摘要:

The construction of a new high‐pressure cell is presented. It is a piston‐cylinder type of apparatus. The entrance and exit of X‐rays or light are possible through two parallel diamond‐plate windows placed on V‐shaped slits covering almost the whole range of reflection angles. This makes the cell particularly suitable for X‐ray diffraction examinations of monocrystals. The potential accuracy of absolute‐bulk‐modulus determination by lattice‐constant measurements using the Bond method [Bond (1960).Acta Cryst.13, 814–816] at pressures of 108–109Pa can be estimated at 1–2%. The cell enables the simultaneous examination of two samples, which makes possible the comparison of their compressibilities with an accuracy of 0.01–0.1%. As examples, the results of compressibility measurements

ISSN:1600-5767    

DOI:10.1107/S0021889892006009

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

A modification is described of the singular value decomposition (SVD) method suitable for underdetermined linear least squares (LLS). When a set of data to be fitted is incomplete and does not allow an independent determination of all model parameters, the modified method automatically merges a previously available approximate solution into the LLS results. The solution so produced is more appropriate to the particular problem than the usual SVD solution, while still being a LLS estimate of the whole set of parameters. The method is discussed with reference to the LLS determination of unit‐cell dimensions during the step‐by‐step assignment ofh,k,lindices of a diffraction pa

ISSN:1600-5767    

DOI:10.1107/S0021889892006010

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

The use of position‐sensitive area detectors with high‐flux radiation sources demands high‐rate data‐acquisition systems. Ideally, such devices should be commercially available and machine independent. In this paper, the basic features of such a system are described. The steps are highlighted that determine the counting rates of present‐day addressing‐logic and computer‐bus speeds. The improvements expected with a new industrial standard bus system are brie

ISSN:1600-5767    

DOI:10.1107/S0021889892007088

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

The interpretation of difference Patterson maps for heavy‐atom positions is an important step in the structure determination of biological molecules. In the absence of a large number of noncrystallographically related subunits, the first step is usually a Harker‐vector search to locate single‐site solutions, which is followed by a cross‐vector search to locate two‐atom solutions and subsequent searches to locate more sites. Crystals of biological macromolecules frequently contain assemblies obeying noncrystallographic symmetry. The information about their noncrystallographic symmetry can be incorporated in search procedures to aid the Patterson‐map interpretation. In the presence of noncrystallographic symmetry, a self‐vector search is useful for locating the positions of heavy atoms relative to the center of the noncrystallographic symmetry point group and a cross‐vector search is then used to locate that center in the crystal unit cell. An automated procedure is presented which is a generalized Patterso

ISSN:1600-5767    

DOI:10.1107/S0021889892007295

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

The microstructure of ZnO powder, obtained from thermal decomposition of the oxalate and studied previously by electron microscopy and adsorption calorimetry, was investigated by means of X‐ray powder diffraction pattern decomposition. A Williamson–Hall plot revealed that some lines were broadened solely due to the effects of crystallite size, whereas other breadths included a contribution due to stacking faults. Spherical and cylindrical models are used to describe the form of the crystallites and procedures are presented for separating 'size' effects from `mistake' broadening. This leads to estimates of the mean dimensions of the crystallites and the stacking‐fault probability. The analysis demonstrates that, with good‐quality data for a large number of reflections, a considerable amount of detailed information can be obtained about microstructure. On the other hand, it reveals some of the limitations of current procedures for modelling diffraction line p

ISSN:1600-5767    

DOI:10.1107/S0021889892007684

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

X‐ray diffraction under grazing‐incidence and ‐exit angles was measured on 100 keV Si+‐implanted silicon. In comparison with model calculations based on dynamical and kinematical scattering theory, depth‐resolved information on the distortions of the crystal lattice is obtained. The thickness of amorphous and damaged crystalline regions, the degree of damage and the extension of the amorphous/crystalline interfaces are determined and are found to be in good agreement with the results of transmission electron microscopy (TEM) measurements. Furthermore, recrystallization after rapid optical annealing at different temperatures has been studied and quantitative results concerning the improvement of crystal structure, the sharpening of interfaces and the decrease in the heavily distorted surface‐layer thickness a

ISSN:1600-5767    

DOI:10.1107/S0021889892007799

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

Highly pure dendritic single crystals of cadmium iodide, variously (2 to 8%) doped with lead iodide, have been grown from the vapour phase in a vacuum. The study of polytypism and related phenomena in the crystals has been carried out by X‐ray diffraction. An uneven high rate of increase in the occurrence of the 12‐layered rhombohedral polytype 12Rhas been found to exist when the amount of PbI2in the crystals rises from 2 to 8%. Simultaneously, the relative intensity of the 12Rreflections has been found to rise rapidly. Another curious feature has been an unusually high frequency of occurrence of a particular high polytype 48Hin the heavily (8%) doped crystals. The percentage occurrences of streaking and arcing have been found to be low and to vary slowly with the PbI2content of the crystals. The observed polytypism has been explained in terms of the nucleation of two different thermodynamically stable end‐member polytypes, 4Hand 12R, of CdI2and PbI2, respect

ISSN:1600-5767    

DOI:10.1107/S0021889892007921

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

Branden&Jones state, inNature: `Protein crystallography is an exacting trade, and the results may contain errors that are difficult to identify. It is the crystallographer's responsibility to make sure that incorrect protein structures do not reach the literature.' [Branden&Jones. (1990).Nature(London),343, 687–689.] One of several available methods of checking structures for correctness is the evaluation of atomic contacts. From an initial hypothesis that atom‐atom interactions are the primary determinant of protein folding, any protein model can be tested for proper packing by the calculation of a contact quality index. The index is a measure of the agreement between the distributions of atoms around each residue fragment in the model and equivalent distributions derived from the database of known structures solved at high resolution. The better the agreement, the higher the contact quality index. This empirical test, which is independent of X‐ray data, is applied to a series of successively refined crystal structures. In all cases, the model known or expected to be better (the one with the lowerR‐factor) has a better contact quality index, indicating that this type of contact analysis can be used as an independent quality criterion during crystallographic refinement. Modelled proteins and predicted mutant structures can also be ev

ISSN:1600-5767    

DOI:10.1107/S0021889892008240

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

In this paper, the principle of a new neutron diffractometer is presented. In this diffractometer are combined the advantages of a high‐resolution time‐of‐flight (TOF) diffractometer and some essential characteristics of a classical double‐axis spectrometer (DAS), specific disadvantages of both instruments being eliminated. Its name, MARTIN (multiple‐angle high‐resolution time‐of‐flight instrument for neutrons), is derived from the particular instrumental set‐up. With the MARTIN diffractometer, a neutron pulse from a narrow wavelength band (0.2–0.5 Å) impinges on the sample. The diffracted radiation is registered and time‐analyzed by detectors located at several scattering angles. The angular arrangement of the detectors is such that each detector samples a different portion of the diffractogram, with the portions detected by adjacent detectors partially overlapping. These partial diffractograms can be conjoined to yield the complete diffractogram of interest. This new machine has all the advantages of a TOF diffractometer: no contamination of the primary beam by higher orders, higher counting rates because of the possible utilization of the entire Debye–Scherrer ring, higher flexibility because no monochromators are needed and less strict requirements of mechanical precision as a result of the fixed detector arrangement. In addition, the use of a narrow wavelength band reduces the problem of absorption and extinction corrections in TOF diffractometry to one solvable by the well known data treatment of two‐axis diffractometry. A prototype of the MARTIN diffractometer was designed for powder diffraction and installed as an extension of the TOF diffractometer at the Munich Research Reactor (FRM) in Garching, where its characteristics were tested

ISSN:1600-5767    

DOI:10.1107/S0021889892008252

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

A review of the symmetry of proteins has revealed that in many cases the metric symmetry exceeds the reported crystal symmetry. Regardless of the reason, this observation has important implications for experimental protein crystallography. Standard laboratory procedure should always include a direct determination of the lattice metric symmetry. With full knowledge of the highest possible symmetry, the experimentalist is then able to determine in a logical and accurate manner the Laue group and the space group. For those proteins in which it has been proved that the metric symmetry exceeds the crystal symmetry, the protein crystallographer must proceed with caution (e.g.in relating multiple sets of data, positional parametersetc. on the same or related crystals), because a given lattice will have metrically similar unit cells that are not symmetrically equivalent.

ISSN:1600-5767    

DOI:10.1107/S0021889892008434

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY