Showing papers in "Acta Crystallographica in 1960"

Precision lattice constant determination

Abstract: A single crystal method of lattice constant determination is presented by Bond in which the several errors are minimized or corrections computed. The apparatus measures the angle between two reflecting positions of the crystal. With nearly perfect crystals the lattice constants can be measured to a few parts in a million.

The crystal chemistry of the amphiboles

Abstract: The major differences in unit-cell shape of the different amphibole varieties, and the respective composition ranges of the orthorhombic and monoclinic amphiboles, are related to the effects of the ionic radius of the ions occupying two of the sets of ionic sites in the structure. These are the ions which serve to link together the talc-like strips from which the structure may be regarded as being built. The reason for the existence of both limited and unlimited isomorphous substitution ranges for the same ions in different parts of the composition field is shown to depend on the different effects on chain packing of the radius of the ions occupying these two sites as compared with each other and with the sites which are wholly inside the talc-like strips.

Electron diffraction study of evaporated carbon films

Abstract: T h e s t r u c t u r e of e v a p o r a t e d c a r b o n f i lms ha s b e e n s t u d i e d b y e l ec t ron d i f f r ac t i on u s ing a c a m e r a w i t h a r o t a t i n g sec tor . F i l m s of 100 A t h i c k n e s s were used . E l e v e n ha los w e r e o b s e r v e d in t h e r a n g e of s = (4~/)l) s in 0 < 33 A -1. B o t h t h e r ad ia l d i s t r i b u t i o n a n d t h e co r r e l a t i on m e t h o d s were a p p l i e d to t h e ana lys i s of d a t a . R e s u l t s o b t a i n e d a re as fol lows. T w o k i n d s of b o n d d i s t a n c e s ex i s t in t h e f i lm. One is t h e g r a p h i t e l ike 1.41 A, a n d t h e o t h e r is t h e d i a m o n d l i k e 1-55 A. T h e n u m b e r of t h e d i a m o n d l i k e d i s t a n c e s is s o m e w h a t l a rge r t h a n t h a t of t h e g r a p h i t e l i k e d i s t ances . M e a n d e v i a t i o n s of t h e s e e q u i l i b r i u m d i s t a n c e s a re 0.11 N 0.12 A, w h i c h are a b o u t twice as la rge as t h o s e f o u n d in free molecu le s . T h e a t o m i c d i s t r i b u t i o n a r o u n d a n y a t o m b e c o m e s u n i f o r m b e y o n d severa l A n g s t r 6 m un i t s . T h e p r o b a b l e a t o m i c a r r a n g e m e n t in t h e f i lm is a t h r e e d i m e n s i o n a l r a n d o m n e t w o r k c o n s i s t i n g of g r a p h i t e l i k e a n d d i a m o n d l i k e c o n f i g u r a t i o n s . A m o d e l of s u c h a t o m i c a r r a n g e m e n t is p r o p o s e d a n d c o m p a r e d w i t h s t r u c t u r e s of o t h e r a m o r p h o u s ca rbons .

Determination of lattice parameters with the aid of a computer

Abstract: A least squares method is described which permits the determination of lattice parameters and standard errors for the orthorhombic system and all other crystal systems of higher symmetry. The angular measurements of lines or spots of several different wave lengths may he used. Provision was made for using as many as three separate correction terms for eccentricity, absorption, divergence, and other variables. Experience has indicated that a weighting factor usually gives a better fit with a smaller standard error. Since a problem can be solved in a short time, it is usually advantageous to obtain several solutions to the problem using various combinations of correction terms and weighting factors in order to verify the fitting process and the accuracy of the lattice parameters. (B.O.G.)

Slit height corrections in small angle X-ray scattering

Abstract: A method of correcting experimental small angle x-ray scattering curves for the effects of the height of the collimating slits is developed. The method has the advantage of eliminating the need for numerical differentiation of the experimental curve. An expression suitable for numerical calculation is given, and an analysis is made of the error in the numerical approximation. The results of some applications of these slit correction techniques are discussed. (auth)

The structure of the intermetallic phase θ(Cr-Al)

Abstract: The monoclinic theta -phase in the chromium-aluminum system was shown to be isomorphous with alpha '(V-Al). The structure was refined and the interatomic distances are discussed and compared with those in alpha '(V - Al) and with those in two ternary alloy phases containing chromium and aluminum. (auth)

The accurate determination of the position and shape of heavy‐atom replacement groups in proteins

Abstract: Green, Ingrain & Perutz (1954) introduced the method of isomorphous replacement with heavy atoms to the X-ray analysis of the centro-symmetric zone of horse haemoglobin. In order to determine the phase angle of general reflections multiple isomorphous replacement has to be used, involving the preparation of a series of isomorphous compounds each bearing one or more heavy atoms placed on different sites (Kendrew et al., 1958). The 'heavy atoms' in proteins may often consist of sizeable organic residues to which one or more heavy atoms are attached, producing irregular 'lumps' on an electron-density map. Patterson coefficients ([FHeavy I --IFl)e can be used to determine the position of the heavy atoms in centrosymmetric projections. However, for non-centrosymmetric crystals there exists the problem of determining the position of one atom relative to another without a fixed symmetry element as origin. The first solution of this problem is due to Harker (1956) who considers trial-and-error techniques based on selected reflections. Bragg (1958) suggested a method of fitting sinusoidal curves to the envelope of the differences plotted as a function of sin 0, which can then be shown to correspond to the structure-factor graph of the heavy-atom content of the unit cell. Perutz (1956) proposed two correlation functions, later modified and improved by Blow (1958), which depend on calculating certain Fourier summations from which the heavyatom positions can be deduced. The great advantage of the Perutz-Blow approach is that an overall picture of the cell is obtained, so that subsidiary sites can be discovered. The present paper deals with another Fourier technique which is believed to possess the advantage of giving less background effect, thus leaving no doubt in placing the atomic centre within about 0.4 A from data with 6A resolution.