Powder diffraction

Powder diffraction is a scientific technique using X-ray, neutron, or electron diffraction on powder or microcrystalline samples for structural characterization of materials [2].

Explanation

Ideally, every possible crystalline orientation is represented equally in a powdered sample. The resulting orientational averaging causes the three dimensional reciprocal space that is studied in single crystal diffraction to be projected onto a single dimension. The three dimensional space can be described with (reciprocal) axes x*,y* and z* or alternatively in spherical coordinates q,φ*,χ*. In powder diffraction intensity is homogeneous over φ* and χ* and only q remains as an important measurable quantity. In practice, it is sometimes necessary to rotate the sample orientation to eliminate the effects of texturing and achieve true randomness.

When the scattered radiation is collected on a flat plate detector the rotational averaging leads to smooth diffraction rings around the beam axis rather than the discrete Laue spots as observed for single crystal diffraction. The angle between the beam axis and the ring is called the scattering angle and in X-ray crystallography always denoted as 2θ. (In scattering of visible light the convention is usually to call it θ). In accordance with Bragg's law, each ring corresponds to a particular reciprocal lattice vector $G$ in the sample crystal. This leads to the definition of the scattering vector as:

$G\ =\ q\ =\ 2\ k\ \sin(\theta) = 4\ \pi\ \sin(\theta)\ /\ \lambda$

Powder diffraction data are usually presented as a diffractogram in which the diffracted intensity I is shown as function either of the scattering angle 2θ or as a function of the scattering vector q. The latter variable has the advantage that the diffractogram no longer depends on the value of the wavelength λ. The advent of synchrotron sources has widened the choice of wavelength considerably. To facilitate comparability of data obtained with different wavelengths the use of q is therefore recommended and gaining acceptability.

An instrument dedicated to perform powder measurements is called a powder diffractometer, but there is a variety of systems each with their strong and their weak points.

Uses

Relative to other methods of analysis, powder diffraction allows for rapid, non-destructive analysis of multi-component mixtures without the need for extensive sample preparation [3]. This gives laboratories around the world the ability to quickly analyse unknown materials and perform materials characterization in such fields as metallurgy, mineralogy, forensic science, archeology, condensed matter physics, and the biological and pharmaceutical sciences. Identification is performed by comparison of the diffraction pattern to a known standard or to a database such as the International Centre for Diffraction Data's Powder Diffraction File (PDF) or the Cambridge Structural Database (CSD). Advances in hardware and software, particularly improved optics and fast detectors, have dramatically improved the anaytical capability of the technique, especially relative to the speed of the analysis. The fundamental physics upon which the technique is based provides high precision and accuracy in the measurement of interplanar spacings, sometimes to fractions of an Ångström, resulting in authoritative identification frequently used in patents, criminal cases and other areas of law enforcement. The ability to analyze multiphase materials also allows analysis of how materials interact in a particular matrix such as a pharmaceutical tablet, a circuit board, a mechanical weld, a geologic core sampling, cement and concrete, or a pigment found in an historic painting. The method has been historically used for the identification and classification of minerals, but it can be used for any materials, even amorphous ones, so long as a suitable reference pattern is known or can be constructed.

Phase identification

The most widespread use of powder diffraction is in the identification and characterisation of crystalline solids, each of which produces a distinctive diffraction pattern. Both the positions (corresponding to lattice spacings) and the relative intensity of the lines are indicative of a particular phase and material, providing a "fingerprint" for comparison. A multi-phase mixture, e.g. a soil sample, will show more than one pattern superposed, allowing for determination of relative concentration.

J.D. Hanawalt, an Analytical Chemist who worked for Dow Chemical in the 1930s, was the first to realize the analytical potential of creating a database. Today it is represented by the Powder Diffraction File (PDF) of the International Centre for Diffraction Data (formerly Joint Committee for Powder Diffraction Studies). This has been made searchable by computer through the work of global software developers and equipment manufacturers. There are now over 550,000 reference materials in the 2006 Powder Diffraction File Databases, and these databases are interfaced to a wide variety of diffraction analysis software and distributed globally. The Powder Diffraction File contains many subfiles, such as minerals, metals and alloys, pharmaceuticals, forensics, excipients, superconductors, semiconductors etc., with large collections of organic, organometallic and inorganic reference materials.

Crystallinity

In contrast to a crystalline pattern consisting of a series of sharp peaks, amorphous materials (liquids, glasses etc.) produce a broad background signal. Many polymers show semicrystalline behavior, i.e. part of the material forms an ordered crystallite by folding of the molecule. One and the same molecule may well be folded into two different crystallites and thus form a tie between the two. The tie part is prevented from crystallizing. The result is that the crystallinity will never reach 100%. Powder XRD can be used to determine the crystallinity by comparing the integrated intensity of the background pattern to that of the sharp peaks. Values obtained from powder XRD are typically comparable but not quite identical to those obtained from other methods such as DSC.

Lattice parameters

The position of a diffraction peak is independent of the atomic positions within the cell and entirely determined by the size and shape of the unit cell of the crystalline phase. Each peak represents a certain lattice plane and can therefore be characterized by a Miller index. If the symmetry is high, e.g. cubic or hexagonal it is usually not too hard to identify the index of each peak, even for an unknown phase. This is particularly important in solid-state chemistry, where one is interested in finding and identifying new materials. Once a pattern has been indexed, this characterizes the reaction product and identifies it as a new solid phase. Indexing programs exist to deal with the harder cases, but if the unit cell is very large and the symmetry low (triclinic) success is not always guaranteed.

Expansion tensors, bulk modulus

Cell parameters are somewhat temperature and pressure dependent. Powder diffraction can be combined with in situ temperature and pressure control. As these thermodynamic variables are changed, the observed diffraction peaks will migrate continuously to indicate higher or lower lattice spacings as the unit cell distorts. This allows for measurement of such quantities as the thermal expansion tensor and the isothermal bulk modulus, as well determination of the full equation of state of the material.

Phase transitions

At some critical set of conditions, for example 0 °C for water at 1 atm, a new arrangement of atoms or molecules may become stable, leading to a phase transition. At this point new diffraction peaks will appear or old ones disappear according to the symmetry of the new phase. If the material melts to an isotropic liquid, all sharp lines will disappear and be replaced by a broad amorphous pattern. If the transition produces another crystalline phase, one set of lines will suddenly be replaced by another set. In some cases however lines will split or coalesce, e.g. if the material undergoes a continuous, second order phase transition. In such cases the symmetry may change because the existing structure is distorted rather than replaced by a completely different one. E.g. the diffraction peaks for the lattice planes (100) and (001) can be found at two different values of q for a tetragonal phase, but if the symmetry becomes cubic the two peaks will come to coincide.

Crystal structure determination

Although in many cases this is neither desirable nor possible, Rietveld refinement does allow the full fitting of the intensity profile of a diffractogram to determine the crystal structure[4] as well as other characteristics like preferred orientation (texture), crystallite size and -in the case of neutron patterns- even the magnetic structure, i.e. the spatial distribution of magnetic moments in the structure if the material shows magnetic order.

There are many factors that determine the width B of a diffraction peak. These include:

1. instrumental factors
2. the presence of defects to the perfect lattice
3. differences in strain in different grains
4. the size of the crystallites

It is often possible to separate the effects of size and strain. Where size broadening is independent of q (K=1/d), strain broadening increases with increasing q-values. In most cases there will be both size and strain broadening. It is possible to separate these by combining the two equations in what is known as the Hall-Williamson method:

B.cosθ = /D + η sinθ

Thus, when we plot B.cosθ vs. sinθ we get a straight line with slope η and intercept /D.

The expression is a combination of the Debye-Scherrer formula for size broadening and the Stokes and Wilson expression for strain broadening. The value of η is the strain in the crystallites, the value of D represents the size of the crystallites. The constant k is typically close to unity and ranges from 0.8-1.39.

Magnetic structures and the detection of hydrogen

X-ray photons scatter by interaction with the electron cloud of the material, neutrons are scattered by the nuclei. This means that the scattering power for XRD is roughly linear with atomic number, but fluctuates from isotope to isotope for neutrons. Both protons and deuterons (the stable nuclei of the element hydrogen) are strong scatterers for neutrons. The one electron of the atom makes it difficult to detect by X-rays. This makes neutron powder diffraction an attractive way to gain better insight of the precise location of hydrogen in a structure.

Neutrons can also undergo scattering if the material has an ordered magnetic structure. Neutrons have a spin and thus a small magnetic moment which interacts with the magnetic moments of electrons in an incomplete shell. The powder pattern can be used to determine the magnetic structure.

Aperiodically-arranged clusters

Predicting the scattered intensity in powder diffraction patterns from gases, liquids, and randomly-distributed nano-clusters in the solid state[5] is (to first order) done rather elegantly with the Debye scattering equation[6]:

$I_{powder}[g]=\sum_{i=1}^{N}\sum_{j=1}^{N}f_i[g]f_j[g]\frac{\sin[2\pi g r_{ij}]}{2\pi g r_{ij}},$

where g = q/(2π) is the magnitude of the scattering vector q in reciprocal lattice distance units, N is the number of atoms, fi[g] is the atomic scattering factor for atom i and scattering vector g, while rij is the distance between atom i and atom j. One can also use this to predict the effect of nano-crystallite shape on detected diffraction peaks, even if in some directions the cluster is only one atom thick.

Devices

Cameras

The simplest cameras for X-ray powder diffraction consist of a small capillary and either a flat plate detector (originally a piece of X-ray film, now more and more a flat-plate detector or a CCD-camera) or a cylindrical one (originally a piece of film in a cookie-jar, now more and more a bent position sensitive detector). The two types of cameras are known as the Laue and the Debye-Scherrer camera.

To promote randomization the capillary is usually spinning around its axis.

For neutron diffraction vanadium cylinders are used as sample holders. Vanadium is all but transparent for neutrons. The element hardly scatters at all.

A later development in X-ray cameras is the Guinier camera. It is built around a focusing bent crystal monochromator. The sample is usually placed in the focusing beam., e.g. as a dusting on a piece of sticky tape. A cylindrical piece of film (or electronic multichannel detector) is put on the focusing circle, but the incident beam prevented from reaching the detector to prevent damage from its high intensity.

Diffractometers

Diffractometers can be operated both in transmission and in reflection configurations. The reflection one is more common. The powder sample is filled in a small disc like container and its surface carefully flattened. The disc is put on one axis of the diffractometer and tilted by an angle θ while a detector (scintillation counter) rotates around it on an arm at twice this angle. This configuration is known under the name Bragg-Brentano.

Another configuration is the theta-theta configuration in which the sample is stationary while the x-ray tube and the detector are rotated around it. The angle formed between the tube and the detector is 2theta. This configuration is most convenient for loose powders.

The availability of position sensitive detectors and CCD-cameras is making this type of equipment more and more obsolete.

Neutron diffraction

Sources that produce a neutron beam of suitable intensity and speed for diffraction are only available at a small number of research reactors and spallation sources in the world. To compensate for the low flux of neutrons, they usually have a battery of individual detectors arranged in a cylindrical fashion around the sample holder, and can therefore collect scattered intensity simultaneously on a large 2θ range.

X-ray tubes

Laboratory X-ray diffraction equipment relies on the use of an X-ray tube, which is used to produce the X-rays.

For more on how X-ray tubes work, see for example here or X-ray.

The most commonly used laboratory X-ray tube uses a Copper anode, but Cobalt, Molybdenum are also popular. The wavelength in nm varies for each source:

 Element Kα (weight average) Kα2 (strong) Kα1 (very strong) Kβ (weak) Cr 0.229100 0.229361 0.228970 0.208487 Fe 0.193736 0.193998 0.193604 0.175661 Co 0.179026 0.179285 0.178897 0.162079 Cu 0.154184 0.154439 0.154056 0.139222 Mo 0.071073 0.071359 0.070930 0.063229

Other sources

In house applications of X-ray diffraction has always been limited to the relatively few wavelengths shown in the table above. The available choice was much needed because the combination of certain wavelengths and certain elements present in a sample can lead to strong fluorescence that interferes with the observation of the diffraction pattern. A notorious example is the presence of iron in a sample when using copper radiation. In general elements just below the anode element in the period system need to be avoided.

Another limitation is that the intensity of traditional generators is relatively low, requiring lengthy exposure times and precluding any time dependent measurement. The advent of synchrotron sources has drastically changed this picture and caused powder diffraction methods to enter a whole new phase of development. Not only is there a much wider choice of wavelengths available, the high brilliance of the synchrotron radiation makes it possible to observe changes in the pattern during chemical reactions, temperature ramps, changes in pressure and the like. One could say that X-ray data are no longer limited to stills but now come as movies.

The tunability of the wavelength also makes it possible to abserve anomalous scattering effects when the wavelength is chosen close to the absorption edge of one of the elements of the sample.

Neutron diffraction has never been an in house technique because it requires the availability of an intense neutron beam only available at a nuclear reactor. Typically the available intensity requires relative large samples, which has traditionally limited the technique taking stills of powders. There are important developments afoot however (spallation sources) that may change this situation and facilitate single crystal work and movies.

Although it possible to solve crystal structures from powder X-ray data alone, its single crystal analog is a far more powerful technique for structure determination. This is directly related to the fact that much information is lost by the collapse of the 3D space onto a 1D axis. Nevertheless powder X-ray diffraction is a powerful and useful technique in its own right. It is mostly used to characterize and identify phases rather than solving structures.

The great advantages of the technique are:

• simplicity of sample preparation
• rapidity of measurement
• the ability to analyze mixed phases, e.g. soil samples

By contrast growth and mounting of large single crystals is notoriously difficult. In fact there are many materials for which despite many attempts it has not proven possible to obtain single crystals. Many materials are readily available with sufficient microcrystallinity for powder diffraction, or samples may be easily ground from larger crystals. In the field of solid-state chemistry that often aims at synthesizing new materials, single crystals thereof are typically not immediately available. Powder diffraction is therefore one of the most powerful methods to identify and characterize new materials in this field.

Particularly for neutron diffraction, which requires larger samples than X-Ray Diffraction due to a relatively weak scattering cross section, the ability to use large samples can be critical. In neutron diffraction therefore the powder technique is more the norm and the single crystal one the exception, although new more brilliant neutron sources are being built that may change this picture. Structure determination by neutrons has the great advantage that hydrogen is a strong scatterer whereas for X-rays it is almost invisible.

Since all possible crystal orientations are measured simultaneously, collection times can be quite short even for small and weakly scattering samples. This is not merely convenient, but can be essential for samples which are unstable either inherently or under x-ray or neutron bombardment, or for time-resolved studies. For the latter it is desirable to have a strong radiation source. The advent of synchrotron radiation has therefore done much to revitalize the powder diffraction field because it is now possible to study temperature dependent changes, reaction kinetics and so forth by means of time dependent powder diffraction