EXPERIMENTAL METHODS FOR X-RAY STRUCTURE DETERMINATION

 Various experimental methods are used for analysing the structure of crystals.

1. The Powder Method

Debye and Scherrer (1916) and Hull (1917) devised a method of X-ray crystal analysis which permits the use of a substance in powder form. In this method, the wavelength is fixed by using monochromatic X-rays, with respect to the incident beam. A fine crystalline powder is taken in a thin walled capillary tube. Each particle in the powder acts as a tiny crystal oriented randomly and hence the angle & varies. Due to random orientation of particles in a crystal, the X-rays are scattered from all sets of planes. The scattered rays are further detected by using an X-ray sensitive film.

X-rays are generated in a tube and these are made to fall on a monochromatic crystal to get monochromatic beam. The beam is now passed through a slit and made incident on a sample powdered film in a thin walled glass capillary tube. Due to reflection from the various lattice planes, diffraction maxima arise. The diffraction maxima are photographed on a film (Fig. 4.10). Since the powder sample consists of microcrystal in all possible orientations, the rotation of crystal sample is not necessary.

 The diffracted X-rays form concentric cones originate from the powder. On a narrow strip of film, ares appear on either side of the bright spot at the centre. Each pair of arcs which is equidistant from the central bright spot gives the position of a reflection of a definite order from a particular plane and the value of glancing angle is calculated by taking into account the distance of the are (say S meter) from the central spot and its distance from the capillary i.e., the radius of the are. From this data, the d-spacings between the planes can be calculated.

 The glancing angle, 20 =S/r or 0 = S/2r

 0sa obtained will be in radians which can be converted into degrees.

 0 in radiation = 0× 180/pi degrees.

The photographic plate is then developed. Spots of different sizes and intensities are obtained which is characteristic of a crystal. The spots. (except

Applying Bragg's equation, nd 2d sin d can be calculated.

Laue's Method 

In this method. Dis fixed and the wavelength of X-rays in varied 2 rays from the X-ray tube are collimated into a beam by a lead shield having a hole in it moved on the circular crystal in mounted in the path of the X-rays beam. When the X-rays are incident on the pasily jonizable gos alu crystal, each atom in a plane acatters some X-rays which in turn give an impression on thisnization troduced i photographic plates The photographic plate is then developed. Spots of different sizes and intensities are obtained which is characteristic of a crystal. The spots. (Except) represent the scattering part of the original beam through various characteristic angles Different arrangements of spots.