X-ray Diffraction- Illuminating the Secrets of Crystals

X-ray Diffraction 

Introduction 

A non - destructive method knon as X-ray diffraction analysis (XRD) can give precise details on a material's crystal structures, elemental makeup, and physicochemical properties. It really is dependent on the constructive interference of crystal and monochromatic X-rays. A frequent method for figuring out the composition or crystalline structure of a sample is X-ray diffraction. It can be utilized to ascertain the atomic structure of sample materials that are bigger crystals, like macromolecules and inorganic compounds. The sample composition, crystallinity, and phase purity can be determined if the crystal structure is too tiny. 

To use this method, x-ray beams are passed through it. Instead of using much larger wavelengths, which would not be affected by the spacing between atoms, X-ray beams are used because their wavelength is comparable to the spacing between atoms in the sample. This means that the angle of diffraction will be affected by the spacing of the atoms in the sample.

The analysis used in XRD techniques ranges from qualitative to semi-quantitative to fully quantitative. According to the amount of effort necessary to increase accuracy through sample preparation, instrument operation, and analysis, each technique will produce varying degrees of accuracy and precision.

History of XRD

In 1895 Roentgen discover X-rays and in 1914 crystals diffraction pattern was made by Knipping and Von Laue. Brags develop the theory to determine the crystal structure by using an X-ray diffraction pattern in 1915. In 1953 DNA structure was also determined by using XRD by Watson and Crick. In these times diffraction pattern was improved by technological resources. Diffraction patterns are used to determine atomic structure and are frequently used in the medical field for various applications.

Diffraction:

Waves spreading outward around obstructions are known as Diffraction. Sound, electromagnetic radiation like light, X-rays, and gamma rays, as well as very small moving particles like atoms, neutrons, and electrons that exhibit wavelike qualities all exhibit diffraction.

Bragg’s Law:

This law is used to find out the relation between the crystal atomic structure planes and incidence angles at which atomic planes form the powerful reflection of radiation of electromagnetic regions like rays that are related to neutrons and electrons i.e., gamma rays, X-rays, and waves of particles. Bragg^’s law provides the necessary conditions for constructive interference.

When measuring wavelengths and figuring out how far apart crystal lattices are spaced, the Bragg law is helpful. The radiation beam and the detector are both adjusted to a specific angle θ in order to measure a specific wavelength. After then, the angle is adjusted until a strong signal is picked up. The wavelength is then directly derived from the Bragg law by the so-called Bragg angle. The primary method for taking accurate X-ray and low-energy gamma-ray readings is Braggs law.

Braggs law formula,

                                                        nʎ = 2dsinθ

            d = Distance between slits

            θ = Diffraction angle

            n = Order number for the maximum

            ʎ = Wavelength

 

Bragg’s Diffraction Angle

The path difference between two rays Ray 1 and Ray 2 is equal to 2dsinθ

If constructive interference occurs than 
 nʎ = 2dsinθ

X-rays:

X-rays are the high Energy Electromagnetic rays. They cause the ionization of molecules.

Types of XRD:

Single crystal and powder XRD techniques are the two different categories. Scale is where these approaches diverge.

1. Single crystal XRD

The exact atomic locations of a single, well-ordered crystal or powder are the focus of a single-crystal study. 

2. Powder XRD

A sample of bulk material can be characterized using XRD. Powder samples are examined in powder XRD.

Components of XRD:

An x-ray diffractometer is used to perform X-ray diffraction, which comprises a

·    👉    X-ray source

·    👉  “Goniometer”, It is a device that is used for restricting wavelength range

·    👉     Specimen holder

·     👉     Detector

·      👉   Readout device

Diffraction pattern showed on photographic film.

Parts of XRD

Working of XRD:

X-rays are the high energy radiations that ionize of the sample. These are electromagnetic radiations and are used in XRD for diffraction pattern formation.

In a cathode tube, X-rays are made by heating a filament, which releases electrons. Then, these electrons are propelled toward a target made of a metal, usually copper. The copper target is the source of monochromatic x-rays that are produced when the electrons interact with it (a single wavelength). The generated x-rays are then collimated—that is, they are made to run parallel to one another—and pointed toward the sample.

The intensity of the diffracted x-rays is measured while the x-ray tube and detector revolve around the sample. An XRD pattern known as a diffractogram is produced when the geometry of crystallographic planes interacts with x-rays. XRD is used to identify minerals, measure crystallite size, gauge the thickness of atomic layers, detect crystal deformation, and other phenomena.

 

Working of XRD

 

How to interpret data:

The outcome of X-ray diffraction plots the signal's strength for different diffraction angles at each of their respective two theta points. As determined by the angle of diffraction from the incident x-ray beam transmitted into the sample, the two theta positions correspond to a specific spacing between the crystals or atoms in the specimens. The number of molecules in that phase or with such spacing affects the peak's strength. The quantity of crystals or molecules with that particular spacing increases with peak intensity.

The size of the crystal has an inverse relationship with the peak width. A larger crystal is associated with a thinner apex. A wider peak indicates that there may be a smaller crystal, a flaw in the crystalline structure, or that the sample is amorphous in nature, i.e., not perfectly crystalline. The composition of smaller samples can be ascertained from the patterns discovered by XRD analysis. The diffraction patterns for the many elements, compounds, and minerals are stored in a sizable database. By matching the position, width, and relative heights of the diffraction patterns, it is possible to identify an element by comparing the pattern for an unknown compound to values found in the literature and in experiments.

Interpret the result of XRD

Diffraction methods

Why X-rays are used in XRD? 

In diffraction methods, we use X-rays, electrons, and protons but these have advantages and disadvantages.
If we use X-rays than
It measures wavelength to about 1A
Have energy 10⁴ev
Interact with the sample electrons and penetrate

If we use neutrons in diffraction than

Neutrons interact with the sample nuclei and have more penetration power. 

These have an energy of approximately 0.08ev

If we use electrons in diffraction than 

Because of their high energy of 150 EV, these are less penetration power. 
Interact with the sample electrons. 
So, from the above discussion, we see X-rays have properties that are good for diffraction methods. This method so-called the X-ray Diffraction method. 
Typically spacing between atoms is in the order of 2-3 A, hence X-rays are suitable radiations for crystal study.