X-ray diffraction (XRD) knowledge summary(4)

1. What should I pay attention to when collecting XRD? (e.g., what are the requirements for collection angle, perimeter, speed, etc.?

The intensity of the diffraction peaks is related to many factors, such as the diffractive ability of the sample, the nature of the sample, as well as the power of the instrument, the test method, the sensitivity of the detector, etc.

The smaller the grains in the sample, the lower the peak intensity of the diffraction peaks, but the broader the peaks. In fact, the broadening of the X-ray diffraction peaks is used to analyze the crystalline granularity of the sample according to this principle (Scherrer's formula).

There is a relationship between grain size and particle size, but there is a difference in their respective meanings. A grain may also be a particle, but it is more likely that the grains hold together and aggregate secondarily to become particles. A particle is not the fundamental unit of diffraction, but tiny particles can produce scattering. The finer you grind, the stronger the scattering. For grains, if you grind them too much, the crystal structure is destroyed, and they become amorphous, and the ability to diffract is gone. If you grind too hard, some of the peaks may disappear, and the neighboring diffraction peaks will be superimposed on each other due to broadening, and will eventually become one or several “bulges”. Generally, the peaks of the grain spacing is large by the grain refinement will be more obvious, because the d value of the grain surface is easy to be destroyed.

3. Is the weakening of the diffraction intensity essentially due to smaller crystal grains or smaller sample grains?

The intensity is not only related to the grain size, but also to the surface state of the grains. Generally, the finer the grain, the larger its surface area, and the more serious the defects in the surface layer structure. Structural defects will lead to lower diffraction intensity and broadening of the diffraction peaks, XRD should be the study of grains, crystal problems, and crystal structure related problems, not the size of the sample particles, Xerox formula should also be the size of the grains. The size of the sample particles should be determined by other methods. For example, light scattering, X-ray scattering, electron microscopy, and so on.

4. The repeatability of XRD of fine acicular microcrystalline powder samples is very poor. （How can I avoid selective orientation?

It is very difficult to avoid the selective orientation, but we can only try to minimize its effect.

First of all, you have to grind the sample as fine as possible (but moderately, taking care that the crystal structure of the sample is not damaged by over-grinding);

Don't press strongly on the smooth glass plate (when pressing the sample, you can line a rough paper on the glass plate), sample forming as loose as possible;

The sample making process can also be mixed with some glass powder, or add some glue to blunt the corners of the sample.

Of course, there are other methods, which can be found at http://www.msal.net

5. For crystal diffraction analysis using X-rays, the diffraction pattern is recorded using the photographic method. 1. How does the diffraction pattern change when the polycrystalline grains are refined? 2. How does the diffraction pattern change when there are macroscopic stresses in the polycrystalline specimen?

Whether it is a powder sample or a (poly)crystalline sample, if the powder particles or grains are too coarse, there are fewer grains participating in the diffraction, which will cause the diffraction lines to flatten, but if the powder particles or grains are too fine, the diffraction lines will widen, which is not conducive to the analytical work.

The effect of the presence of macroscopic internal stresses is to change the position of the diffraction rings or diffraction peaks, resulting in a widening of the diffraction lines on the negative, which is not conducive to analytical work.

6. What causes the overall shift of the XRD peak to the right?

It may be that elements with small ionic radii are replacing elements with large ionic radii.

It is also possible that the sample surface is higher than the sample holder plane or the zero point of the instrument is not accurate when you make the sample. It is recommended that you correct your data with a standard sample.

7. Place the sample back so that the sample is about 1mm off the center axis of the goniometer, how does the diffraction peak change?

The peak displacement is shifted to a lower angle. With the sample surface 0.1mm off the goniometer's rotary axis, the diffraction angle measurement will have an error of about 0.05 degrees (2θ) (for a Cu target, at a position near 2θ 20 degrees)

8. Does the resolution of an instrument's measurements depend only on θ?

There are many factors that affect the resolution of an instrument measurement: the radius of the goniometer; the focal spot size of the X-ray source; the size of the various slits in the optics; the adjustment of the instrument (2:1 relationship); the width of the picking step; and the positioning of the sample.

9. How is the target (X-ray tube) selected for diffraction analysis? I have a polycrystalline sample of Cr, and all I know is that a Cr target is best for diffraction analysis, but I don't know why.

For all elements, X-rays and possibly their characteristic X-rays are produced by the bombardment of high-speed electrons. Elements irradiated with higher energy X-rays can also excite their characteristic rays, called secondary X-rays or fluorescent X-rays, and show strong absorption and attenuation of the incident X-rays.

For a certain wavelength, the attenuation coefficient increases with the increase of the atomic number in the periodic table, but suddenly decreases to a certain atomic number; for a certain element, the mass attenuation coefficient increases with the increase of the wavelength, but suddenly decreases to a certain limit, which can occur several times.

The wavelength value at which the mass attenuation coefficient of each element suddenly changes is called the absorption edge or absorption limit of the element. By utilizing this sudden change in the absorption properties of an element, we are able to select for each X-ray target a substance to be made into a “filter”, which strongly absorbs only the Kβ line generated by the target and only partially absorbs the Kα wavelength, so that we can obtain a diffraction pattern that is basically generated by the Kα wavelength. A substance with an atomic number 1 smaller than that of a target is a Kβ filter for this target.

However, the Kβ filter does not remove the effect of fluorescent X-rays from the sample on the diffractogram. The intensity of the fluorescent X-rays will be superimposed on the background of the diffractogram, resulting in a high background that is not conducive to the analysis of the diffractogram. Therefore, for an X-ray diffractometer that is not equipped with a curved crystal ink monochromator and uses only Kβ filters, the main consideration for wavelength (or target) selection is that the main constituent elements of the sample will not be excited to produce intense fluorescent X-rays. If the atomic number of the element in the analyzed sample is 1 to 4 smaller than the atomic number of the element in the target, strong fluorescence scattering will occur.

For example, the use of Fe targets to analyze the main constituent elements of Fe, Co, Ni samples is appropriate, but not suitable for the analysis of substances containing Mn Cr V Ti; Cu targets are not suitable for the analysis of Cr, Mn, Fe, Co, Ni, these elements of the material. Therefore, for the Cr is the main constituent elements of the sample, can only choose Cr target X-ray tube.

The graphite monochromator not only removes the Kβ diffraction lines from the incident beam, but also prevents the fluorescent rays of the sample and the diffraction of “white light” from the sample from being superimposed on the background of the diffraction pattern, so that a strictly monochromatic diffraction pattern is obtained for the Kα wavelength. Therefore, when working on a diffractometer equipped with a curved graphite monochromator, the interference of fluorescent X-rays generated by the sample can be disregarded, and the Cu-target X-ray tube can be used universally for a wide variety of samples, including samples composed mainly of elements such as Cr, Mn, Fe, Co, Ni, etc. However, samples with wavelengths greater than the CuKα wavelength can be diffracted with the CuKα wavelength.

However, targets with wavelengths greater than CuKα (e.g., Cr, Fe, Co, etc.) can be valuable for small-angle X-ray diffraction studies or for the accurate determination of crystal plane spacing. The increase in wavelength reduces the overlap of the diffraction peaks; all the diffraction peaks are shifted to a higher angle.

10. What is the reason for the asymmetry of the diffraction peaks?

The diffraction peaks (precisely the diffraction lineprofile) obtained by the diffractometer are asymmetric, especially in the low-angle region (2θ < 30°). The asymmetry of the peaks is due to a number of factors, notably the geometry of the diffractometer optical path, the adjustment of the instrument, and the absorption properties of the sample.

11. How does a high-precision goniometer achieve θ/2θ angular rotation?

Is it possible to ensure strict angular synchronization? θ/2θ are two coaxial gardens, θ is the gardens that drive the sample and 2θ is the gardens that the detector rotates in order to ensure that the focal point of diffraction of the sample is always on the large gardens that the detector rotates in. Modern diffractometers use two stepper motors to independently control the rotation of the θ and 2θ gardens, and the control circuit ensures that the two gardens rotate at a 1:2 ratio, which ensures that the rotation of the two gardens is strictly synchronized by a multiple angle. Is it possible that magnetic materials such as NdFeB or NdFeN powders are preferentially oriented due to the presence of magnetism? No. Magnetic materials are definitely most selectively oriented, otherwise they would be non-magnetic and should be ground to a powder to suppress this orientation. The “preferential orientation” will cause many diffraction peaks that would otherwise be present to fail.

##12. Why are there XRD data with (200) and (400) facets but not the basic (100) facet data? Why are there (220) but not (110) in the XRD data?

Powder diffraction does not necessarily show all the facets, and many substances do not necessarily show (100) and (110) in powder diffraction, which is related to the structure. There is a phenomenon called “extinction” in crystal diffraction. The “extinction law” of a crystal is determined by the symmetry of its structure, and the “extinction law” is different for different space groups. If diffraction does not occur when it should, it is caused by the preferred orientation of the sample. Furthermore, the angle of the (100) plane is low, and sometimes it is not swept or submerged in the low angle background.