# AN INTRODUCTION TO X-RAY CRYSTALLOGRAPHY WOOLFSON PDF

An Introduction to X-ray Crystallography. An Introduction to X-ray . Michael M. Woolfson, University of York. Publisher: . pp i-vi. Access. PDF; Export citation. An Introduction to X-Ray Crystallography - Ebook download as PDF File .pdf), Text File was shown by Lipson and Woolfson () and by Rogers and Wilson. X-ray crystallography. Contents. 1. The geometry of the crystalline state; 2. The scattering of X-rays; 3. Diffraction from a crystal; 4. The Fourier transform; 5.

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Cambridge University Press, Cambridge, UK - - p. This is a textbook for the senior undergraduate or graduate student beginning a. Mai M. M. WOOLFSON. An Introduction to X‐Ray Crystallography. Cambridge University Press , Seiten, Abbildungen Preis £ First published: July resourceone.info About. Figures; References; Related; Information. ePDF PDF. PDF · ePDF PDF · PDF.

We show that much experimental detail of dynamical motion is already present in X-ray crystallographic data, which arises from being solved by different research groups using different methodologies under different crystallization conditions, which then capture an ensemble of structures whose variations can be quantified on a residue-by-residue level using local density correlations. We show that measuring structural variations across an ensemble of X-ray derived models captures the activation of conformational states that are of functional importance just above TD and they remain virtually identical to structural motions measured at K.

It provides a novel analysis of large X-ray ensemble data that is useful for the broader structural biology community. PLoS Comput Biol 6 8 : e This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Department of Energy. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Introduction It has been suggested that at temperatures below the protein dynamical transition temperature, TD, there is a dominant native basin in which a protein's dynamics is largely controlled by harmonic motions [1].

Above this temperature, a sudden activation of new anharmonic protein motions that are thought to be dependent on a more fluid solvent environment [2] , correlates with a rapid enhancement of enzymatic function in most cases. We shall refer the scattering from each component array of atoms to that which would be obtained from an array of point electrons at the origin points of the unit cell. Hence there will be a diffracted beam from the whole crystal only when the Laue equations are satisfied and the Laue equations can thus be seen to be the controlling factor for diffraction from any three-dimensional periodic arrangement of atoms.

The total scattered amplitude from the crystal will thus be that from an array of electrons. Imaginary axis a Phase-vector diagram for a structure factor with contributions from four atoms. Thus for a centrosymmetric unit cell. A phase-vector diagram for the structure factor of a centrosymmetric structure shows very convincingly why the F is real. This only confirms what we know from physical considerations anyway. An important case. At this time it had been deduced indirectly from the geometry of crystals that they were almost certainly in the form of a three-dimensional periodic array of atoms.

From the known values of Avogadro's number. The two points we shall consider are A and A' separated by the distance d which is the interplane spacing [fig. To do this we must bring in the distance between the parallel reflecting planes. This inspired prediction was confirmed by Friedrich and Knipping whose X-ray pictures.

In However this condition would be true for any value of 6 and we must now explain why it is only for special angles that the reflection can occur.

## Woolfson M.M. An introduction to X-ray crystallography

Since reflections from all points in one plane are in phase. When a parallel wave train falls on a specularly reflecting surface the conditions are such that reflection from any two points of the surface will give rays in phase in the reflected beam.

Bragg gave the first mathematical explanation of the actual positions of the X-ray diffraction spots. We can follow the main lines of his explanation by considering diffraction by a two-dimensional array of scatterers as infig.

A number of sets of equi-spaced parallel lines are shown which pass through all the scatterers and. A selection of sets of parallel equally spaced lines on which lie all points of an array of scatterers.

For reasons which will be clearer later we shall only Fig. From fig. If we look at the atom-rich lines in fig. It can be seen now why we refer to the diffracted beams as 'reflections' and why the angle between the incident and diffracted beam is denoted by 29 and not simply by 9. These we shall refer to as the indices of the X-ray reflection and the spacing of the planes so defined is denoted by dhkl.

Now the latter lines are not all atom- rich. Let us see if we can now relate Bragg's law to the Laue equations for. For three dimensions this generalizes to being able to consider reflection from planes with indices nh nk nl as the nth-order reflection from planes with indices hkl. Successive hkl reflecting planes with Direction of interplanar spacing dhkl. If we look again at fig.

## 1. Introduction

We could construct a reciprocal lattice by first. If the cell edges are denoted by the vectors a. This situation is shown in fig. In the foregoing description of Bragg's law we have treated only the case of one atom per unit cell.

Thus we say that an atom has coordinates x. Whether or not one is formally justified in thinking of a diffraction process in terms of specular-type reflection is not too certain from the physical point of view but. Thinking of X-ray diffraction in terms of Bragg's law is often very convenient and we shall have recourse to this description of the diffraction process from time to time. A two-dimensional structure with three different atoms showing interleaved reflecting planes.

From this and the relationships 3. This important intensity relationship is known as Friedel's law. The form of the structure-factor equation. Equation 3. The non- centrosymmetric monoclinic space group is P2V The cell dimensions are: If the X-ray wavelength is 1.

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Garcia-Blanco Ada Cryst. The projection down the b axis has the centrosymmetric two-dimensional space group p2. It contains facilities. Perales and S. Find what orders of diffraction will be produced and the angle the diffracted beams will make with the row. This file should be kept as it may be used as input for Problems to other chapters.

Problems to Chapter 3 3. Calculate the observable structure factors for this projection taking all necessary information from Problem 3. Problems to Chapter 3 75 Table 3. Coordinates for Problem 3. We might look at the form of some of the terms which are included in the summation.

We need not be too troubled by the various mathematical restrictions placed on the function f X. Other results follow from equations 4. What we are doing here is to extend the range of indices to all integers. The relationships between the Fourier coefficient which are given in equation 4. If we are dealing with one- dimensional functions then the equations we shall want to use are 4.

To illustrate the application of these relationships we shall consider the function shown infig. If one formally goes through the process of using equation 4.

This could be predicted from the fact that f X is an even function of X i. In the early days of crystallography. Simulation of the function shown in fig. It is a very simple program. The summations shown infig. An early and important contribution to this problem was made by Beevers and Lipson who precalculated components of individual contributions to the series on strips of cardboard which were provided in convenient-to-use wooden boxes.

The computed values are shown by dots. Often it is convenient to define a function in the range of X from 0 to a and it is clear that the Fourier series can be applied to this range.

Thus the summation gives a periodic function of spacing a and the Fourier coefficients Ah corresponding to the f X shown infig. Let us now examine the properties of the Fourier series. By placing strips under each other. A typical Beevers-Lipson strip is shown infig.

Such a function. The periodic function reproduced by the Fourier series which is derived from the function i X shown in fig. Let us suppose that we have a periodic function f x but that f x is now not an analytical function but one which is known in graphical or tabular form.

This replaces an integral N h by. For example if we have a two-dimensional. Using this method the integrals in equation 4. The small disagreements are due to rounding off the calculated values of Ah and Bh and the termination of the series. These results may be compared with the tabulated values of the original function given in fig. A non-analytical function f x shown graphically and by a table. Simpson's Rule is equivalent to adding the areas of parabolae fitted to three function values at points defining successive pairs of intervals.

The coefficients found by equations 4. Y defined in a parallelogram whose sides are the vectors a and b. A reconstruction of the function shown in fig. It is not quite so straightforward to represent the terms in a two. Z defined in the parallelepiped given by the vectors a. If in equations 4.

The final equations corresponding to 4. Negative regions are shaded. But if a is very large. Sometimes the integral in equation 4. The sum in equation 4. However we may now take out the 'real-part' symbol within the integration 4. The Fourier transform of f r is F s. The two signs can both be reversed. We can see the mutually reciprocal nature of the two quantities r and s from the relationships 4. This is a characteristic relationship between a function and its Fourier transform.

The position of a point in the parallelepiped whose sides are the vectors a. We shall now generalize our description of Fourier transforms to the three-dimensional function given in equations 4. The Gaussian function 4. This function has the following properties: The inverse relationship of the width of these functions can be readily seen.

We can see this by considering the transforms of two functions fx X and f2 X. The function f r and F s are related by the equations 4.

## An Introduction to X-Ray Crystallography

This result can be extended to three dimensions. D We can have a 5-function located at a point other than the origin. Then if we take 4. A set of 5-functions located at the nodes of an infinite three-dimensional lattice defined by the vectors a.

D ]j is the amplitude of scattering at a distance D from thejth scatterer at an angle 26 with the incident radiation. If we have a continuous distribution of electron density p r. For a particular value X the formation of the function g X — u is also shown in fig.

From this we find that the total scattered amplitude from the distribution of electron density p r.

## Diffraction Techniques in Structural Biology

The relationship between diffraction and Fourier-transform theory has now been established and the application of Fourier-transform ideas to the consideration of X-ray diffraction will be found to be most useful.

Thus the scattered amplitude from such a small volume will be p r dv times as much as that from an electron at the same position. We can also infer from equation 4. Then the integral 4. The result of doing this for all values of X completely defines the function c X and.

If equation 4. The operation by which g X — u is produced from g u is referred to as folding g w about X. Another method. The steps in the process are: I b The superposition of 85 the functions 90 f w. There is a theorem. In b the eight weighted and displaced images of f w are shown and in c they are added to give the total function c X. From equation 4. In a there is shown the division of g w into eight strips and the relative areas are given.

The proof of this result is fairly straightforward and is now given. We can express this as F s G s exp. This can also be found by applying the expression 4. If the function g X is normalized to have unit area under the curve then the convolution c X differs very little from f X. Thus we have the general result that the Fourier transform of a product of functions is the convolution of the transforms of the individual functions.

Extending the application of equation 4. This is. This process is taken another stage by considering the convolution of f X with an infinite linear array of 8 functions given by. This point is illustrated in fig. Thus it may be seen that any periodic pattern of spacing a may be considered as the convolution of one unit of the pattern with a 5-function at the nodes of a lattice of spacing a.

The result may be extended to three dimensions. A three-dimensional periodic function may be considered as the convolution of one unit of the pattern, contained within the parallelepiped defined by the vectors a, b and c, with a set of Fig.

In particular, the electron density within a crystal may be considered as the convolution of the density within one unit cell with the three-dimensional real-space lattice of spacing a, b and c.

The scattered amplitude, as a fraction of that scattered by a point electron, is the Fourier transform of the electron density. Thus, from equation 4. The structure factor is also given by equation 4. Thus we may take equation 4.

In this equation if Fhkl is the normal structure factor then p r will be the electron density expressed in electrons per unit volume.

The electron. The electron-density equation can take different forms depending on the space group of the crystal structure. The various forms of the structure-factor equation 3.

The structure-factor equation 4. If we now imagine that we project all the electron content of the unit cell along z on to the xy plane then the electron density at the point xy , expressed as electrons per unit area, will clearly be given by , i.

It can be seen that Fhk0 is the Fourier transform of the two-dimensional function, the projected electron density pp xy.

Consequently, by analogy with our previous results, we may write. The X-ray crystallographer frequently wishes to compute electron density since this gives a picture of the atomic arrangement in the crystal. At the resolution corresponding to the X-ray wavelengths usually used, the electron density has its highest value at the location of the atomic centre and the separate atoms of the structure are usually well resolved.

The electron- density equation 4. From the equations 4. In equation 4. The z-axis projection of D-xylose. The positions of carbon and oxygen atoms are shown by the line framework. From Woolfson, If the phases of the structure factors are known then the crystal structure is known, for one can compute the electron density from equation 4.

The electron density is usually computed at the points of a uniform grid covering the unit cell and contours of constant electron density may then be drawn. An example of such a contour map is given in fig.

It will be noticed that the carbon c and oxygen o atoms may all be clearly seen but no trace of the hydrogen atoms is evident. The contribution of the single electron of the rather diffuse hydrogen atom has been swamped by the contributions of the heavier atoms.

However, with careful experimental measurements and special computing techniques hydrogen atoms can be detected and we shall return later to this problem.

It is more difficult to present an electron-density map in three dimensions. This can be done by computing two-dimensional sections keeping one coordinate constant. These can be examined section-by-section or reproduced on transparent material glass or plastic and stacked so as to get a direct three-dimensional view of the electron density.

The advent of modern computer graphics has given the crystallographer very powerful ways of examining and utilizing three-dimensional maps. On the computer screen there can be projected stereoscopic pairs of images of surfaces of constant electron density, reproduced by cage-like structures, and these are accompanied by viewing glasses. In one system the stereo images are projected on the screen alternately and glasses worn by the viewer are equipped with opto-electronic devices synchronized with what is happening on the screen so that the left and right eye see different images.

If the switching rate is high enough, greater than about 30 Hz, then there will be no flicker and a constant three-dimensional image will be seen. With a three-dimensional view of the electron density available it is then possible.

To give an idea of the power of a stereoscopic image there is shown infig. By holding the figure at a comfortable distance many readers will be able to fuse the two images to obtain a stereoview of the molecules. The most efficient programs for carrying out this kind of calculation employ the so-called Fast Fourier Transform algorithm.

However, although FOUR2 is less sophisticated in its approach it does have some efficient features such as factorizing the calculation in the x and y directions and using some of the symmetry of cosine and sine functions. Transforming equation 4. One of the first decisions to be made when computing an electron-density map is the grid spacing to be used along each of the unit-cell axes.

If the grid is too fine a great deal of needless computing is done whereas if the grid is too coarse then interpolation between grid points will be uncertain. The output of FOUR2 is scaled values of projected electron density at grid points. To visualize the actual projected density, contours of constant projected density can be drawn. An example of part of a map with density contours is shown in fig.

Part of the projected a. I level of 20 and 30 in the arbitrary scaled units of projected density V — 27T. Over what portion of the unit cell must p xyz be computed in order to completely determine the structure? Periodic function for Problem 4. Problems to Chapter 4 4.

If the incident X-rays make an angle jpi — 6 with s then a diffracted beam is produced coplanar with the incident beam and s. The motion of the Rotation axis scattering vector s as A the crystal rotates.

As the crystal rotates about OA so does the vector s and the angle between s and — S o decreases until the Fig. Since the vector — s corresponds to the reflection with indices hk T which is. As it does so the angle between the direction of the incident beam. Let us see how the crystal can be systematically moved to a position for a particular diffracted beam to be produced. Path of end of vector s. Further rotation of the crystal and of s reduces the angle between s and — S o until finally s is again in the plane defined by 10A and the angle between s and — S o is equal to n — a [fig.

It is. The variation of these angles as the crystal is rotated through 2it is plotted in fig. Perpendicular planes containing all directions shown ib position shown in fig. The situation we have considered here. There is an interesting geometrical way of looking at the diffraction condition which can be very helpful in considering more general axes of rotation.

Angle between Angle between o L s and. When the crystal is in such a position that the vector s is along OP or OQ a diffracted beam is produced with indices hkl appropriate to the vector s. The sphere of reflection.

By rotation of the crystal about some axis any diffracted beam can be produced as long as the end of the scattering vector can be made to lie on the surface of the sphere. The direction of the diffracted beam can also be found from this geometrical construction. Sphere of Incident beam When the tip of the reflection direction vector s touches the sphere the corresponding reflection is produced. The condition for this is that Fig. One can now follow the process of rotating the crystal about an axis not perpendicular to the incident beam by using the concept of the sphere of reflection.

Hence CP is parallel to OD and gives the direction of the diffracted beam. The condition for a diffracted beam to be produced is now seen to be that the end of the scattering vector s.

If a possible reflection is not produced by rotation of the crystal about some particular axis then the situation must be like that shown in fig. With these ideas at our disposal we can now consider the various types of apparatus by which X-ray diffraction data may be collected. The powder is usually formed into the shape of a cylinder either by containing it in a plastic or thin-walled glass container or by mixing the powder with an adhesive material and shaping it before it.

It should be noticed that the reflection corresponding to — s i. The end of the vector s describes a circular path which does not intersect the sphere.

Such a specimen will be a myriad of tiny crystallites arranged in random orientations. This is necessary as otherwise this radiation would be scattered by thefilmand fog it over a considerable area thereby obscuring some of the diffraction pattern at low Bragg angles. Powder cameras vary somewhat in their design but the arrangement of components for a fairly typical type of camera is shown schematically in fig. When thefilmis straightened out it appears as in fig.

The direct beam. Any crystallite giving the reflection will produce a diffracted beam making an angle 20 with the incident beam and. Frequently the data required by the X-ray crystallographer are a Fig. If the powder is fairly coarse so that the crystallites are larger in size and fewer in number then the diffraction lines will not be uniform but tend to be rather spotty as infig.

This can be overcome by rotating the specimen as then many more of the crystallites contribute to each of the diffraction lines. It should be mentioned here. The locus of the Diffracted beams from diffracted beams for a different crystallites given hkl from all the crystallites of a powder specimen. The diffraction pattern is recorded by thefilmas the intersection of cones similar to the one shown infig.

Let us examine what happens when a monochromatic X-ray beam falls on such a specimen. Direct beam. For a particular reflection there will be a large number of the crystallites in a diffracting position. Thefilmsshown infig. A beam of monochromatic X-rays is collimated by passing it through afinetube or through a slit system and falls on the specimen.

Motor for roiaiing specimen Beam rap Film I c Fig. The overlap problem is so severe that powder photographs are normally not used for the purposes of collecting data for structure determination unless the alternative of obtaining data from a single crystal is. In fact many diffracted beams will have similar 26 values similar magnitudes of s.

A combination of new needs. For space groups of higher symmetry there may be fourfold. The total volume within the limiting sphere is and the volume of the reciprocal unit cell will be. Many materials of great potential technological importance.

Their powder diffraction patterns may be collected with a powder diffractometer. If two or more beams fall on top of one another then it is impossible to ascertain the intensities of the individual components of the composite line and valuable information is lost.

The resultant intensity profile. To get some idea of this problem let us consider how many reflections might be obtained for a crystal structure with a moderate-sized unit cell - say of volume A3. The incident highly monochromatic X-ray beam is very well collimated which means that the angular spread of the powder lines is reduced as much as possible.

The convolution of the two gives the profile shown infig. However the cross-sections of the diffraction lines will all be of the same shape. There have been a number of successful computer-based methods for carrying out the deconvolution of powder patterns but.

For example for a cubic crystal with cell dimension a it is found from equation 3. The theoretical basis of this line spread will be found in equation 3. The problem is one of deconvolution. This is because the small crystallites of which the powder specimen is formed will diffract over some small angular region around the Bragg angle.

The deconvolution problem is to find fig. There are some deconvolution problems which cannot be solved by any form of analysis. Although the incident X-ray beam is well collimated and highly monochromatic which. Powder cameras are often used for the identification of materials. This will occur with pairs of indices [ An important contribution to the use of powder patterns was made by H. Rietveld in who showed how it was possible to use the whole pattern. Most commercial powder diffractometers will be provided with a range of software for deconvoluting the diffraction patterns and also for carrying out Rietveld refinement.

The number of parameters which could be accommodated by this method is of the order of For a cubic crystal. A large number of materials of all types have the major features of their X-ray powder patterns listed in a set of ordered cards known as the Powder Diffraction File. The relative intensities of powder lines may depend on the shape of the crystallites in the specimen and this may differ from sample to sample.

If the material to be identified is a mixture of two or more materials in unknown proportions then identification of the components can often be extremely tedious.

Consequently identification of materials by the Powder Diffraction File is sometimes not straightforward and requires some patience and ingenuity. The crystal is glued to a suitable Fig. Collimator — Goniometer head Housing for rotating and oscillating mechanism. Powder photographs can also be used to identify completely unknown materials. The information on one of the cards includes the d-spacings which can be derived from the Bragg angle by equation 3.

It may be necessary to ascertain the form of alumina being produced under different sets of conditions and the most convenient way of doing this is by taking X-ray powder photographs. Film A typical arrangement for an oscillation camera. The strongest three lines are displayed at the top of each card and they are listed in the order of the d-spacings of their strongest lines.

These can be compared with a set of photographs covering the complete range of forms of alumina and identification would usually be straightforward and unambiguous. The X-ray beam is collimated and is perpendicular to the axis of rotation of the crystal. There are errors of measurement of distances on the film which are aggravated by thefinitesize of the spots due to the divergence of the X-ray beam.

Adjustments of the arcs and slides enable one to set a prominent axis of the crystal coincident with the axis of rotation of the goniometer head. The goniometer head consists of two perpendicular sets of arcs for rotating the crystal and two perpendicular slides for lateral motion of the crystal. The camera diameter 2r was From the distances between these straight lines. As the crystal is rotated about the a axis then. As the crystal rotates so the X-ray reflections are produced one by one and are recorded by thefilm.

We shall assume for now that a prominent axis has been so set and that it corresponds to the direction of one of the edges of the unit cell. This prominent axis may be one of the main edges of the crystal so that the axis can be set quite accurately by observation through the microscope which is usually attached to the camera. When thefilmis developed and straightened out the diffraction spots are found to lie on a series of straight lines which are the locus of the intersection of the cones of diffraction with the film see fig.

A typical oscillation photograph two-thirds natural size. Although the separation of the diffraction data into layers makes the situation better than for powder photographs there is still nearly always some overlap of diffraction spots.

Plane perpendicular b Locus of zero layer to rotation axis in relation to cylindrical film. Changes in the physical dimensions of thefilmswhen they are processed are also important sources of error. Axis of a Location of the zero rotation layer when the crystal axis is tilted from the Sphere rotation axis in a of reflection vertical plane containing the incident beam. The cameras are provided with mechanisms operated by cams which enable one to oscillate the crystal to-and-fro through a few degrees.

The effect of mis-setting of the crystal in. The photograph in fig. In fact one rarely takes rotation photographs where the crystal rotates continuously. The crystal can also be accurately set about an axis on an oscillation camera and then transferred on its goniometer head to one of the more elaborate cameras which are described later.

With a small angle of oscillation there are many fewer spots on thefilmand the chances of overlap are thereby much reduced.

The cams are so designed that. Oscillation cameras are rarely used for the collection of intensity data but are sometimes used for the preliminary examination of crystals. Let usfirstconsider a crystal the prominent axis of which is displaced by an angle s from the axis of rotation in a plane containing the rotation axis and the X-ray beam.

The intersection of the zero layer with the sphere of reflection is shown for this situation in fig. When thefilmis opened out and placed so that we are looking at it in the direction travelled by the incident X-ray beam then the zero layer appears as infig.

Because the axis of rotation is slightly displaced from the crystal axis. For a tilt in the opposite sense the appearance of the zero layer is shown in fig. Such an appearance indicates that the crystal axis is tilted in a fore-and-aft direction such that the top of the crystal is tilted towards the viewer. The figs.

The sphere of reflection with centre C is shown. Now in this case the layer line in the central region of the film is almost straight and makes an angle s with the 'ideal' line corresponding to a perfectly set crystal.

The indexing of oscillation photographs can best be done by a graphical. If the mis-setting has the major characteristics shown in fig. The other simple type of mis-setting is when the crystal axis is displaced from the rotation axis in a plane perpendicular to the direction of the incident beam.

If both layer lines are recorded on the samefilmand if one. In practice.

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This 'ideal' line does not actually appear on the film. If the mis-setting has the major characteristics offig. By taking a succession of photographs and correcting the setting each time the crystal axis can be set along the rotation axis with a high degree of precision. The angle between the two straight sections of the layer lines is 2s which can be measured easily and the crystal is known to be in the position corresponding to the second weaker layer line.

If we transfer to the actual camera. If P is a reciprocal-lattice point then a diffracted beam is produced in a direction given by CP. As the crystal rotates so does the reciprocal lattice and reciprocal-lattice points pass through the circle of reflection giving diffracted beams as they do so. The relative motion of the circle and the reciprocal lattice can also be shown by keeping the reciprocal. Sphere of b Locus of zero layer reflection in relation to cylindrical Plane film.

Axis of a Location of the zero rotation layer when the crystal Zero layer axis is tilted from the of reciprocal lattice rotation axis in a plane Diametral plane perpendicular to the containing incident beam incident beam. Now let us see how to solve the converse problem.

We know that the points must lie within two areas arranged as in fig. Such afitof arcs to points not all perfect because of errors of measurement is shown in fig.

If a transparent replica of fig. The reader may try for himself the problem of fitting the arcs in fig. In addition it will have demonstrated how useful is the concept of the reciprocal lattice in interpreting the results which one obtains.

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Volume 26 , Issue 4 July Pages Figures References Related Information. Email or Customer ID. Forgot password?Volume 26 , Issue 4 July Pages Thus for a centrosymmetric unit cell.

For space groups of higher symmetry there may be fourfold. First let us derive an expression for the volume of the unit cell. The diffracted beam will then be observed. If we have any repeated pattern in space.

If P is a reciprocal-lattice point then a diffracted beam is produced in a direction given by CP. In a there is shown the division of g w into eight strips and the relative areas are given.

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