摘要:

Crystal structure parameters have been obtained for α‐Al2O3,β‐PbO2and (Mg, Fe)2SiO4by Rietveld analysis of CuKα X‐ray powder diffraction data collected on a conventional diffractometer using counting times ranging from 0.01 to 5 s per step. For all but the 0.01 s data collected on (Mg, Fe)2SiO4the structural parameters obtained at different counting times are statistically identical at the 3σlevel, and the spread in the values is essentially the same as that obtained by sample repacking at a fixed counting time of 1 s per step. The parameter e.s.d.'s and conventional agreement indicesRwpandRBdecrease to values limited by residual model errors as the counting time increases, but the goodness‐of‐fit parameter becomes unacceptably larger than its ideal value of unity. When more than a few thousand counts are accumulated for the maximum step intensity in the diffraction profile a weighting scheme based solely on counting variance is inappropriate and the parameter e.s.d.'s are no longer a reflectio

ISSN:1600-5767    

DOI:10.1107/S0021889884011547

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

The many‐beam dynamical theory of moiré fringes of Gevers [Phys. Status Solidi(1963).3, 2289–2297] has been applied to the parallel moiré case and the results have been verified on epitaxically grown Sn/SnTe specimens showing the sensitivity of moiré patterns to the presence of systematic reflections. The Fourier analysis of the calculated intensity profiles showed that each additional diffracted beam gives rise to a new term in the Fourier series so that the profile can be sinusoidal only in the two‐beam approximation. The same result has been obtained recently by Pardo, Pariset&Renard [Phys. Status Solidi A.(1981).64, 283–295], who applied a slightly different theoretical approach to Sb/Bi specimens. It is shown that the corrections to scattering factors for temperature effects are not important since they do not affect the general character of the moiré profiles. Some experiments were performed showing the dependence of the moiré pattern on the number of beams that are transmitted through the objective aperture. The effect of the gradual thinning of one layer on the moiré pattern could be reproduced by comput

ISSN:1600-5767    

DOI:10.1107/S0021889884011559

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

The Bragg reflection of neutrons by vibrating perfect crystals is considered. The additional possibilities offered by the Doppler effect for shaping neutron beams in thekspace are discussed. A simple model for computing the vibrating crystal reflectivity is proposed.

ISSN:1600-5767    

DOI:10.1107/S0021889884011560

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

The predominant dislocation types in crystals of pentaerythritol tetranitrate (PETN) grown from solution have been observed and characterized using transmission X‐ray topography. The dislocation characterization, carried out by contrast variations, was aided and simplified by comparison of observed dislocation line directions with those calculated from the theory of preferred line directions of Klapper [Habilitationsschrift (1975). RWTH, Aachen]. With few exceptions, the dislocations observed were induced during growth. Eight different types were identified comprising all possible Burgers vectors up to and including (111). Of these, one was pure edge, the remainder being of mixed character. The relative frequency of occurrence of the dislocation types is discussed, together with factors influencing dislocation contrast and characterizatio

ISSN:1600-5767    

DOI:10.1107/S0021889884011572

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

Vapor‐phase ZnS crystals were grown under strict stabilization of temperature and pressure. Many crystals containing wide polytypic regions were found. Hollow tubes were observed, almost exclusively in the wide regions; the regions with no hollow tubes consist of a large number of very narrow faulted sub‐regions. It is suggested that the creation of Shockley partials is associated with the strain in the vicinity of the growth screw dislocations. Those regions where the core of the dislocation is hollow are unstrained and the Shockley partials, moving in from neighboring regions, can climb unimpeded along the helicoidal basal planes, transforming them into the various observed polyty

ISSN:1600-5767    

DOI:10.1107/S0021889884011584

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

A combination of X‐ray powder diffraction and single‐crystal Laue photography was used to determine the unit‐cell parameters for letovicite.μr= 247.25. The unit cell is triclinic,P1 orP, and has dimensionsa0= 5.87(1),b0= 10.17(3),c0= 8.27(1) Å,α= 101.1(4),β= 111.1(1),γ= 89.9(2)°,V= 450.7 Å3,Z= 2,Dx= 1.82 Mg m−3. TheM(20) cell reliability is 23.8. Many weak powder spectra were observed, which preclude the possibility of monoclinic cell symmetry previously reported. Projection of the letovicite Laue data along [001] reveals the striking pseudo‐hexagonal symmetry observed optically. The JCPDS Diffraction File No. for ammonium hydro

ISSN:1600-5767    

DOI:10.1107/S0021889884011596

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

Enhancements in the auto‐indexing procedure for cell determination improve its power and reliability. A proper selection of three basis reflections for construction of an initial subcell is essential. A standard cell‐reduction procedure applied to the subcell, and the calculation of appropriate limits for the direct‐lattice‐vector generation integers, make a comprehensive coverage of the direct space efficient and effective. Refinement of the generated lattice vectors against all the available reflection data makes assignment of the correct unit cell easier and more reliable. Generation of some additional long vectors is recommended in specia

ISSN:1600-5767    

DOI:10.1107/S0021889884011602

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

A high‐resolution small‐angle X‐ray scattering camera has been built, which has the following features. (i) The point collimation optics employed allows the scattering cross section of the sample to be directly measured without corrections for desmearing. (ii) A small‐angle resolution better than 0.5 mrad is achieved with a camera length of 1.6 m. (iii) A high photon flux of 0.9 photons μs−1is obtained on the sample with the rotating‐anode X‐ray generator operated at 40 kV–30 mA. (iv) Incident X‐rays are monochromated by a bent quartz crystal, which makes the determination of the incident X‐ray intensity simple and unambiguous. (v) By rotation of the position‐sensitive proportional counter around the direct beam, anisotropic scattering patterns can be observed without adjusting the sample. Details of the design and performance are pr

ISSN:1600-5767    

DOI:10.1107/S0021889884011614

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

X‐ray absorption spectra about theKedges of Ni, Cu and Ti have been measured at the Stanford Synchrotron Radiation Laboratory. The imaginary part of the atomic scattering factorf′′ was determined using the optical theorem and the real partf′ computed by the Kramers–Kronig dispersion relation. Methods for evaluating this integral as well as the effects onf′ of various experimental errors are investigated. Thef′ results for Cu and Ni are compared to data from interferometr

ISSN:1600-5767    

DOI:10.1107/S0021889884011626

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

A modification of the Rietveld refinement program is introduced by replacing the Gaussian profile function with a Voigtian. The performance of the program is tested by an application to simulated and measured diffraction patterns of NaTaO3, and excellent results are obtained when the tails of the reflections are included at distances 10 to 20 times the half‐width of the reflections. Comparison between Gaussian and Voigtian refinements show large differences in the thermal parameters. This is due to the high level of background that is assumed in the Gaussian refinement to compensate for the lack of tail overlap. Application of the Voigtian refinement to the measured pattern of Ni yields a thermal parameter that is close to the literature values, while the Gaussian analysis gives a value which is 35% too large. The isotropic temperature factors of the room‐temperature structure of NaTaO3, respectively, drop down to the valuesB(Ta) = 0.09(2),B(Na) = 1.21(3),B(O1) = 0.59(3) andB(O2) = 0.61(3) Å2, when the Voigtian analysis with a proper background is carried out. The role of background in powder pattern refinement is discussed, and it is suggested that a calculation of the average TDS should be included in the refi

ISSN:1600-5767    

DOI:10.1107/S0021889884011638

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY