Mosaic Crystals Research Articles

In-situmeasurement of rocking curves during lysozyme crystal growth

The rocking curve of protein crystals contains a lot of useful information concerning crystal quality, most of which is lost owing to the superimposition of spurious features appearing in these fragile materials after growth, during handling and mounting. To minimize such data spoiling, an experimental setup to perform in situ X-ray diffraction experiments during crystal growth has been designed. The setup, which includes video observation to allow the correlation of crystal shape, size and growth rate with X-ray data, has been used to assess the mosaicity of tetragonal lysozyme crystals during crystal growth. The full width at half maximum (FWHM) of diffraction peaks collected from these crystals changes during the growth process as a (directly proportional) response to the growth rates and the different development of different domain blocks. These changes in the domain distribution and FWHM with time involve a 'zonation' of the crystals, which show very different rocking curves in different parts of their volume. The rocking curves recorded in situ from growing crystals are easier to understand than those from crystals that have suffered even minor handling.

Effects of traveling surface acoustic waves on neutron Bragg scattering from perfect crystals

We have studied the effects of a traveling surface acoustic wave (SAW) disturbance on symmetric Bragg case neutron diffraction from a perfect crystal of lithium niobate. Generally, the reflectivity is increased as the SAW distortion creates an effective moving mosaic crystal in the near surface region. Unlike the corresponding x-ray situation in which the crystalline distortion is effectively stationary, thermal neutron and SAW velocities are comparable. Thus the interaction between the neutron and the coherent SAW phonons involves significant Doppler effects that also modify the reflected neutron intensity. While the observed effects can be explained qualitatively, as yet no quantitative picture exists of the interaction between the neutron and the coherent surface phonons in this situation. {copyright} {ital 1999} {ital The American Physical Society}

Influence of the nanostructure and morphology of (VO)2P2O7 on its catalytic reactivity

The reoxidability of vanadyl pyrophosphate is studied by thermal analysis and by temporal analysis of product (TAP). The samples differ by surface area, oxidation state of vanadium, crystallite size, and degree of crystallinity. The morphology ranges from platy to prismatic and the microstructure from mosaic crystals exhibiting {100} faces to bulky crystals. The reactivity of these samples during oxidation as well as their catalytic reactivity studied in flow reactor is accounted for by the microstructure. The highest reoxidation capability and the best catalytic properties in oxidation of n-butane to maleic anhydride are obtained with mosaic crystals.

Pseudospherulitic fibrous calcite in paleo-groundwater, unconformity-related diagenetic carbonates (Paleocene of the Ager Basin and Miocene of the Madrid Basin, Spain)

ABSTRACT Massive, decimeter-thick intervals formed by mosaics of subspherical calcite crystals displaying an intracrystalline fibrous microfabric have been found in Paleocene and Miocene strata in Spain. The fibrous microfabric is marked by inclusion-defined fibers that radiate from the central parts of the crystals, which can be termed pseudospherulites and not spherulites, because they are single crystals and not polycrystalline aggregates. The horizons of pseudospherulitic calcite appear in meter-scale, laterally persistent levels of crystalline limestones that exhibit complex pseudospar fabrics, they are interbedded with peritidal and/or lacustrine deposits, they grade downward into bedded dolomicrites, and they can be linked to unconformities several meters higher in the section, above intervening unaltered limestones. The pseudospar zones are interpreted as early-diagenetic, paleo-groundwater alteration products after dolomicrites. The pseudospherulites are composed of low-Mg calcite with low contents of Fe and Mn, and they have a stable-isotope composition indicative of precipitation from meteoric water. They form xenotopic mosaics in which goethite, palygorskite, and sepiolite may be present as inclusions and in the intercrystalline matrix. Cathodoluminescence microscopy reveals peripheral growth of the pseudospherulites, which results from several phases of precipitation and even of dissolution. The first phase of crystal growth may show rhombic habits and may contain corroded dolomite inclusions, interpreted as an indication of growth at the expense of dolomicrite by a dissolution/precipitation process. The second phase of crystal growth is fibrous sensu stricto, exhibits lobate growth laminae, and shows indications of displacive growth. The pseudospherulitic fabric is not relict and the pseudospherulites do not represent speleothems, recrystallized Microcodium, or neomorphosed spherulitic carbonate. Instead, they are interpreted as split crystals that grew within dolomicritic host sediments, in part by dissolution-precipitation and in part by displacement. The hypothesis of a dolomicritic host is supported by the geological and diagenetic setting, by the presence of corroded dolomite inclusions within the pseudospherulites, and by the relationship between their growth and the authigenesis of sepiolite, palygorskite, and goethite. The pseudospherulites grew in the shallow subsurface, at or near the water table, from predominantly oxidizing meteoric water, and under periodically fluctuating geochemical conditions (saturation, redox) caused by fluctuations of the water table during periods of subaerial exposure. The presence of fossil bacteria within the pseudospherulites, the anastomosing pattern of the inclusion-defined fibers, and the lobate growth forms of the fibrous calcite suggest some bacterial influence in the genesis of the pseudospherulitic fabric.

Measurement of diffraction efficiencies relevant to crystal lens telescopes

The Toulouse/Argonne collaboration is working on a novel detector for gamma-ray astronomy based on a crystal lens that uses Bragg reflection of Ge crystals to concentrate photons onto a small detector in the image plane. The diffraction efficiency of the crystals is decisive for the energy bandpass, the field of view and the effective area of the crystal lens telescope. We have measured diffraction efficiencies of Ge crystals from 200 to 500 keV at the Advanced Photon Source synchrotron at Argonne National Laboratory in two experiments carried out at the sector 1 bending magnet beamline. The high brilliance and high particle energy of third-generation synchrotrons permit measurements at gamma-ray energies. The low angular divergence of synchrotron radiation is similar to that of an astronomical point source. The efficiencies ranged from 20% to 31% for diffraction of Ge (1 1 1) and (2 2 0) using crystals with a mosaic width of a few arc seconds. Neglecting absorption, the values ranged from 31% to 49%. The continuum flux diffracted in second-order diffraction (4 4 0) was a factor of 2.4–2.6 less than in first-order diffraction (2 2 0) . For third-order diffraction (the (3 3 3) and the (1 1 1) planes), the flux was reduced by a factor of 6.8. The peak efficiency and the rocking curve integral for mosaic widths ranging from a few arc seconds to 0.4° were measured. The results agree with the expectation of the Darwin model for mosaic crystals. This gives us confidence in the calculations of the crystal lens telescope performance, and allows an optimization of a balloon-borne and a satellite-based telescope configuration. Measurements were carried out with a wedged crystal, that was used to increase the diffraction efficiency of the lens crystals for a source at finite distance. The efficiency increase is due to both a bending and a straining of the crystal. We measured rocking curve widths of a crystal before and after etching to determine the effect of surface damage on the diffraction efficiency. The mean decrease in rocking curve width due to the etching was 1.7 arc sec in the center of the crystal and 6.9 arc sec at the crystal edge.

Parametric X-rays and diffracted transition radiation in perfect and mosaic crystals

The amplitude of X-ray emission by relativistic electrons in a single crystal, calculated in the kinematical approach, is decomposed unambiguously in Diffracted Transition Radiation (DTR) and Parametric X-rays (PXR). DTR becomes significant for γ ≳ ω P ,γ being the Lorentz factor and ω P the plasma frequency. It is more collimated than PXR and, above threshold, its flux increases logarithmically with γ. However, it saturates with thickness at the Bragg primary extinction length l e. This saturation is accounted for only in the dynamical approach, the formulas of which are compared to the kinematical ones. The respective contributions of DTR and PXR are calculated for a simple model of mosaic crystal, taking into account saturation of DTR with thickness. The PXR flux is basically the same as in a perfect crystal. If the size of the domains is larger than l e, the DTR flux is multiplied by the number of domains crossed by the electron. For domains smaller than l e and γ ≳ ω P , the DTR and PXR fluxes are of the same order of magnitude, up to logarithmic factors. In any case, mosaicity increases the total yield of X-ray photons.

Polarization of parametric X radiation

Polarization properties of parametric X radiation (PXR) have been investigated theoretically and experimentally at the S-DALINAC. For the first time analytical expressions describing the polarization properties of PXR have been derived. PXR is – under ideal conditions – 100% linearly polarized in every single point of its angular distribution. In reality, however, multiple scattering of the electrons passing through the crystal, the emittance of the electron beam, the crystal mosaicity and the detector acceptance decrease the polarization degree. In the present experiment PXR was produced by E 0=80.5 MeV electrons interacting with a (2 2 0) plane of a 13 μm silicon crystal and observed at an angle of about 20° by a CCD polarimeter. For the comparison of the experimental results with the theoretical predictions a Monte Carlo simulation taking into account the experimental conditions has been performed. The comparison shows very good agreement.

Characterization of the crystalline quality on GaN on sapphire and ternary alloys

The growth of GaN on sapphire by OMVPE using a low-temperature deposited buffer layer was characterized by in situ TEM observation. The crystalline quality, especially tilting and twisting of the mosaic GaN crystal, was characterized by conventional X-ray diffraction together with grazing-incidence X-ray diffraction. Strain and relaxation of the AlGaN and GaInN on GaN were also characterized by asymmetrical X-ray diffraction. © 1998 Scripta Technica, Electron Comm Jpn Pt 2, 81(10): 48–54, 1998

Macromolecular crystal annealing: overcoming increased mosaicity associated with cryocrystallography.

Although cryogenic data collection has become the method of choice for macromolecular crystallography, the flash-cooling step can dramatically increase the mosaicity of some crystals. Macromolecular crystal annealing significantly reduces the mosaicity of flash-cooled crystals without affecting molecular structure. The process, which cycles a flash-cooled crystal to ambient temperature and back to cryogenic temperature, is simple, quick and requires no special equipment. The annealing process has been applied to crystals of several different macromolecules grown from different precipitants and using a variety of cryoprotectants. The protocol for macromolecular crystal annealing also has been applied to restore diffraction from flash-cooled crystals that were mishandled during transfer to or from cryogenic storage. These results will be discussed in relation to crystal mosaicity and effects of radiation damage in flash-cooled crystals.

A flash-annealing technique to improve diffraction limits and lower mosaicity in crystals of glycerol kinase.

A flash-annealing method has been developed that increases the diffraction limits, and simultaneously decreases the mosaicity of glycerol kinase crystals. This technique utilizes brief thawing and rapid freezing cycles of the crystal in the cold nitrogen stream. The effective resolution limits increased almost by 0.8 A, from 3.6 to 2.8 A, and mosaicity values halved.

Mosaic Crystals of Vanadyl Pyrophosphate Obtained by Oriented Nucleation and Growth

A study of the pseudomorphic transformation of the VOHPO4·0.5H2O precursor (P/V=1) to (VO)2P2O7catalyst is reported. Attention is paid to the nucleation and growth stages, which are discussed in connection with the concept of topotaxy. The transformation mechanism which accounts for the textural change is established using electron microscopy techniques and X-ray diffraction. The morphology change to a prismatic shape at high temperature, approximately 870°C, is also discussed.

Response to Mathieson's (1998) comment on A universal treatment of X-ray and neutron diffraction in crystals. I. Theory

There is an obvious difference between the behaviour of the dependence of the integrated reflection power ratio on the asymmetric parameter for plane crystals predicted by the H–D equations on mosaic crystals and that predicted by the theory of Hirsch & Ramachandran on perfect crystals. It is most important to get rid of the surface layer effect of the crystal sample as much as possible for the verification of such a difference by experiment. This paper is in response to the comment by Mathieson [Acta Cryst. (1998), A54, 251].

Standing waves and Kossel line patterns in structure determination

Synchrotron radiation excited Kossel and standing wave line patterns of a NiO mosaic crystal are presented. It is shown that crystallographic parameters such as symmetries, crystal orientation and lattice parameters can be obtained from this type of measurement.

Two-and three-dimensional conduction channels at block boundaries in (CdHg)Te mosaic crystals

Additional electronic conduction has been observed along the block boundaries in mosaic n-type CdxHg1−xTe crystals. Conduction along block boundaries is two-dimensional (2D) in one group of samples and three-dimensional (3D) in another group. The main parameters of the 2D and 3D channels—electron density cyclotron mass, relaxation time, and mobility—were determined by analyzing the Shubnikov-de Haas oscillations. In strong magnetic fields at liquid-helium temperatures conduction occurs mainly along a network of conducting channels at block boundaries. The resistance of such a network of 2D channels in a 6×1.5×0.5 mm3 sample equals approximately 110 Ω and is virtually temperature-independent.

Flat and Spherically Bent Muscovite (Mica) Crystals for X-ray Spectroscopy

Essential parameters for the application of crystals toquantitative X-ray spectroscopy are the upper wavelength limit and thequantitative reflection properties (intrinsic resolving power,luminosity) of the crystal. Due to the large lattice constant,muscovite, a mica group mineral, can be used in the wavelength rangeup to about 2nm. Muscovite crystals can be bent to small radii ofcurvature due to their favourable cleavage and elastic properties.Characteristic reflection properties at reflections 002 – 00 24were investigated theoretically and experimentally. The integratedreflectivity was calculated for various reflections of perfect flat aswell as spherically bent muscovite crystals with curvature radii R = 100 and R = 186mm. It was measured for flat crystalsin the reflections 00 10 – 00 26 using CuKα- andMoKα-radiation from X-ray tubes and compared with calculationsfor both perfect and mosaic crystals. Available high-quality muscovitecrystals have a mosaic structure with a mosaic spread of about 1arcmin. This mosaic spread limits the spectral resolving power forhigh reflection orders.

Calcified root cells in Miocene pedogenic carbonates of the Madrid Basin: evidence for the origin of Microcodium b

Calcified root cells, forming microspar and pseudospar mosaics of calcite and/or dolomite crystals, constitute a major component of calcretes and dolocretes from the Miocene of the Madrid Basin. The calcified cells occur in massive nodules or fill root tubes in the calcrete-dolocrete profiles. The arrangement of the cells within the mosaics and their internal features, together with the isotopic data, clearly indicate that the crystals formed through the calcification of root cells and not through recrystallization or dolomitization. Calcified root cells formed in a favourable microenvironment caused by biochemical phenomena associated with plant growth. In these examples, the calcification is incomplete as only in the innermost part of the root the cells were totally calcified, whereas in the root cortex only the cell walls were calcified. The distribution of the calcified cells within the roots was controlled by the different ionic environments which prevail within an active root system. In the inner part the ionic conditions were mostly controlled by the cellular activity of the root creating a suitable microenvironment for the biomineralization of the cells. This differs notably from most published examples of calcified root cells in which it is usual for the cortical cells of roots to be completely calcified. The calcified root cells of the Madrid Basin resemble unequivocally the problematic Microcodium (b), which suggests that this type of Microcodium formed through calcification of root cells.

High-Resolution Crystals of Methionine Aminopeptidase fromPyrococcus furiosusObtained by Water-Mediated Transformation

The monoclinic crystal form of methionine aminopeptidase fromPyrococcus furiosus(MAP-Pfu) has been crystallized from four different conditions. Native crystals belong to space group P21with typical unit-cell dimensionsa= 53.4,b= 85.1,c= 72.7 Å, β = 107.7° and diffract to 2.9–4.5 Å resolution. However, there is a problem of nonisomorphism among the crystals. Water-mediated transformation to low-humidity form occurs by reduction of the relative humidity of crystal environment to 79%. The unit-cell dimensions of transformed crystals area= 51.9,b= 83.3,c= 70.3 Å, β = 105.9°, and the calculated solvent content is 3.9% less than in original crystals. Transformation to low-humidity form is accompanied by 1.7 times reduction of overall temperature factors, extension of diffraction resolution up to 1.75 Å, without change or reduction of crystal mosaicity, and improvement in stability to X-ray radiation. The water-mediated transformation also appears to relieve the problem of nonisomorphism among the original MAP-Pfu crystals.

A Novel Method of Analyzing Peak Broadening Due to Mosaicity for Cubic Crystals on (001) Substrate using Double-Crystal X-ray Diffraction Methods

A novel method of analyzing X-ray peak broadening due to mosaicity for double-crystal X-ray diffraction measurements is proposed, which is an improved version of a conventional method using azimuthal angle dependence. The method is applicable to cubic crystals grown on a (001) substrate. The novelty of the method lies in the fact that the dependence of the full width at half-maximum (FWHM) on various reflection indices hkl is analytically derived in detail. The geometric relation between a tilted epilayer and the hkl measurement configuration is clarified, and a theoretical formula is derived. The resultant value of the FWHM divided by cos θc, where θc is the angle between (001) and (hkl) planes, is independent of the reflection index when the broadening is due to local tilt distribution caused by crystal mosaicity. The analytical method can be used as a criterion to judge whether or not X-ray peak broadening is due to the mosaicity. It is demonstrated that this formula holds for InAsP/InGaAsP SLS (strained layer superlattice) samples. The presented method is especially useful for samples with very weak peak intensity due to a high degree of mosaicity, to which reciprocal lattice-mapping measurement cannot be applied.

Effects of travelling surface acoustic waves on neutron Bragg scattering from perfect crystals

We have studied the effects of a travelling surface acoustic wave (SAW) disturbance on symmetric Bragg case neutron diffraction from a perfect crystal of lithium niobate. Generally, the reflectivity is increased as the SAW distortion creates an effective moving mosaic crystal in the near surface region. Unlike the corresponding X-ray situation in which the crystalline distortion is effectively stationary, thermal neutron and SAW velocities are comparable, giving rise to significant Doppler effects that modify the reflected neutron intensity. No quantitative picture exists of the interaction between the neutron and the coherent surface phonons in the present situation. Several of our observations contradict even the qualitative expectations of a moving mosaic crystal analogy.

CCD video observation of microgravity crystallization of lysozyme and correlation with accelerometer data.

Lysozyme has been crystallized using the ESA Advanced Protein Crystallization Facility onboard the NASA Space Shuttle Orbiter during the IML-2 mission. CCD video monitoring was used to follow the crystallization process and evaluate the growth rate. During the mission some tetragonal crystals were observed moving over distances of up to 200 micrometers. This was correlated with microgravity disturbances caused by firings of vernier jets on the Orbiter. Growth-rate measurement of a stationary crystal (which had nucleated on the growth reactor wall) showed spurts and lulls correlated with an onboard activity: astronaut exercise. The stepped growth rates may be responsible for the residual mosaic block structure seen in crystal mosaicity and topography measurements.