BACKGROUND

XRD (X-Ray Diffraction) is a non-destructive technique to characterize crystalline materials. It provides information on structures, phases, preferred crystal orientations (texture), and other structural parameter. XRD analysis is done by shooting a focused X-Ray beam at the sample at a specific angle of incident, which results a diffraction pattern which can be compared to diffraction pattern database. Every crystalline substance gives a pattern. The same substance always gives the same pattern and in a mixture of substances each produces its pattern independently of the others. XRD analysis can be applied to inorganic samples such as salts, minerals, metals, etc and organic samples (biological molecules) such as vitamins, drugs, proteins, and also nucleic acids such as DNA.

HOW DOES IT WORK?

1. XRD analysis, a focused X-Ray beam is shot at the sample at a specific angle of incidence.

2. The X-Rays deflect or "diffract" in various ways depending on the crystal structure (inter-atomic distances) of the sample. The locations (angles) and intensities of the diffracted X-Rays are measured.

3. Crystalline substances have an ordered three-dimensional arrangement with a particular spacing of atoms.

4. When x rays strike the atoms within the crystal, the atoms absorb and reemit the energy from the x rays in the form of spherical wave fronts emanating from each atom. The waves travelling outward from each atom interact with other waves in the processes known as constructive and destructive interference

which results a diffraction pattern. The patterns are controlled by the spacing of atoms within the matrix and are unique to that substance.

Every compound has a unique diffraction pattern. In order to identify a substance, the diffraction pattern of the sample is compared to a library database of known patterns.

Instrument component

Part of XRD Machine An XRD Machine consist of :

1. Source ( X – ray tube)2. Primary Optics ( Lead Screen)3. Sample Holder4. Secondary Optics ( Optional)5. Detectors ( Photographic plate)

1. Source

The source come from the X- ray Tube.What does the source produce?The source produces X- ray that will be used for analyzing the sample.What is an X – ray Tube?An X – ray tube is usually made from evacuated ceramic or glass vessel. A x – ray tube contain a tungsten filament which is located in the cathode. From the

cathode, electron are emitted. The x ray tube also contain an anode where these electrons are accelerated with a potential of several ten thousand volt.

Component of an X – ray tubeCathode - conduct the high voltage current- focus the emitted electrons

Filaments- Very tightly wound coil wire

- When an electric current is passed through the filament, it heats up to such an extent that some of the electrons have enough (thermal vibrational) energy to break free of the attractive (electrostatic) forces holding them inside the filament.

- amount of electrons that are emitted from the filament = amount of electrons flowing inside the filament (i.e., the current).

Anode- positive end of the x-ray tube, target for stream of electron that are

emitted by the cathode

- The anode assembly is important as the source of the x-rays, as the primary conductor of heat out of the tube and as an integral part of the high voltage circuit.

The processaccelerated electrons hit inner shell electrons from atoms of the target metal material, remove them and leave holes behind. These holes are quickly filled from higher level electrons of the same atom. On falling down to the lower energy level the atoms emit characteristic radiation ( X ray) with sharply defined frequencies associated with the difference between the atomic energy levels of the target atoms.

2. Primary Optic

Function

controls the beam produced by the X-ray source, and manipulates it into forms more useful for diffraction experiments

Components of Primary Optics• Soller Slit

reduce the axial divergence of the X- Ray beam to less than 6° = to collimate the X – ray Beam

The divergence slit

It reduces the height divergence (beam spread) and increases the resolution of the output.

Monochromator

Prevent the passing of other lights ( in nm) except for the chosen wavelength

3. Sample holderFunctionPlace to put the sample.The best result is obtained when the sample is a rotating sample holder. In the sample holder, the most common error that might occurred is due to overfilled or underfilled of the sample in the sample holder. This will cause error on the result.

4. DetectorsThe detectors based on the image above is the photographic plate.

Function• allows the diffracted X-rays to be detected

MechanismThe photographic plate is sensitive to light, this happened since there is coating of emulsion of silver salts. When the x- rays emits its beam, the photographic plate will absorbs it beams and undergo chemicals changes which can capture the diffracted X – ray patterns

INTERPRETATION OF XRD DATA

Data of XRD is in the form of Diffraction Pattern The data obtained from XRD are in the form of DIFFRACTION PATTERN,

which is also shown in the X-Ray detector. How the data is obtained?

o When X-Ray is passed through a crystal, it will bent at various angles/diffracted

o It is because the electromagnetic radiation will strike the lattice atom of the crystal in the lattice plane

o As a result, the electromagnetic radiation is diffracted off sets of planes from where the X-Rays attack and interact with the lattice atom of the crystal in the lattice plane

The set of planes itself is defined by the shape/geometry of the unit cell (smallest component of the crystal)

o The result of the diffracted X-rays is shown as diffraction pattern in the detector, which we use to determine what kind of crystal present in our sample.

What happen during XRD analysis?What happen when X-ray attack the lattice atom of the crystal plane?

Diffraction pattern obtained from XRD

IN ORDER FOR THE DIFFRACTION PATTERN TO BE ANALYZED WE SHOULD UNDERSTAND MORE AOBUT THE DIFFRACTED LIGHT

How diffraction happen? - Inside of Crystals • The regular arrangement atom molecules of the crystal can be described

by using framework-like model called crystal lattice. Each intersection of the lines will represent the position of the actual arrangement of atoms in a crystal.

• During XRD analysis the X-ray will attack the atom in the crystal lattice and then diffracted off set of planes that are present inside the unit cell (smallest constituent that make up the crystal lattice or repeating unit of crystal lattice).

Condition for light to be reflected There should be a constructive interference between the waves Constructive interference occurs when the reflection of light satisfy the

Bragg equation How constructive interference occurs?

During analysis, same wavelength of X-ray will attack different atom in different planes of the crystal.

However some X-ray will attack atom that is located in the planes that is deeper into the atomic plane.

Therefore for the same wavelength of the X-ray to attack the atom located in a deeper atomic plane and thus reflected, it needs to travel extra 2dsinθ

If the extra distance is equal to the wavelength of two waves they will constructively interfere when exit both the crystal

The result of the waves when they exit the crystal is seen as one whole wave.

Direction of diffraction depends on the orientation of planes in the crystal

In general the direction where the light are reflected depends on the orientation of planes inside the unit cell. Different orientation of planes will make the X-Ray to be diffracted in various directions.

The set of planes of planes in crystal are labeled according considering how the plane (or indeed any parallel plane) intersects the main crystallographic axes of the solid.

In mathematical terms the set of planes is labeled in the form of Miller Indices

Which is a set of numbers that show the intercepts of the planes in crystal and thus may be used to uniquely identify the plane or surface.

o It is because different miller indices represent planes with different orientation.

The result of the difference in the direction of diffraction from different planes result different in the location of the dots in the diffraction pattern

This would mean that each dots in the diffraction pattern will represent the planes in the crystal from where the x-ray is diffracted.

Analysis of Diffraction Pattern

The analysis of Diffraction Pattern primarily involves the calculation of d spacing between the crystal lattice planes of atoms that produce the constructive interference by using Bragg Equation.

How?

• Wavelength is known• Measurement is made at angle 2θ

where constructive interference occur

• Solving Bragg's Equation gives the d-spacing between the crystal lattice planes of atoms that produce the constructive interference

The d-spacing interplanar spacing (d-spacing) of a crystal is used for identification and characterization purposes in X-ray diffraction (XRD). It is because the d spacing of the atom is unique to each type of crystal although the diffraction may come from the same planes of crystal with the same miller indices.

Example: comparison between d spacing value of aluminum and copper

Applications of XRD

The main function of XRD application are:

To identify the unknown crystalline materials

To determine the charcteriation of crystalline materials

To measure the samplepurity.

There are 4 example of XRD application in real life:

Pharmaceutical insustryo X-ray diffraction (XRD) can be used to unambiguously

characterize the composition of pharmaceuticals. An XRD-pattern

is a direct result of the crystal structures, which are present in the

pharmaceutical under study. As such, the parameters typically

associated with crystal structure can be simply accessed. For

example, once an active drug has been isolated, an indexed X-

ray powder diffraction pattern is required to analyse the crystal

structure, secure a patent and protect the company’s investment.

o For multi-component formulations, the actual percentages of the

active ingredients in the final dosage form can be accurately

analysed.

Forensic Sccienceo XRD is used mainly in analysis. Examples of contact traces are paint flakes,

hair, glass fragments, stains of any description and loose powdered

materials. Identification and comparison of trace quantities of material can

help in the conviction or exoneration of a person suspected of involvement in

a crime.

Geological Applicationo XRD is the key tool in mineral exploration. Mineralogists have

been amongst the foremost to develop and promote the new field

of X-ray crystallography after its discovery. Thus, the advent of

XRD has literally revolutionized the geological sciences to such a

degree that they have become unthinkable without this tool.

Nowadays, any geological group actively involved in

mineralogical studies would be lost without XRD to

unambiguously characterise the individual crystal structures.

Each mineral type is defined by a characteristic crystal structure,

which will give a unique x-ray diffraction pattern, allowing rapid

identification of minerals present within a rock or soil sample. The

XRD data can be analysed to determine the proportion of the

different minerals present

Microelectics Industryo As the microelectronics industry uses silicon and gallium arsenide

single crystal substrates in integrated circuit production, there is a

need to fully characterise these materials using the XRD. XRD

topography can easily detect and image the presence of defects

within a crystal, making it a powerful non-destructive evaluation

tool for characterising industrially important single crystal

specimens.

Glass industry o While glasses are X-ray amorphous and do not themselves give X-ray

diffraction patterns, there are still manifold uses of XRD in the glass industry.

They include identification of crystalline particles which cause tiny faults in

bulk glass, and measurements of crystalline coatings for

texture, crystallite size and crystallinity