1- First routine contact with a material a priori unknown

The material can be any solid (inorganic, organometallic,..., synthetical or natural).

In this scenario, nothing should be done by diffraction methods before a careful examination of the sample by optical microscopy (polarized and natural light). This examination can reveal inhomogeneities in the sample (to a point where a polyphasic nature could be suspected) and the presence of single crystals sufficiently large for being characterized by using a four-circle diffractometer. The scenario is highly dependent of this first examination : powder or not powder diffraction, that is the question. A routine pattern takes 15-30 minutes on an automated powder diffractometer. Searching in a powder pattern databank will take a few minutes more. The powder diffraction technique is preferred for material identification because it may be really fast. Nevertheless, the success will depends on your know how and on the databanks exhaustivity, we will discuss later on this point. Even if the sample is constituted of sufiiciently large single crystals, an identification by powder diffraction may be preferable to a preliminary study by Weissenberg, Laue or Buerger techniques or even to a fast automated search for the cell with a four circle diffractometer. If the optical examination has revealed inhomogeneities, then you should make several powder patterns on selected different parts (isolated crystals for instance). We will see some practical aspects of the first contact in the next part.


Copyright © 1997- Armel Le Bail