X ray diffraction (XRD) knowledge summary(2)

1. What is the difference between a PDF2 card and a JCPDS card?

There are the same thing, PDF2 is ICDD (International Center for Diffraction Data) products, ICDD formerly JCPDS (Joint Committee on Poder Diffraction Standards).

2. Why do I use pcpdfwin to check the electronic PDF card and my school library to check the PDF card is different, but the relative intensity value is not the same, this is why?

Relative intensity is estimated, there is an error is normal, may be different sources of data caused by, and note that, because the card can not be changed after the production, so some of the cards were corrected by the results of the later, which in the printed card is no such information, but in the database of the card has a description of the card, which card has been deleted.

3. How come some of the relative intensities in the diffraction cards are greater than 100 (e.g. 999)?

The intensity data of powder diffraction card is provided by relative intensity, generally the strongest line is 100, but the intensity value of the strongest peak of the calculated powder diffraction data is taken as 999, in fact, the value of the relative intensity is not important, you just need to take the largest as 100, and compare the others with it, for example, 999 is taken as 100, then 500 is 50, and 353 is 35, in this case, because it is an estimation, it does not matter if there is an error. In this case, since it is an estimation, there is an error, and it doesn't matter.

4. Is it normal to have two cards for the same substance?

This is normal because the two cards were made at different times or by different people. You can call up the card by card number and the original literature from which the data on the card came can be found on the card.

6. In the data table of X-ray powder diffraction, there is Rel.Int.[%] in the Peak List and Scale Fac in the Pattern List.

“Int.[%]” means ‘Relative Intensity, %’, and ‘Scale Fac’ is the ‘scale factor’ (of intensity).

7. Is there any software that can draw the spatial structure of crystals based on fractional coordinates? It is the kind with octahedra or tetrahedra.

According to the structural data of the crystal, the space structure of the crystal can be drawn with a professional crystal structure drawing software such as diamond or atoms.

8. Where can I find crystal data for natural products published in the literature?

The ICSD database or the PDF database of the ICDD. In addition to this, the mineral database and the American Mineralogist's Crystal Structure Database (AMCD) can be found free of charge at the link provided in the “Crystallographic Databases” section.

9. Are small angle X-ray scattering and small angle X-ray diffraction the same thing?

In the early days, Small Angle X-ray Scattering only referred to the scattering of X-rays from ultrafine particles in the low angular range , whereas nowadays, Small Angle X-ray Scattering generally refers to the scattering of X-rays in the low angular range.

The physical phenomenon of coherent scattering (existence of phase relations) of X-rays onto a crystal is called diffraction, and it is diffraction even at low angles. For example, a phase with a d-value of 31.5 Å has a corresponding diffraction of 2.80° (Cu-Kα), and if the phase has a high degree of crystallinity, the 31.5 Å peak is still very sharp. Thin films can also produce X-ray diffractions that depend on the thickness of the film and the microstructure of the film and are concentrated in the small angular range. In these cases, the small-angle X-ray scattering intensity of the sample comes primarily from the diffraction of the sample, called angular X-ray diffraction. For such samples, the interest is in their maximum d-value or film thickness and structure, which must be studied for small-angle X-ray diffraction.

Coherent scattering also occurs when X-rays are irradiated into ultrafine powder particles (with a particle size of less than a few hundred angstroms, whether crystalline or amorphous), also in the low-angle region. However, the characteristics of the coherent scattering pattern produced by fine particles are quite different from those of the small-angle X-ray diffraction pattern produced by very large crystal spacings or thin films, as described above. Small-angle diffraction is generally applied to the determination of ultra-large crystal plane spacing or film thickness as well as the micro-periodic structure of thin films and the distribution of periodically arranged pores; small-angle scattering is applied to the determination of the properties related to the distribution of pores in ultra-fine powders or loosely packed porous materials.

X-rays irradiated to the sample will also occur non-coherent scattering, its intensity, respectively, are also mainly concentrated in the low-angle range, Compton scattering belongs to this category, the result is to increase the background.

10. I am working on mesoporous materials. The mesopores (pore size 2-50 nm) are arranged in the material in an orderly fashion, like a crystal lattice, and the pore walls and material are amorphous. Why is XRD able to roughly measure the spacing between holes? What I understand is that the holes are arranged in an orderly fashion, so there are diffraction peaks at small angles, and the peaks on the (001) side are related to the hole diameter. But I don't know why?

It has to do with the long period: large holes require large periods, or “pore face” spacing, similar to “crystal face spacing”.” A “hole” means that the area of the crystal is not filled with atoms, and without filled atoms there are no diffraction peaks. The boundaries of the holes are closely packed with atoms, which have a relatively high atomic density, corresponding to strong diffraction and higher intensity.

Large holes with large apertures and large spatial repetition periods (i.e., long periods) correspond to large crystal plane distances, producing diffraction in the small angular region. The said (001) has a strong line corresponding to a material with a longer crystal C-axis, and if the first line is (100) then the A-axis is longer.