In Search of the True Structure of the Sodium Chromium Alum: Crystal Growth and Structure of the Double Salt NaCr(SO4)2(H2O)6

Z. Naturforsch. 2012, 67b, 1229-1234 / DOI: 10.5560/ZNB.2012-0225 Received August 23, 2012 The cubic structure of the sodium chromium alum [NaCr(SO4)(2)(H2O)(12)] has been published in 1968 with a lattice parameter a which is by 0.17 angstrom larger than the one of the potassium chromium alum. In order to investigate this "lattice parameter anomaly", the growth of Na-Cr alum crystals was attempted with the aim to re-determine their structure. However, instead of cubic alum crystals, monoclinic crystals of composition NaCr(SO4)(2)(H2O)(6) were obtained. Their structure consists of one-dimensionally infinite [Na(SO4)(2)](3-) chains and "isolated" [Cr(H2O)(6)](3+) complexes, held together by electrostatic attraction and numerous hydrogen bonds. The compound is erroneously addressed as the sodium chromium alum in the Powder Diffraction File PDF-2.