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| 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
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| 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.
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| 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.
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| 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.
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| 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.
