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It is obvious and well known that, when starting studies on a given sample, it is necessary to be sure of the chemical and phase composition of the sample. The most ideal method is to verify this by X-ray (or neutron) diffraction and subsequent structural analysis. Such initial analysis must use the well-known set of assumptions based on 100 years of X-ray diffraction studies. Without the use of these assumptions or rules, the subsequent results of any studies will not be valuable and may be erroneous. The most important rule is that the diffraction pattern is a kind of fingerprintof the given crystal/compound. Thus, the second rule is that the diffraction pattern of the mixture of phases is a simple sum of diffraction patters from the components.

In a recent paper (J. Solid State Chem.148, 316 (1999)) Solans et al.presented the structure of different phases of LiKSO4and a new phase transition at about 226 K (268 K on heating). We believe that most of the results commented on are not credible.