X-ray emitters for diffraction experiments are usually referred to as "tubes" not "lamps". In other words the fact that they contain a tungsten filament (like in a lamp) is secondary to the fact that the third component of the device, the emitter, alias the target, is made of a metal whose characteristic X-ray emission (K-alpha line) is utilized for the diffraction experiment. The tube is in fact a sort of a triode where the filament is the cathode, the target is the anode (also called anticathode) and the intermediate element (the grid; in actual fact a focusing cup) is also a cathode.  These three elements are contained in a vacuum tube out of which the X-rays emerge through a thin window made of beryllium ( a low absorption material).

An X-ray tube is thus characterized by the metal with which the anode (anticathode) is made. The choice of tube depends on the wavelengths of X-rays one wants to use for the experiment. The most commonly used tubes are those with Cu (lambda = 1.5418 angstrom) and Mo (lambda = 0.7107 angstrom) targets. Other possible alternatives include Fe (1.9373), Co (1.7902) and Cr (2.2909). The lower limit for the interplanar distance explored by the X-rays is equal to ½ lambda (see Braggs equation). Therefore, the first criterion for the choice of tube will be the amount of reciprocal space in the crystal one needs to explore (i.e. the total number of reflections collected). This number is the highest possible when collecting single crystal data for structure determinations, hence the tube in this case is usually a Mo tube. When the separation between diffraction lines (in terms of 2theta angle) becomes more crucial, such as in the case of the powder diffractometer, a longer wavelength (i.e. Cu) is normally used.

Other criteria will sometimes dictate the choice, especially when a major element contained in the sample may produce strong X-ray fluorescence effects when hit by X-rays with a characteristic energy just above its absorption edge. Such is the case, for instance, of Fe with a Cu tube. In this case, one may want to switch to an Fe or Cr tube in order to "clean" the experiment from unwanted x-ray fluorescence radiation which may obscure the diffractions.

Hello Mohammad,

The main parameter which is associated with different XRD lamps is the so-called lambda which corresponds to characteristic wavelength of lamps radiation (and is specified mostly in angstroms).

Generally the lower lambda parameter the more accurate diffraction because one can only 'see' interplanar distances that are at least comparable with lambda/2.

For typical Cu-Kalpha radiation lambda is 1.54 A. In comparison the Molybdenum lambda is about 0.7 A. So when you use lamp with lower lambda the radiation is more energetic (and therefore can penetrate better studied material)

Another way to look at this is when you get RDF (or PDF - pair distribution function). The lower lambda in this case can show bonds with smaller distances.

Sometimes you must take care of energetics of your radiation to ensure you dont influence your studied material.

Dear Konrad Gruszka

Thanks for your reply

All mentioned above are OK. But what about florescence effect?

If you will examine Bragg equation you will see that diffraction angles depends on the wavelengths x-rays, thus you will improve separation in diffraction pattern changing Lambda Mo(0.71Angtr/)=>Lambda Cu(1.54 Angstr.) for over two times.

radiation from lighter elements with relatively low energy (long wavelength)has low penetrating power through samples with heavy elements, thus absorption is increased(but scattering also) when you using long wavelengths in experiment.

X-ray emitters for diffraction experiments are usually referred to as "tubes" not "lamps". In other words the fact that they contain a tungsten filament (like in a lamp) is secondary to the fact that the third component of the device, the emitter, alias the target, is made of a metal whose characteristic X-ray emission (K-alpha line) is utilized for the diffraction experiment. The tube is in fact a sort of a triode where the filament is the cathode, the target is the anode (also called anticathode) and the intermediate element (the grid; in actual fact a focusing cup) is also a cathode.  These three elements are contained in a vacuum tube out of which the X-rays emerge through a thin window made of beryllium ( a low absorption material).

An X-ray tube is thus characterized by the metal with which the anode (anticathode) is made. The choice of tube depends on the wavelengths of X-rays one wants to use for the experiment. The most commonly used tubes are those with Cu (lambda = 1.5418 angstrom) and Mo (lambda = 0.7107 angstrom) targets. Other possible alternatives include Fe (1.9373), Co (1.7902) and Cr (2.2909). The lower limit for the interplanar distance explored by the X-rays is equal to ½ lambda (see Braggs equation). Therefore, the first criterion for the choice of tube will be the amount of reciprocal space in the crystal one needs to explore (i.e. the total number of reflections collected). This number is the highest possible when collecting single crystal data for structure determinations, hence the tube in this case is usually a Mo tube. When the separation between diffraction lines (in terms of 2theta angle) becomes more crucial, such as in the case of the powder diffractometer, a longer wavelength (i.e. Cu) is normally used.

Other criteria will sometimes dictate the choice, especially when a major element contained in the sample may produce strong X-ray fluorescence effects when hit by X-rays with a characteristic energy just above its absorption edge. Such is the case, for instance, of Fe with a Cu tube. In this case, one may want to switch to an Fe or Cr tube in order to "clean" the experiment from unwanted x-ray fluorescence radiation which may obscure the diffractions.

Dear Romano Rinaldi

Really thanks for your kind reply.

Great