A double axis X-ray diffractometer for magnetostriction measurements

Effect of high-temperature neutron irradiation on the crystallites of graphite materials

Convergent beam electron diffraction (CBED) patterns have been used in several different applications to materials problems (summarised by Steeds) and has more recently been used to quantify shifts in the lattice parameters of cubic materials (e.g. 2). This technique has proved highly sensitive to such changes, capable, via the use of very simple geometrical computer simulations, of the detection and quantification of changes in lattice parameters of hexagonal alloys.Alloy systems based on zinc and titanium were used. A simple geometrical computer simulation was used to quantify the shifts, ignoring dispersion surface effects, in the higher order Laue zone (HOLZ) lines upon change in the lattice parameters. X-ray diffraction data from each of the alloys was obtained to provide a standard for comparison. The sensitivity of each zone axis pattern to changes in either a or c was determined from the magnitude of the HOLZ line shifts in the CBED pattern.

Experimental techniques to measure x-ray lattice parameter changes to 1 ppm in quenched aluminum single crystal foils are described. Included in the description is a cryostat designed to allow the mounting of a sample cooled to liquid nitrogen temperature after quenching. X-ray diffraction measurements are performed on the sample at liquid nitrogen temperature. In between successive measurements, the sample is annealed at temperatures up to 260°C. The measurement of simultaneous lattice parameter and Bragg line shape changes is shown to be possible for quenched samples.

This study has characterised the oxidation products of a fine-grained single domain magnetite which was made synthetically by a colloidal method. Changes in the intrinsic magnetic properties (saturation magnetisation, saturation remanence, and coercive force) during progressive oxidation are correlated with lattice parameter changes and an oxidation mechanism. It is proposed that magnetite oxidises to hematite via at least two metastable maghemites. The first of these, formed on low temperature oxidation by the formation of a magnetite/maghemite solid solution, is a face centered maghemite with lattice parameter a= 8.3419±0.0006 A. A second maghemite, produced on oxidation at higher temperatures, has a primitive cubic structure and a lattice parameter a = 8.3505±0.0005 A. Maghemite cation distributions are derived to explain the reduced saturation magnetisations of between 56 and 74 Am2 kg-1 observed, and a maghemite structure containing an increase in tetrahedral Fe3+ ions and up to 3 octahedral vacancies per 32 oxygen unit cell is proposed.

Mechanism of ethanol electrooxidation on numerous materials has been widely investigated in the past. It is well known that in acidic conditions the electrooxidation of ethanol on platinum surface leads to strongly adsorbed surface groups (i.e. CO), poisoning the surface of catalyst and inhibiting further reactions. Presence of metals other than platinum will make it possible to oxidize adsorbed CO by surface OH groups, present on the other metal (so called “bi-functional mechanism”). Such bi-metallic surface is more resistant to poisoning and as a result more catalytically active. Hovewer it is still not well understood, how the presence of the other metal influences the surface’s electronic properties, which in turn alter adsorption properties, and reaction mechanism. Electronic properties of the catalyst can be modified by the following methods: they can be related directly to charge transfer from other component of the system, such as another component of the alloy, from substrate, or they can be induced by geometric factors, such as change in lattice parameter. Change in lattice parameter influences the degree of overlapping of valence orbitals forming the conduction band. When lattice is contracted, the conduction band becomes broader. When the conduction band is filled in more than half the broadening of the conduction band leads to shift of the d-band center towards Fermi edge. The lattice parameter can be (to some degree) modified, and as a result, the degree of orbital overlapping can be changed, for instance by alloying. It is known that in alloys the atom-atom distance changes linearly as a function of composition, in the range determined by inter-atomic distances of the pure elements (Vegard’s law). Similarly, when thin layers are formed, the layer forming metal will exhibit the lattice parameter of the substrate (so called “pseudomorphic monolayer”). Consequently experimental investigation of materials, where lattice parameter has been intentionally modified, allows for experimental determination of the relation between reaction mechanism and electronic properties of the material used.

Structural, tribological and resistivity studies of Ga substituted (Al71−xGax)Pd21Mn8 icosahedral and other intermetallic phases

Synchrotron x-radiation in a double-crystal geometry has been used for topography and rocking-curve measurements to study imperfections in single-crystal HP/HT synthetic diamonds grown from (111) seeds by the reconstitution technique. The influence that clusters of small opaque particles (sometimes called `pin-point-like inclusions') have upon lattice parameter differences, as well as upon relative lattice tilts within each sample, was of particular interest in this study. Rocking curves measured with a large beam diameter (illuminating entire specimens) gave widths (FWHM) in the range 5 - 30 seconds of arc in a non-dispersive Bragg - Bragg diffraction geometry in which good diamond crystals give and for which the calculated theoretical minimum width is under . Rocking curves were also recorded for small regions of each crystal, using a translatable pin-hole, and these showed that some diamonds had relatively narrow rocking curves in selected areas (of diameter). Epochs of crystal growth were recorded in series of x-ray topographs, taken at intervals of 2.5 seconds of arc. Such series were taken for all four principal azimuths (, 90, 180 and ) of the incident beam by rotating the sample about the normal to the Bragg plane. This procedure yielded important information about lattice parameter distribution and lattice tilt across entire samples. For the samples heavily populated with pin-point-like inclusions it was concluded that there is a possible slight variation in lattice parameter values in some regions of the crystal but most of it falls below the adopted angular step resolution, although better accuracy can be achieved. Relative tilt measurements revealed that there are large misorientations of those parts of the crystal which are close to external outcrops of {111} boundaries. In this case of (111) oriented diamond samples, three separate regions with different tilts can be attributed to the three parts of the crystal close to optically visible {111} growth-sector boundaries.

X-ray diffraction studies of irradiated beryllium oxide

Electron-doped $\mathrm{SrTi}{\mathrm{O}}_{3}$ (where the dopant can be Nb or La) has been widely investigated for both its fundamental interest in condensed matter physics and for industrial applications. Its electronic properties are closely related to the Ti-O bonding states in the $\mathrm{SrTi}{\mathrm{O}}_{3}$ crystal. To further develop and control these properties, it is crucial to understand the factors controlling the change in lattice parameters and local lattice distortion upon doping with various atoms. Herein, we report the changes in lattice parameters and local lattice distortion in Nb- and La-doped $\mathrm{SrTi}{\mathrm{O}}_{3}$ single crystals, investigated by in-plane x-ray diffraction and first-principles calculations. The lattice parameter of Nb- and La-doped $\mathrm{SrTi}{\mathrm{O}}_{3}$ single crystals increased with dopant concentration. The broad intensities around the Bragg peak observed in the in-plane x-ray-diffraction experiments indicated that the local lattice expansion and contraction, or local lattice distortions, in the crystal were caused by the dopant atoms. First-principles calculations similarly showed that the lattice expansion and local lattice distortions in the $\mathrm{SrTi}{\mathrm{O}}_{3}$ crystals were caused by doped Nb and La atoms. Atoms surrounding Schottky pairs of O and Sr vacancies were displaced both away from and towards the vacancies, resulting in a reduction in the lattice expansion of donor-doped $\mathrm{SrTi}{\mathrm{O}}_{3}$. Donor atoms and Schottky pairs thus play an important role in determining the lattice parameters of $\mathrm{SrTi}{\mathrm{O}}_{3}$ crystals. These fundamental structural analyses provide a useful basis to further investigate the electronic conductivity of electron-doped $\mathrm{SrTi}{\mathrm{O}}_{3}$.

Effects of non-uniformities on the hardening of crystals

Measurements have been made of the changes in X-ray lattice parameter which result when a PdHx alloy is taken on a path in the PdHx phase diagram which traverses the mixed ( alpha + beta )-phase region to reach beta -phase concentrations (0.70<x<0.92). The changes were followed by comparing the lattice parameter values for alloys which had been taken along such a path with lattice parameter values of alloys taken along paths avoiding the mixed-phase region in reaching beta -phase concentrations of comparable value. From previous experience with the mixed ( alpha + beta )-phase region and the observed diffraction line broadening and magnitude and direction of the lattice parameter shifts, it can be inferred that the lattice parameter changes are due to internal stresses resulting from the progression of the alpha to beta phase change as the alloy is moved through the mixed-phase region. A discussion is given of some implications of the observations for the use of lattice parameter measurements in locating coexistence curves for M-H alloys.

Variational method applied to determine the lattice parameter profiles in semiconductor heterostructures

Various methods have been used to fabricate vanadium dioxide (VO2) thin films exhibiting polymorph phases and an identical chemical formula suited to different applications. Most fabrication techniques require post-annealing to convert the amorphous VO2 thin film into the VO2 (M1) phase. In this study, we provide a temperature-dependent XRD analysis that confirms the change in lattice parameters responsible for the metal-to-insulator transition as the structure undergoes a monoclinic to the tetragonal phase transition. In our study, we deposited VO2 and W-doped VO2 thin films onto silica substrates using a high repetition rate (10 kHz) fs-PLD deposition without post-annealing. The XRD patterns measured at room temperature revealed stabilization of the monoclinic M2 phase by W6+ doping VO2. We developed an alternative approach to determine the phase transition temperatures using temperature-dependent X-ray diffraction measurements to evaluate the a and b lattice parameters for the monoclinic and rutile phases. The a and b lattice parameters versus temperature revealed phase transition temperature reduction from ∼66 to 38 °C when the W6+ concentration increases. This study provides a novel unorthodox technique to characterize and evaluate the structural phase transitions seen on VO2 thin films.

High energy α-particles which are produced by the decay of actinide compounds collide with the ions of fuels in nuclear reactors. Isolated Frenkel pairs are produced in the path of α-particles and lattice parameter changes in the nuclear fuel (i.e., uranium dioxide or plutonium dioxide). Some of the cation (uranium) and anion (oxygen) defects is randomly distributed in the crystal, results increase in some peaks in radial distribution function. Remaining cation defects have their special positions, trapped in octahedral cages, surrounded by six cations. These interstitial cations give rise to a significant, new peak, smaller than the principle peak of nearest neighbor cation-cation distance. Area below the observed pre-peak, give us information about the number of cations in the octahedral position which is also linearly dependent on the relative change in lattice parameter.

The ability to dynamically reconfigure superlattices in response to external stimuli is an intriguing prospect for programmable DNA‐guided nanoparticle (NP) assemblies, which promises the realization of “smart” materials with dynamically adjustable interparticle spacing and real‐time tunable properties. Existing in situ probes of reconfiguration processes have been limited mostly to reciprocal space methods, which can follow larger ordered ensembles but do not provide access to real‐space pathways and dynamics. Here, in situ atomic force microscopy is used to investigate DNA‐linked NP assemblies and their response to external stimuli, specifically the contraction and expansion of on‐surface self‐assembled monolayer superlattices upon reversible DNA condensation induced by ethanol. In situ microscopy allows observation and quantification of key processes in solution, e.g., lattice parameter changes, defects, and monomer displacements in small groups of NPs. The analysis of imaging data uncovers important boundary conditions due to DNA bonding of NP superlattices to a substrate. Tension in the NP–substrate DNA bonds, which can elastically extend, break, and re‐form during contraction/expansion cycles, counteracts the changes in lattice parameter and causes hysteresis in the response of the system. The results provide insight into the behavior of supported DNA‐linked NP superlattices and establish a foundation for designing and probing tunable nanocrystal‐based materials in solution.