Hi, The definition for "unknown" is here "not included in the Cambridge Structure Database (CSD) nor in the Inorganic Crystal Structure Database (ICSD)" meaning that the X-ray crystal structure has not been determined. Most catalogs (Web-searchable or paper versions) of chemical companies do not indicate this essential point for solids : does the crystal structure has been determined from X-ray single crystal (or powder) diffraction data ? And this is a pity. It seems that these companies consider as sufficient that the X-ray powder pattern fits with one JCPDS-ICDD card (in case of inorganic products, because powder diffraction is not normally used for checking organic solids). The problem is that many JCPDS-ICDD cards even not give cell indexation (they also don't care with the information : is the crystal structure known ?). Examples can be found of compounds bought at a chemical company of which the structure was determined ab initio from powder diffracton data. Frequently the formulae are bizarre with an unprecise number of water molecules (want one example ? see for instance Zr(OH)2(NO3)2 4.7H2O J. Solid State Chem. 94 (1991) 27-35). Incidently, this means that sometimes you may buy rather undefined products...:-( So here is my question. I was searching for unknown compounds, inorganic and organic stable solids, available in large quantity from chemical companies, preferably exclusively in powder form. These companies were unable to produce for me the list of their "unknowns" compounds (i.e. with undetermined crystal structure). Has somebody yet compared the CSD and ICSD databanks with catalogs from chemical companies, and found "unknowns" ? More efforts should be asked from these companies. We, buyers, should require that they add one clear mention in their catalog : "unknown (or known) crystal structure". This is not difficult to check against the CSD and ICSD databanks. This would be the info that maybe the true composition is not as accurate as suggested by the vendor... The add could also consist in the entry number in CSD and/or ICSD (analogous to the CAS number) If you have identified such "unknowns" in catalogs please contact me. The target is the organisation of a Round Robin on ab initio structure determination from powder diffraction data. Armel Le Bail - Universite du Maine, Laboratoire des Fluorures, CNRS ESA 6010, Av. O. Messiaen, 72085 Le Mans cedex 9, France http://fluo.univ-lemans.fr:8001/ http://www.cybercable.tm.fr/~cristal/welcome.html chemweb: A list for Chemical Applications of the Internet. To post to list: mailto:chemweb@ic.ac.uk Archived as: http://www.lists.ic.ac.uk/hypermail/chemweb/ To (un)subscribe, mailto:majordomo@ic.ac.uk the following message; (un)subscribe chemweb List coordinator, Henry Rzepa (mailto:rzepa@ic.ac.uk)