Difference between XRD polycrystalline and single crystal materials

X-ray diffraction (XRD) is a powerful technique used to analyze crystal structures. In XRD analysis, the sample is irradiated by X-rays, which produces diffraction phenomenon, so that information about the crystal structure of the sample can be obtained; according to the type of sample, XRD analysis can be divided into polycrystalline diffraction and single crystal diffraction.

1. Single-crystal materials

Single-crystal material means that there is only one crystal in the whole sample, and the size of the crystal is large, usually between tens of micrometers and several millimeters. In XRD analysis, single-crystal material shows a series of sharp diffraction peaks, which correspond to the diffraction of different crystal planes; the number and distribution of the peaks in the single-crystal diffraction pattern are related to the crystal structure, crystal cell parameters and orientation of the sample.

The diffraction pattern of single crystal material has the following characteristics:

(1) Fewer diffraction peaks: Since single crystal material has only one crystal, the number of diffraction peaks is less, which is manifested as a series of sharp diffraction peaks.

(2) Narrower distribution of diffraction peaks: single-crystal materials have a narrower distribution of grain orientation, resulting in a narrower distribution of diffraction peaks in the two-dimensional diffraction pattern.

(3) Regular shape of diffraction peaks: The regular shape of the diffraction peaks of single-crystal materials is related to the symmetry of the crystal plane and the cell parameters.

Figure 1: Simple single crystal electron diffraction patterns

Figure 2: A single-crystal electron diffraction pattern of ordered chalcogenides with a sixfold period along the [111]p direction (electron diffraction pattern of ordered phases)

2. Polycrystalline materials

Polycrystalline materials are composed of many small crystals, these small crystals are called grains, the grain size of polycrystalline materials is generally between a few nanometers to tens of micrometers; in XRD analysis, polycrystalline materials are manifested as a series of diffraction spots, which are corresponding to the diffraction of different crystalline surfaces, the number and distribution of spots in the polycrystalline diffraction patterns are related to the sample's crystalline structure, grain size and orientation distribution.

The diffraction pattern of polycrystalline materials has the following characteristics:

(1) Higher number of diffraction spots: Since polycrystalline materials are composed of many small crystals, the number of diffraction spots is higher, which is manifested as a series of discrete diffraction peaks.

(2) Wide distribution of diffraction spots: the grain orientation of polycrystalline materials is widely distributed, resulting in a wide distribution of diffraction spots in the two-dimensional diffraction pattern.

(3) Various shapes of diffraction spots: The diffraction spots of polycrystalline materials have various shapes, which are related to the orientation of the grains and the spacing of the crystal surfaces.

Figure 3: Diffraction patterns of polycrystalline electrons

3. Differences between polycrystalline and single crystal materials

(1) The number and distribution of diffraction spots: polycrystalline materials have a larger number of spots in the diffraction pattern and a wider distribution; while single-crystal materials have a smaller number of peaks in the diffraction pattern and a narrower distribution.

(2) Shape of diffraction spots: the diffraction spots of polycrystalline materials have various shapes, which are related to the orientation of the grains and the spacing of the crystal planes, while the diffraction peaks of single-crystalline materials have regular shapes, which are related to the symmetry of the crystal planes and the parameters of the crystal cells.

(3) Diffraction intensity: the diffraction intensity of polycrystalline materials is weaker because the grains are smaller and the diffraction phenomenon is not significant; while the diffraction intensity of single-crystalline materials is stronger because the crystal size is larger and the diffraction phenomenon is obvious.

(4) Crystal orientation: the crystal orientation of polycrystalline materials has a wider distribution, resulting in a wider distribution of diffraction spots in the two-dimensional diffraction pattern; while the crystal orientation of single-crystal materials has a narrower distribution, resulting in a narrower distribution of diffraction peaks in the two-dimensional diffraction pattern.

4 Application

Take lithium-ion battery cathode materials as an example, polycrystalline materials usually have better electrochemical performance, but poorer cycling stability; while single-crystalline materials have excellent cycling stability and electrochemical performance, although it is more difficult to prepare; through the XRD analysis, it can accurately determine whether lithium-ion battery cathode materials are polycrystalline or single-crystalline, which can provide the basis for optimizing the design of the material and the preparation process.