Question about Powder X-ray diffraction experiment

In summary, powder diffraction is used in experiments because the Bragg peak occurs at a specific angle/direction between the incident beam and scattering direction. By having a random orientation of scatterers, the Bragg peaks will only occur for a small percentage of crystals in the powder. This method also allows for easier analysis of hard-to-crystallize samples and does not require large crystals.
  • #1
Clara Chung
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I don’t understand why do we use power in this experiment. If we use powder, the d spacings of each crystalline will be oriented randomly, why won’t the angle of reflection changes? I don’t understand the idea of using powder in the experiment.

Thank you so much
 

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  • #2
I believe the answer is because the Bragg peak occurs at at specific angle/direction between incident beam and scattering direction. Bragg scattering is a combination of specular reflection off of a single crystal plane (basically a zeroth order diffraction maximum) as well as constructive interference between adjacent planes. By having random orientation of the scatterers, the Bragg peaks will only occur for a very small percentage of the small crystals of the powder. (I believe I have this part correct). Most of the crystals in the powder do not have the proper incident angle to make a Bragg peak. ## \\ ## When you get a Bragg peak from a given crystal, the scattering will be in a ring, (i.e. for each single small crystal, it will occur somewhere in the ring), because the scattering is no longer in a single plane for all of the crystals that satisfy the Bragg condition.
 
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  • #3
Charles Link said:
I believe the answer is because the Bragg peak occurs at at specific angle/direction between incident beam and scattering direction. Bragg scattering is a combination of specular reflection off of a single crystal plane (basically a zeroth order diffraction maximum) as well as constructive interference between adjacent planes. By having random orientation of the scatterers, the Bragg peaks will only occur for a very small percentage of the small crystals of the powder. (I believe I have this part correct). Most of the crystals in the powder do not have the proper incident angle to make a Bragg peak. ## \\ ## When you get a Bragg peak from a given crystal, the scattering will be in a ring, because the scattering is no longer in a single plane for all of the crystals that satisfy the Bragg condition.
cool!
 
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  • #4
One additional item=and I think I have this correct=the Bragg peak does not require the incident beam to precisely have the proper angle of incidence to get a Bragg peak. Because the process works off of diffraction, (and perhaps also because of the thermal motion of the individual atoms), the Bragg condition only needs to be met within a small ## \Delta \theta ## for a given small crystal to have the proper angle of incidence to get a specular reflection that also results in a Bragg peak. Also, since the powder consists of small crystal particles, the diffraction pattern from a reflection will cover a finite ## \Delta \theta ##, (still small though), rather than the infinitesimal ## \Delta \theta ## that would result from a larger crystal. ## \\ ## And this is my own deduction, but I believe I have this part correct, is smaller particles of the powder would make for a larger ## \Delta \theta ##. In addition, the solid angle of cone of acceptance ## \Delta \Omega ## for the incident beam to be such that the crystal particle meets the ## \Delta \theta ## requirement is proportional to ## (\Delta \theta )^2 ##. Thereby, it would be advantageous to have a somewhat fine powder, in order to increase ## \Delta \theta ## to have a much higher percentage of crystal particles meet the ## \Delta \theta ## requirement to participate in the ring of Bragg scattering that occurs. ## \\ ## And here is a "link" that agrees with my statement of post 2 that only a small percentage of the particles participate in making the Bragg ring: http://prism.mit.edu/xray/oldsite/Basics of X-Ray Powder Diffraction.pdf The powder diffraction method is discussed around page 25. (The pages are not numbered).
 
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  • #5
Clara Chung said:
View attachment 224555
I don’t understand why do we use power in this experiment. If we use powder, the d spacings of each crystalline will be oriented randomly, why won’t the angle of reflection changes? I don’t understand the idea of using powder in the experiment.

Thank you so much

Powder diffraction makes it easier to analyze hard-to-crystallize samples. All crystals will have some disordering, using a powder allows 'averaging' over the disorder, leaving only the lattice contributions. Powder diffraction also does not require large crystals, so sample preparation is often cheaper and faster.

https://en.wikipedia.org/wiki/Powder_diffraction
 

Related to Question about Powder X-ray diffraction experiment

1. What is powder X-ray diffraction (XRD) and how does it work?

Powder X-ray diffraction is a technique used to analyze the structure of crystalline materials. It works by directing a beam of X-rays onto a powdered sample, which causes the X-rays to diffract or scatter. The resulting diffraction pattern provides information about the arrangement of atoms in the sample, allowing for the identification of the crystalline phases present.

2. What equipment is needed for a powder XRD experiment?

To perform a powder XRD experiment, you will need an X-ray source, a sample holder, a detector, and a data analysis software. The X-ray source is usually an X-ray tube that produces a focused beam of X-rays. The sample holder is a flat plate or capillary that holds the powdered sample. The detector collects the diffracted X-rays and converts them into an electrical signal, which is then analyzed by the data analysis software.

3. What types of materials can be analyzed using powder XRD?

Powder XRD is commonly used for analyzing crystalline materials, such as minerals, metals, ceramics, and pharmaceuticals. It can also be used to determine the crystal structure of proteins and other biological molecules. However, it is not suitable for amorphous materials, such as glasses or liquids.

4. How is the data from a powder XRD experiment interpreted?

The data from a powder XRD experiment is typically presented as a graph called a diffractogram, which shows the intensity of the diffracted X-rays as a function of the scattering angle. The positions and intensities of the peaks in the diffractogram can be compared to a database of known patterns to identify the crystalline phases present in the sample. The peak positions can also be used to calculate the lattice parameters and unit cell dimensions of the crystal structure.

5. What are the advantages of using powder XRD over other analytical techniques?

Powder XRD is a non-destructive and relatively fast technique that can provide information about the crystal structure of a material without the need for extensive sample preparation. It is also highly sensitive and can detect small amounts of different phases present in a sample. Additionally, it is a widely available and cost-effective method for characterizing crystalline materials.

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