n = 3,4,5,6). The resulting crystal structure [FW = 419.13, monoclinic, P2₁/n, a = 9.0096(3) Å, b = 8.1371(2) Å, c = 11.6954(4) Å, V = 852.45 ų, Z = 4] was determined by single crystal X-ray diffraction and fluorescent properties were investigated. Additionally, the complete series of uranium-2,n-pydc materials was examined on a basis of fluorescent behaviors and architectural similarities.     

Structure and properties of La(Zn1/2Ti1/2)O3

(La,Nd)(Zn,Mg)_0.5Ti_0.5O₃ compounds are of great interest for microwave dielectric applications, although the structure of some of these complex perovskites is still uncertain. The perovskite tolerance factors in this family of compounds range from 0.916 to 0.952, suggesting a very high degree of tilting in the oxygen octahedra for compositions throughout the system. The effects of this tilting in La(Zn_0.5Ti_0.5)O₃, for example, have been observed in this study by transmission electron microscopy in the form of 1/2{3 1 1}_c and 1/2{1 3 1} α superlattice reflections and 1/2(2 3 1)_c γ reflections corresponding to anti-phase and in-phase tilting, respectively. Systematic La⁺³ displacement was also detected by the appearance of {h + 1/2, k, l}_c (β) reflections. Reflections of the type 1/2{1 1 1}_c in neutron diffraction patterns indicate 1:1 cation ordering on the B-site. Electrical tests show no piezoelectricity in La(Zn_0.5Ti_0.5)O₃, indicating a centrosymmetric point group. Rietveld refinements of neutron diffraction patterns indicate that the space group is P2₁/n, with lattice constants a = 7.8943 Å, b = 5.5959 Å, c = 5.5805 Å, and β = 90.0291°.     

Preparation of Ni1−xMnxFe2O4 ferrites by sol–gel method and study of their cation distribution

The spinel ferrite nanoparticles of the system Ni_1?xMn_xFe₂O₄ with x=0.0, 0.1, 0.3, 0.5, 0.7 and 0.9 were prepared by sol–gel auto combustion technique using chlorides of Ni, Mn and Fe as a source with citric acid as chelating agent. The structure of the ferrite materials and the particle size were determined by XRD, it was observed that the structure was a single phase, face centered cubic with lattice parameter ranging from 8.365 Å to 8.394 Å and the particle size ranging from 23.86 Å to 38.30 Å. The lattice parameter showed a linear dependence on concentration in accordance with the Vegard's law. By analyzing XRD patterns, the cation distribution over A and B-sites was estimated through the R-Factor method. The magnetic moment for each sample was determined from cation distribution on the two sites. An enhancement in the net magnetic moment was observed with gradual increase in the Mn content.     

Anisotropic X-ray peak broadening and twin formation in hematite derived from natural and synthetic goethite

Hematite obtained by dehydration of goethite at temperatures between 250 °C and 1000 °C was studied by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). Starting materials were goethite mineral and synthetic goethite, respectively. Hematite derived from natural goethite shows narrow X-ray diffraction peak widths with little variation between the reflections and marginal dependence on dehydration temperature. TEM imaging reveals the existence of large (>200 nm) hematite twin domains, which is attributed to slow dehydration kinetics associated with the formation of few hematite nuclei.In contrast, strong anisotropic X-ray diffraction peak broadening is observed for hematite obtained from synthetic needle-shaped goethite at low dehydration temperatures (T < 500 °C); the peak widths significantly decrease with increasing dehydration temperature. Anisotropic peak broadening is observed only for reflections, which structure factors are dominated by the iron sub-lattice and which are not common to both twin variants of hematite. In hematite producing the strong anisotropic X-ray peak broadening extremely small twin domains with sizes ranging from 5 nm to 10 nm could be imaged by high-resolution TEM. Further, DSC and TEM observations indicate that dehydration kinetics in coarse-grained natural goethite and fine-grained synthetic goethite differ considerably. It is concluded that during the dehydration reaction taking place at the large surface area of synthetic goethite crystals hematite nuclei with ambient orientation are rapidly formed, thereby creating a finely twinned dehydration product. The experimental results prove that XRD peak broadening is mainly caused by fine twinning.