In the realm of scientific discovery, X-ray diffraction serves as a fascinating window into the microscopic world. It’s a technique that allows scientists to see beyond what the naked eye can perceive, unveiling the hidden structures of materials with incredible precision.
Imagine X-rays as tiny messengers that interact with the atoms in a substance. When these X-rays encounter a crystalline structure, they produce a unique pattern – a kind of fingerprint that holds the key to understanding how the atoms are arranged. This is the essence of X-ray diffraction.
Our journey into the world of X-ray diffraction begins with its pioneers, who cracked the code of this powerful technique. Developed in the early 20th century, X-ray diffraction has become a cornerstone in various scientific fields.
In this exploration, we’ll break down the basics of how X-ray diffraction works, discover the tools that make it possible, and explore its real-world applications. From decoding the structure of minerals to aiding in the development of new materials and medicines, X-ray diffraction opens doors to a multitude of scientific possibilities.
Process of X-Ray Diffraction
X-rays are electromagnetic radiations having the range of wavelength 0.1-100Aº, however, the analytical range is (0.7-2.0) Aº. X-ray radiations are produced when a high-velocity electron impinges or collides with the target. The process is:
Fast-moving electrons collide with the atoms of the target
Loss of electrons from the inner shell of the atoms results in vacant
The electron from the penultimate outer shell falls into the inner shell
Emission of X-rays
The energy of x-rays is equal to the difference in energies of the two shells. For example, if an electron from K shell is lost and the electron from the L shell falls into K shell then,
E = Eₗ – Eₖ
E = Energy of x rays
Eₗ = Energy of L shell
Eₖ = Energy of K shell
The other factors that are associated with x-rays are the frequency of the x-rays, wavelength of x-rays, and atomic number of the metal atoms. The relationships are as follows:
𝛎 = (Eₗ – Eₖ)/h
𝛎 = frequency of x rays
h = Plank’s constant
𝛌min = hc/Ve
𝛌min = minimum wavelength of x rays
c = speed of light
V = voltage applied
e = electronic charge
c/𝛌 = a(Z-𝛔)²
a = a constant
Z = atomic number of the atom
𝛔 = a constant that depends upon a series of lines
Interaction of X-rays With Matter
The X-rays when come in contact with the matter, result in three characteristic processes, namely- absorption, diffraction, and fluorescence.
I. X-ray Absorption
X-rays like other radiations also show the properties of absorption. When the electrons of atoms constituting the matter interact with the x-rays, they absorb the radiations and get excited which results in the emission of secondary radiations, characteristic of those atoms. The process follows the Beer’s law. The extent of absorption by a given element depends on the number of atoms of that element in the path of X-rays but is independent of the physical or chemical state of that element.
II. X-ray Fluorescence
When a sample is irradiated with an X-ray beam, the sample sometimes emits other X-ray beams, which is called X-ray fluorescence. The frequency of X-ray fluorescence gives quantitative analysis whereas the wavelength of X-ray fluorescence enables qualitative analysis.
III. X-ray Diffraction
Diffraction is the property of X-rays exhibited only by crystalline substances. When the x-rays are incident on a substance, the electrons of atoms of the substance become small oscillators. The oscillators oscillate at the same frequency as that of incident X-ray radiations. The electrons arranged in a crystal lattice emit electromagnetic radiation in all directions at the same frequency as that of incident X-rays. Constructive interference of the scattered radiations causes diffraction of X-rays. The x-ray diffraction process follows Bragg’s Law, which states:
n𝛌 = 2d sin𝚹
n = integer
𝛌 = wavelength of the x-rays
d = spacing between crystal planes
𝚹 = angle of incidence
X-ray diffraction is a common technique that determines a sample’s composition or crystalline structure. For larger crystals such as macromolecules and inorganic compounds, it can be used to determine the structure of atoms within the sample, whereas, for small-sized crystals, it can determine sample composition, crystallinity, and phase purity.
Types of X-ray Diffraction Methods
The following X-ray diffraction methods are used to study the structure of the crystal:
- The Laue method
- Bragg’s spectrometer method
- Rotating crystal method
- Powder crystal method
Instrumentation of X-Ray Diffraction
The XRD instrument is composed of:
- X-ray source
I. X-ray Source
The X-ray source is a circuit composed of an anode, a cathode, and a battery. The electric current from the battery heats the cathode which is made up of a tungsten filament, that emits thermionic electrons. The emitted electrons hit the anode which has the target metal, transferring the energy to the surface of the target that produces X-rays.
The target material selected should have an atomic number greater than that of the elements being examined in the sample.
The energy of x-rays emitted by the target material should be greater than that required to excite the elements being irradiated.
The target material produces the x-rays which are always randomly directed. They are converted into a narrow beam of X-rays by a collimator. The collimator consists of two metal plates separated by a small gap. It absorbs all the X-rays except the narrow beam that passes through the gap.
There is a wide range of wavelengths possessed by the x-rays but only a short range is useful for the analysis. Monochromators help to pass only the X-rays with desirable wavelengths. There are usually two types of monochromators used in X-ray diffraction instruments:
Filters act as a window of material that absorbs undesirable radiations but allows the radiation of the required wavelength to pass. This method makes use of the large difference in the mass absorption coefficient on the other side of an absorption edge. For example, a zirconium filter is used for molybdenum radiation.
- Crystal Monochromators
A crystal monochromator is positioned in the path of the X-ray beam so that the angle reflecting planes satisfy Bragg’s equation for the required wavelength. The beam is split up by the crystalline material into the component wavelengths in the same way as a prism splits up white light into rainbow colors. This is called analyzing crystals. They can be flat crystal or curved crystal monochromators made up of lithium fluoride, sodium chloride, or quartz.
The detectors are placed after the monochromators to detect the signals and transfer them to the recorder. The detectors used in XRD can be based on the following methods:
a) Photographic Method
In order to record the position and intensity of an x-ray beam, a plane cylindrical film. The film is developed after being exposed to X-rays. The blackening of the developed film is always expressed in terms of density units D.
D = log I0 / I Where,
I0 = Incident intensities
I = Transmitted intensities
D = Total energy that causes the blackening of the film.
b) Counter Method
There are 5 types of detectors used in XRD which are based on the counter method.
- Geiger-Muller Tube Counter
This detector consists of a tube, two electrodes, and a counter. The tube is filled with argon gas. The cathode used is a grounded metallic casing and the anode is a rod in the middle of the detector. When the X-ray particles enter the tube, they ionize the argon gas, producing a large number of Ar⁺/e⁻ ion pairs. Ar⁺ moves towards the cathode while e⁻ moves to the anode and this movement of charges produces an electric potential which is measured by the counter.
- Scintillation Detector
This detector consists of a scintillator made up of sodium iodide and a photomultiplier tube. The radiations fall on the scintillator which converts the rays into visible radiations. When these visible radiations fall on the photocathode, the photons present in visible radiations get multiplied due to reflection on the dynodes present in the photomultiplier tube. The anode collects these photons that show signals on the readout.
- Semiconductor Detector
This detector is divided into three regions namely: p-type region made up of silicon, n-type region made up of silicon doped with lithium and an intrinsic region made up of silicon doped with lithium ions. X-rays enter the intrinsic region and excite electrons to a conduction band. This results in a resistance decrease in the region which allows the flow of current through the device. The detection is based on the increase in conductivity when struck by radiation.
- Proportional Counter
This detector is similar to the Geiger-Muller tube detector. The tube is filled with heavy gases such as xenon or krypton as they are easily ionized. It is operated at a lower voltage than the Geiger-Muller detector. The output pulse is dependent upon the intensity of x-rays falling on a proportional counter.
- Solid-State Semiconductor Detector
In this detector, the electrons produced by X-ray beams are promoted into the conduction and the current that flows is directly proportional to the incident X-ray energy.
The signals detected by the detectors are then amplified and recorded by a computer system in the form of peaks. The intensity of the peaks is related to the amount of molecules in that phase or with that spacing. The greater the intensity of the peak, the greater the amount of crystals or molecules with that distinct spacing. The width of the peaks is inversely proportional to the crystal size.
Application of X-ray Diffraction in Forensic Analysis
XRD is an analytical instrumental method used in forensic science.
- It is applicable in forensic toxicology where inorganic metallic poisons can be detected from the sample. It is also useful in elucidating the structure of the drugs seized from a place.
- X-ray Diffraction is helpful in the examination of jewelry made up of gold, silver, or any other metal, seized in cases of robbery or burglary.
- Examination of paint is also possible using X-ray diffraction methods.
- In forensic medicine, the XRD is helpful in the determination of any bone disease or deformity in the deceased. X-ray of dental records plays a vital role in the identification process.
- The crystalline structure of the suspected material found at the crime scene can be done by using the XRD method.
X-ray diffraction is a transformative tool that has revolutionized our ability to explore the hidden structures of matter. From its inception with early visionaries to the diverse range of detectors utilized today, X-ray diffraction has become indispensable in unveiling the microscopic world.
This technique not only shapes our understanding of crystallography but also extends its impact across scientific domains, from materials science to biology. As we harness its capabilities to decode the atomic mysteries, X-ray diffraction remains a beacon guiding us toward unprecedented insights into the building blocks of our universe.