**7.1 Precautionary/explanatory notes**

Although the basic idea about chemical composition, cell volume and density are needed to obtain a solution to an unknown phase, search using Le Bail, Pawley or ITO, DICVOL, TEROR, EXPO can always be widened to obtain initial profile parameters. However lower symmetry crystal systems like monoclinic and triclinic should not be included unnecessarily. These programs are likely to give multiple solutions to single set of reflection and it remains up to the user's judgment in these cases to choose a suitable solution. The multiple solutions are more prominent when lower symmetry systems are included, and sometimes the search criteria need to be adjusted to remove unrealistic solutions. In general, unrealistic solutions tend to possess either of the characteristics listed below or their combination:


<sup>1</sup> The case of rhombohedral symmetry which is generally expressed in hexagonal axes format should be treated individually. It is normal in some materials which crystallize in rhombohedral symmetry to have large 'c/a' ratio when expressed in hexagonal axes. Few of the examples are Telurides, Selenides and lannonites.
