Question

Q.1 Differentiate between single crystal and polycrystalline. ( in details)

Answer 1

An single crystal is one single block where all the symmetry operations of the structure are valid for the whole block. However, this is so for an ideal single crystal only. In real single crystal often consists of mosaic distribution of regions which are slightly misoriented. This mosaic spread reduces the extinction of the ideal perfect crystal and the kinematical theory is of X-ray diffraction is valid.

The polycrystalline material consists of single crystals described above but of very small sizes of the order of micrometers but randomly distributed when there is no preferred orientation. The chemists often produce polycrystalline samples in their lab. There exist several methods to grow single crystals from the polycrystalline substance. Now X-ray diffraction signature of a single crystal is the existence of Bragg spots. They will be only seen in a diffractometer whenever the Bragg condition is satisfied. A polycrystalline substance will however show Debye rings rather than Bragg spots. To interpret diffraction effects from powder or polycrystalline substances we must consider only he magnitude of the momentum transfer viz. |Q| because due to random orientation the information of the vector properties of Q has been lost. For single crystal diffraction however we retain the vector charecter of Q. Q is defined as Q = 4\pi sintheta/lambda. The crystallographer use an another vector s = sintheta/lambda but it is basically the same as Q.

All the symmetry properties of the structure are present in an ideal single crystal whereas for polycrystalline substances all the symmetry elements are only retained for a single grain which is also a single crystal but very small

Single crystal will have only one arrangement of atoms and the XRD pattern will have diffraction pattern from only that arrangement that we see in regular single crystal XRD pattern.

Poly crystals have multiple crystal together in which have different arrangement in space. Hence their diffraction spots will overlay and you will find ring pattern.

A special case is twin crystal in which two crystal have the same origin although having different arrangement. In these cases the diffraction spots show regular pattern but if look into the 3-D reciprocal pattern you will find two different spot patterns. Separating those can help you solve the structure