3.1.1- Conventional in-laboratory diffractometer

One can estimate limits by considering the recognized maximum number of parameters refinable by the Rietveld method.

For minimal FWHM of the order of 0.12° 2-theta, 50 to 70 free atomic coordinates (x,y,z) are refinable reasonably .

This corresponds to 17 up to 23 independent atoms in general position .

Corresponding to these limits, cell volumes can be more or less estimated, depending on the crystal system and on the Bravais lattice. For centrosymmetrical space groups, these maximal volumes are the following :

   Vmax(Å3)    Multiplicity of the    Lattice     System 
                general position
   500                2                 P         Triclinic 
  1000                4                 P         Monoclinic 
  2000                8                 C              ,, 
  2000                8                 P         Orthorhombic 
  4000               16              A,B,C,I           ,, 
  8000               32                 F              ,, 
  etc for tetragonal, hexagonal and trigonal
 12000               48                 P         Cubic
 24000               96                 I         Cubic
 48000              192                 F         Cubic
Translated in maximal number of reflections, any of these above maximal possibilities corresponds to approximately 1000 to 1500 reflections for a pattern extending from 5 to 150° 2-theta recorded with a ~1.5 Å wavelength.

One will have approximately 20 reflections per xyz refined parameter.

In a single crystal study, 10 reflections per parameter, including the thermal ones, is something considered as normal.

The fact that a larger value is proposed for powder data is a consequence of reflection overlapping.