X-ray diffraction (XRD) is the only laboratory technique that reveals structural information, such as chemical composition, crystal structure, crystallite size, strain, preferred orientation and layer thickness. Materials researchers, therefore, use XRD to analyze a wide range of materials, from powder X-ray diffraction (XRPD) to solids, thin films and nanomaterials.

Ultimately, the goal is to obtain a good XRD pattern result with clear, sharp peaks with low background noise. Good data collection is so critical towards the next phase, which is data analysis and interpretation. These impact how you draw implications and determine your next steps in terms of your materials research and also process monitoring whether in pharmaceuticals, mining geology, catalysts or specialty chemicals. Earlier, our senior application specialist, Dr. Daniel Lee, discussed the basics to powder X-ray diffraction. During the webinar, he covered the basics:

  • The importance of crystal structure
  • What is measured by X-ray diffractometer
  • Basic crystallography before analyzing XRD pattern
  • The most important equation in X-ray diffraction
  • Practical guidelines for getting a nice XRD pattern

Interested to improve in your XRD data analysis and applications? Join our free series of webinars:

  1. Introduction to powder X-ray diffraction. View on-demand
  2. Studying battery cathode materials using X-ray diffraction View on demand
  3. Expand your powder XRD applications for materials characterization research View on-demand
  4. Knowing the difference between good and bad data: XRD data analysis – (29 May 2020) Register now
  5. Improving your phase search mapping by defining your elemental range: introduction to elemental analysis using X-ray fluorescence. (11 June 2020) Register now
  6. Range of XRD instruments to aid materials characterization research. View on-demand

For your convenience, we’ve consolidated the questions raised during the webinar.

INTRODUCTION TO POWDER XRD

Q: Could you please summarize the meaning of the peak intensity and peak position in the XRD pattern?

Peak position refers to the maxima of intensity around the theoretical Bragg angle and value of this maxima is referred to as peak intensity.

Q: XRD patterns: how do they vary from crystal structures

Difference in crystal structure affect position of diffraction peaks. Hence two substances having different crystal structures will produce different sets of XRD scans.

Q: Why is there a peak broadening when the crystallite size is very small?

X-ray peak is derived from the lattice sum over the entire crystal atoms. In ideal crystals, the sum is over infinite number which generate delta function on coherent spectral wavelength. Nanoparticles have much less atoms hence the lattice sum is not able to converge to a diffraction line but broaden out. The smaller the particle the broader the diffraction peak. Therefore, the broadening can be used to measure the particle size.

XRD SOFTWARE

Q: ­How can I quantify the different phases or different composition? Please shed some more light on this.

A position of the peak comes from the dimension of unit cell, an intensity coming from how many electrons are present and where they are. So, everything can be easily calculated in software. So, once we found out what phases are present in the sample (mixture), because we already know the positions of the peaks, we just need to minimize a difference between observed intensities and calculated intensities by cycling fitting procedures until we get a reliable results.

Q: ­how can we name the miller indices in the XRD pattern figure? ­

If you are asking about software? You need to at least do some refinement job to see the miller indices. If you need more help, contact us.

Q: ­What if we cannot find the crystal structure in ICDD?

It means that nobody has published that material yet. So, you need to solve the structure by yourselves. And publish it!

Q: ­Is there any software to adjust the peak ­?

HighScore and HighScore Plus is the software by Malvern Panalytical. It can be used to analyze XRD patterns from other brands of X-ray diffractometers.

XRD TECHNIQUE

Q: ­In the diffractogram of amorphous materials, such as SiO2, it is common to see an amorphous halo between 20º and 30º. Will this also be observed for example in a nanostructured SiO2 matrix coating?

Yes, it does. But, as you said the thickness of SiO2 is in nanoscale, this would not be seen when you use normal Bragg Brentano geometry. I recommend you use parallel geometry which keeps the incident angle at 1~2 degrees and scan only 2 theta(detector). So, you can effectively observe the diffractogram from the surface(skin) of the film.

Q: ­Does destructive interference caused by element position reduce only the intensity or cause peak shift­?

Only reduces the intensity. A peak shift can be caused by the size of unit cell.

Q: ­Hi, is it important to understand the 7 crystal systems in interpreting XRD graph? ­

Yes of course, but not for all. Firstly, you need to understand about your structure. Then you can expand your knowledge to the others and try to connect it with physics and crystallography.

Q: ­What do you mean by spinning of sample. Is it ball milling or something else? ­

Spinning sample means we can rotate the sample holder in phi axis during the measurement. Not ball milling.

Q: ­Does it mean that XRD technique depends on the reflection of the selected plane? ­

In the Bragg Brentano geometries, the planes parallel to the surface or perpendicular to the surface normally can be involved in the diffraction; Bragg’s law. That is why we can use this technique to reveal where is the preferred orientation of the thin film or bulk of the sample. But, in cases of powder, since there are millions of particles and grains, we can assume that those diffractions can represent all the others.

Q: ­Why do we plot 2 theta in y axis instead of theta ­

2 θ is the angle between transmitted beam and reflected beam. In any experiment the transmitted and reflected beam can be observed, so 2 θ is an experimentally measurable quantity. But the crystallographic plane cannot be observed. So θ cannot be determined directly. (from the google)

Q: ­Will change in search range effect the calibration? ­

I am not sure what the question is meaning for. But, usually, there is no correlations between Calibration and Search.

Q: By having an XRD, how can we differentiate it with Microscopy? ­

XRD measures the crystallographic information directly, but microscopy measures optical properties and refractive index. XRD can measure millions of particles as a result it will have a representative value unlike Microscopy. And very small errors from the operators.

Q: ­Is it possible to detect functional groups using XRD? ­

Please refer to the link: https://www.intechopen.com/books/recent-advances-in-crystallography/crystal-structures-of-organic-compounds

Q: ­How to remove the substrate peaks while measuring?

If you used a single crystalline material as substrate, you could tilt your sample a little bit (around 1 ~ 2 deg) during the sample prep. Then you will not see any peaks from the single crystal substrate.

Q: ­If we use composite materials like brake pad applications, do we need to use spinning method for better accuracy results? your view please? ­

It depends on how small your particle is. But I would say that spinning is always beneficial on polycrystalline material!

Q: How does soller slit affect Inorganic crystalline nanomaterials?

When smaller soller slit are used, we achieve better resolution at the cost of intensity.

SAMPLE PREPARATION

Q: ­Regarding on sample preparation for powder samples. You mentioned about smearing a sample, how do you do that? ­

You can prepare some plate having a rough surface like sandpaper. Then you can put the plate onto the sample and slide a little bit to smear the pressed powder.

Q: ­Talking about materials such as soil cuttings, what could be the impact of chemicals that would have encapsulated/absorbed onto the structure? ­

I am not sure what is the detailed situation, but in any cases, materials can be encapsulated or absorbed onto the surface of the matter under high temperature or pressure. Probably, some triggers were there to make secondary phases on the surface.

Q: ­When we spin our sample, will it damage the morphology of the sample? ­

If your particle(grain) size is more than several tenth or hundreds or microns, spinning a sample will give you better statistics in profile. It does not affect your morphology.

Q: ­What are the possible factors of contamination that can occur while preparing a sample for XRD analysis and what are the ways to overcome those things?

Usually, a contamination issue is very low in XRD world. Sometimes when you grind the sample using Mortar, some contamination can occur. And, if your sample material is air-sensitive, then you better use a dome-sample holder to block the air and moisture.

Q: ­A very diluted liquid sample was quoted on a glass substrate but XRD showed only glass peak. Can you suggest a method to take the XRD of liquid sample? ­

Usually a diluted liquid sample cannot be measured. XRD has a relatively poor detection limit around 0.5 wt%.

Q: ­What’s the optimum particle size of the sample for getting accurate results? ­

Theoretically 1-micron meter is the best. But I could say below 20 um is good enough if you spin the sample!

Q: I would like to know what is the volume of the sample? < 1 gram?

Standard sample holders have circular shaped orifice of various dimensions. Most popular are 27mm and 16mm. They are 2mm deep so accordingly volume can be estimated. The weight of sample needed would depend on density of material.

Q: If we grind the sample very fine what will happen while measuring with XRD and what is the recommended grinding techniques to get the best results?

Sample particles should be ideally 5-10 micron, so grinding is recommended but grinding has some side effects too.

  1. Grinding with great force might make some phases amorphous or nano-crystalline in which case whether they disappear from XRD pattern or their peak profile become very broad respectively. The force which results in such effect depends on mechanical property of phase under question so there is no standard answer to require force during grinding. One can optimize by successive XRD scans after repeatedly changing the force of grinding.
  2. In extreme case, phase change can be induced by grinding resulting in appearance of newer phases. Thus, mechanical energy imparted during grinding should be definitely smaller than energy required for phase change.

XRD TUBE AND DETECTOR

Q: ­For one analysis how many X-rays passes through the samples? ­

It is difficult to estimate in number. But, I could say that at least several millions of photons will be detected from the detector using laboratory source.

Q: ­What is the X-ray beam diameter from the X-ray tube and how many times it passes through the sample? ­

Normally, the size of X-ray beam is ranging from 50 um to several tenth of millimetres in case of point beam. But we use line-focused beam for the powder diffraction which has 12 mm x 0.4 mm size of the beam from the beginning. We can re-size it by using slits and masks depending on sample size and starting angle of the scan in order not to overflow.

Q: The high-resolution tube is used for BBHD in Empyrean. This is different from previous tube in X’Pert Pro. What is the difference?

The filament width of HR tube is more thin than normal Tube. So, HR tube is suitable for mirror-based optics because it is more brilliant and narrower irradiated area.

XRD DATA ANALYSIS AND RESULTS

Q: ­What is the effect of search range on analysis? ­

If you freely do the search and match analysis without setting any restrictions, the software may show you completely wrong results. Because XRD is good at distinguishing polymorphs, not isomorphs. If the structural characteristics are exactly identical but the chemical compositions are different, XRD becomes kind of blind. XRD data analysis and results.

Q: ­Does destructive interference caused by element position reduce only the intensity or causes peak shift­

Only reducing the intensity. A peak shifting can be caused by the size of unit cell.

Q: ­What is the reason for negative counts in XRD spectrum? ­

A negative intensity can be seen after subtracting background. So, please try to get a raw data. A negative intensity never gotten in physics.

Q: ­can the intensity of impurity be of higher than the sample is it possible? ­

Usually not. But if the host material is formed amorphous, then impurities can be seen as peaks.

Q: How can we calculate the strain using XRD pattern of a sample?

Firstly, you need to measure your XRD pattern up to 140 degrees. Then we can decompose the FWHMs of the peaks into size and strain terms by following Williamson-Hall theory. If you need some practical helps, contact us again with data.

Q: you have lot of noise in XRD pattern, what will you recommend for better XRD pattern?

Following recommendations can be followed:

  1. Surface of sample should be as smooth as follows as noisy background might result from scattering from uneven surface.
  2. Optimize the number of data points for a diffraction peak. 8-10 data times are sufficient for a diffraction peak profile to be fitted during analysis. To achieve the same, take a fast scan and find the sharpest peak. Find its FWHM and then divide FWHM by 10. Choose this number to be “step size” parameter during scanning. This ensures the all peaks have close to 10 peaks for defining the profile.
  3. As per dimension of sample, choose divergence slit and mask to maximize the area of X-ray irradiation. Smaller irradiation area defines a smaller volume of sample from which diffraction data is produced and affects the signal to noise ratio for a diffraction peak.

Q: How can we calculate strain using XRD pattern of a sample?

Strain affects the peak broadening of a diffraction peak profile. Using Williamson Hall plot, one can calculate the same.