Crystallographic calculations on the silliac electronic digital computer. II. Structure factors

1958 ◽  
Vol 11 (2) ◽  
pp. 99
Author(s):  
HC Freeman
Keyword(s):  
Crystal Structure ◽  
Special Reference ◽  
Structure Factor ◽  
Digital Computer ◽  
High Speed ◽  
Storage Capacity ◽  
Electronic Digital Computer ◽  
Structure Factors

Two programmes for crystal structure-factor calculations on the SILLIAC computer are briefly described, with special reference to the problem of coding such computations efficiently for a computer of high speed but limited storage capacity.

Obtaining the structure factors for an epitaxial film using Cu X-ray radiation

2013 ◽  
Vol 46 (6) ◽  
pp. 1749-1754 ◽  
Author(s):  
P. Wadley ◽  
A. Crespi ◽  
J. Gázquez ◽  
M.A. Roldán ◽  
P. García ◽  
...  
Keyword(s):  
Crystal Structure ◽  
Thin Films ◽  
Structure Factor ◽  
Tetragonal Symmetry ◽  
X Ray Diffraction ◽  
X Ray ◽  
Transmission Electron ◽  
Structure Factors ◽  
Scanning Transmission

Determining atomic positions in thin films by X-ray diffraction is, at present, a task reserved for synchrotron facilities. Here an experimental method is presented which enables the determination of the structure factor amplitudes of thin films using laboratory-based equipment (Cu Kα radiation). This method was tested using an epitaxial 130 nm film of CuMnAs grown on top of a GaAs substrate, which unlike the orthorhombic bulk phase forms a crystal structure with tetragonal symmetry. From the set of structure factor moduli obtained by applying this method, the solution and refinement of the crystal structure of the film has been possible. The results are supported by consistent high-resolution scanning transmission electron microscopy and stoichiometry analyses.

The use of a high-speed digital computer for the direct determination of crystal structures. I

1955 ◽  
Vol 227 (1171) ◽  
pp. 486-500 ◽  
Keyword(s):  
Crystal Structure ◽  
Fourier Series ◽  
Crystal Structures ◽  
Digital Computer ◽  
High Speed ◽  
Electronic Computer ◽  
Direct Determination ◽  
Direct Methods ◽  
Weighting Factor

A method is described whereby an electronic computer, the EDSAC, may be used to select a set, or sets, of signs for the coefficients F (h) of a Fourier series, such that the Fourier series, satisfies a certain condition. This condition is expressed as X ≡ Ʃ h Ʃ h ' P (h,h') S (h) S (h') S (h+h')≽ X 0 , where S (h) denotes the sign of F (h) and P (h, h') is a weighting factor related to the probability that S (h) S (h') = S (h+h'). In certain circumstances the determination of a crystal structure which is beyond the range of other direct methods is possible by this procedure.

A random procedure for the decomposition of a powder pattern inEXPO

2001 ◽  
Vol 34 (6) ◽  
pp. 704-709 ◽  
Author(s):  
Angela Altomare ◽  
Carmelo Giacovazzo ◽  
Anna Grazia Giuseppina Moliterni ◽  
Rosanna Rizzi
Keyword(s):  
Crystal Structure ◽  
Structure Factor ◽  
Phase Error ◽  
Direct Methods ◽  
Main Obstacle ◽  
Second Stage ◽  
Structure Factors ◽  
Structure Solution ◽  
Integrated Intensity ◽  
Integrated Intensities

The main obstacle toab initiocrystal structure solution by powder data is the lack of accuracy in the single Bragg integrated intensity estimates. The two-stage method proves powerful for solving crystal structures from powder data: in the first stage, the integrated intensities and, consequently, the structure-factor moduli, are extracted from the pattern for each reflection; in the second stage, direct methods may be applied to estimate the phases of the structure factors. The greater the accuracy of the integrated intensities, the more efficient the phasing procedure. A new procedure is described, aiming at overcoming the tendency of the Le Bail algorithm [Le Bailet al.(1988).Mater. Res. Bull.23, 447–452] to equipartition the intensities of strongly overlapping reflections. It proves to be efficient for lowering the phase error in the phasing process. The procedure has been implemented in theEXPOprogram [Altomareet al.(1999).J. Appl. Cryst.32, 339–340].

Crystallographic calculations on the silliac electronic digital computer. I. Fourier syntheses

1957 ◽  
Vol 10 (2) ◽  
pp. 95
Author(s):  
HC Freeman
Keyword(s):  
Electron Density ◽  
Digital Computer ◽  
High Speed ◽  
Electronic Digital Computer

A method is presented whereby SILLIAC, a high-speed electronic digital computer, sums 2-dimensional Fourier syntheses for crystallographic electron-density projections of any symmetry. The trigonometrical form of a particular summation and the area over which the projection is to be calculated are determined by programme-parameters introduced with the data.

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