摘要:

A useful algorithm to find the Bravais lattice on a four‐circle diffractometer is developed based on Delaunay's method of classifying the primitive cell. It consists of the evaluation of scalar products between lattice vectors and successive application of transformations to obtain the Bravais lattice. The procedures are found to be satisfactory in practical application

ISSN:1600-5767    

DOI:10.1107/S0021889886089914

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

The accuracy of structural parameters obtainable from measurements carried out with a Merrill–Bassett diamond‐anvil high‐pressure cell has been tested in an X‐ray diffraction study of cubic CaF2. The measurements were performed at 0.0001, 0.70, 1.70, 2.60, 2.90, 3.66 and 4.42 GPa. It was found that a simple modification of the shape of the gasket can eliminate the problem of partly obscured reflections and yield reliable values for the Ca and F thermal parameters – the only variables in this simple structure, and hence particularly sensitive to any systematic errors in

ISSN:1600-5767    

DOI:10.1107/S0021889886089902

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

The generalized equation of pseudo‐Kossel lines obtained in a back‐reflection divergent‐beam pattern is derived for the general case where the X‐ray point source and the plane of the film are not co‐planar. This treatment of the pseudo‐Kossel lines permits lattice‐parameter determinations even from incomplete diffr

ISSN:1600-5767    

DOI:10.1107/S0021889886089896

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

The structures of the metastable phase Mn2Ga0.5As0.5(m) and the stable phases Fe3GaAs and Co2Ga0.5As0.5are isotypic with Ni2In. X‐ray powder diffraction data are reported. Lattice constants and macroscopic densities were measured in the entire range of homogeneity. Crystal chemical parameters of these ternary phases and of the quasi‐homologous isovalent phases are compared and discussed from the point of view of the valence‐electron concentration and of the number of atoms in the unit cell. It is concluded that the valence‐electron concentration is a very important but not a sufficient energetic factor for the stability of phases with the Ni2In st

ISSN:1600-5767    

DOI:10.1107/S0021889886089884

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

An accurate value of the lattice parameter of the spinel ferromagnetic semiconductor HgCr2Se4has been determined at different temperatures, ranging from 298 to 770 K, by X‐ray powder diffraction. The data have been used to evaluate the coefficient of thermal expansion, α, at various temperatures. The temperature dependence of the coefficient of expansion can be represented by the equation α = 10.059 × 10−6− 1.862 × 10−8T+ 3.017 × 10−11T2, whereTis the temperature in K. The diffraction pattern of HgCr2Se4, obtained at 785(5) K, was completely different from that at room temperature. The high‐temperature phase persisted up to 900 K, and above this the compound decomposed. Furthermore, the new phase was retained when the sample was cooled from 900 K to room temperature. These results suggest an irreversible structural phase transformation in HgCr2Se4at 785 K. An attempt is made to explain this interesting high‐temperature thermal behaviour of HgCr2Se4in relation to its structure and

ISSN:1600-5767    

DOI:10.1107/S0021889886089872

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

The efficient use of crystallographic databases necessitates the use of statistical techniques.CAMALis a library of Fortran subroutines for performing simple and multivariate statistical calculations. The library is intended for use with the Cambridge Structural Database and is distributed by the Cambridge Crystallographic Data Centre.

ISSN:1600-5767    

DOI:10.1107/S0021889886089860

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

Results using parallel‐beam polycrystalline geometry at the Stanford Storage Ring are compared with conventional X‐ray tube focusing. The PC‐automated instrumentation included a pair of vertical‐scan diffractometers for wavelength selection using a Si(111) channel monochromator and for powder specimens which may be measured in reflection or transmission. The dependence of intensity, profile shape, width and position on instrument parameters is described. All the profiles in patterns of well crystallized specimens with a selected receiving slit are symmetrical and primarily Gaussian with nearly the same width, thereby greatly simplifying the algorithms and programs for profile fitting and interpretation of profile broadening. The easy wavelength selection allows the use of a wavelength just longer than the absorption edge of elements in the sample to obtain maximumP/Bratio, and short wavelengths permit access to very highhkls. Because of the absence of theKα doublet and the simple symmetrical profile shape the resolution need not be as good as in focusing geometry to achieve comparable overlap separation. The average precision of lattice‐parameter determination wasΔd/d= 5.6 × 10−5using forwa

ISSN:1600-5767    

DOI:10.1107/S0021889886089859

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

An interactive graphics program for the Evans&Sutherland PS330 has been written to display and index screenless X‐ray oscillation films from macromolecular crystals with unit‐cell edges of at least 500 Å.ALIGN, a Fortran program which works in conjunction with M. G. Rossmann's programOSCMGR, accurately displays densitometered reflection intensities, provides the required transformation between predicted and observed reflection coordinates, and is useful in determining crystal mosaicity and detecting unit‐cell

ISSN:1600-5767    

DOI:10.1107/S0021889886089847

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

A continuous nitrogen‐flow cooling device, generally applicable to X‐ray diffraction studies, is described. The device works in the range 77.4 to 323.0 K with a precision of ±0.1 K and a liquid‐nitrogen constant consumption rate of 0.5 l h−1over the whole temperature range. The supply vessel is unpressurized, so that refilling can be done without any observable influence on the cooli

ISSN:1600-5767    

DOI:10.1107/S0021889886089835

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

Close examination of published measurements of absolute configuration in inorganic crystals has revealed many mistakes, and only a small number of published correlations between optical rotation and chirality of crystal structure can be trusted. The sense of rotation of light is here traced to the hand of specific helices of atoms, provided that the latter are correctly chosen. Four principles are given for choosing the helices. The crystals described are: low‐quartz, α‐AlPO4, α‐HgS, Ca2Sr(C2H5COO)6, Bi12SiO20, NaClO3, NaBrO3and α‐LiIO3. The principles suggest that the published absolute configuration of α‐LiIO3must be incorrect {subsequently proved experimentally by Stadnicka, Glazer&Moxon [J. Appl. Cryst.(1985),

ISSN:1600-5767    

DOI:10.1107/S0021889886089823

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY