Module 15 
                        Powder Diffraction 
        
       Learning Objectives 
           Introduction to Powder X-ray Diffracion  
           Bragg’s Law of Diffraction 
           Application of Powder XRD 
           Determination of an Unknown  
           Strengths and Limitations of X-ray Powder Diffraction 
        
       15.1 Introduction to Powder X-ray Diffraction  
          Powder X-ray  diffraction  (PXRD)  is  perhaps  the  most  widely  used  analytical  technique  for 
       characterizing materials. As the name suggests, the sample is usually in a powdery form, consisting of 
       fine grains of single crystalline material to be studied. The technique is also used widely for studying 
       particles in liquid suspensions or polycrystalline solids (bulk or thin film materials). 
          The term 'powder' really means that the crytalline domains are randomly oriented in the sample. 
       Therefore when the 2-D diffraction pattern is recorded, it shows concentric rings of scattering peaks 
       corresponding to the various d spacings in the crystal lattice.  
        
                                                 
        
        
          The positions and the intensities of the peaks are used for identifying the underlying structure (or 
       phase) of the material. For example, the diffraction lines of graphite would be different from diamond 
        
        
       even though they both are made of carbon atoms. This phase identification is important because the 
       material properties are highly dependent on structure (just think of graphite and diamond). 
        
                                                   
          Powder diffraction data can be collected using either reflection  or transmission geometry, as 
       shown below.  
                                                  
        
          Because the particles in the powder sample are randomly oriented, these two methods will yield 
       the same data. 
          A powder XRD scan typically represents a plot of scattering intensity v/s. the scattering angle 2θ 
       or the corresponding d-spacing. The peak positions, intensities, widths and shapes all provide important 
       information about the structure of the material. 
        
        
                                           
                   The diffraction from ideal crystalline substances are characterized by well defined Bragg peaks in 
       X-ray diffraction. The crystalline substances have long range order. The amorphous substances do not 
       possess this long range order. So the diffraction from them do not show sharp Bragg peaks. In amorphous 
       phase, X-rays will be scattered in many directions leading to a large bump distributed in a wide range 
       (2θ) instead of high intensity narrower peaks. 
          Max  von  Laue,  in  1912,  discovered  that  crystalline  substances  act  as  three-dimensional 
       diffraction gratings for X-ray wavelengths similar to the spacing of planes in a crystal lattice. X-ray 
       diffraction is based on constructive interference of monochromatic X-rays and a crystalline sample. These 
       X-rays are generated by a cathode ray tube, filtered to produce monochromatic radiation, collimated to 
       concentrate, and directed toward the sample. In principle, X-ray diffractometers consist of three basic 
       elements: an X-ray tube, a sample holder, and an X-ray detector.  
                              
                              
        
            
                                                                   
                                              
                        The X-ray wavelengths are characteristic of the target material (Cu, Fe, Mo, Cr). Filtering, by 
           foils or crystal monochrometers, is required to produce monochromatic X-rays needed for diffraction. Kα1 
           and K  are sufficiently close in wavelength such that a weighted average of the two is used. Copper is the 
               α2
           most common target material for single-crystal diffraction, with CuK  radiation = 1.5418 Å. These X-rays 
                                                       α
           are collimated and directed onto the sample. As the sample and detector are rotated, the intensity of the 
           reflected X-rays is recorded. When the geometry of the incident X-rays impinging the sample satisfies the 
           Bragg Equation, constructive interference occurs and a peak in intensity occurs. A detector records and 
           processes this X-ray signal and converts the signal to a count rate which is then output to a device such as 
           a printer or computer monitor. 
                       The  interaction of the incident rays with the sample produces constructive interference (and a 
           diffracted ray) when conditions satisfy Bragg's Law. This law relates the wavelength of electromagnetic 
           radiation to the diffraction angle and the lattice spacing in a crystalline sample. These diffracted X-rays 
           are  then  detected,  processed and counted. By scanning the sample through a range of 2θ angles, all 
           possible  diffraction  directions  of  the  lattice  should  be  attained  due  to  the  random  orientation  of  the 
           powdered material. Conversion of the diffraction peaks to d-spacings allows identification of the material 
           because each material has a set of unique d-spacings. Typically, this is achieved by comparison of d-
           spacings with standard reference patterns.