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Z. Naturforsch. 2012,
67b, 1229 – 1234
doi:10.5560/ZNB.2012-0225
In Search of the True Structure of the Sodium Chromium Alum: Crystal Growth and Structure of the Double Salt NaCr(SO4)2(H2O)6
Kristallographie, Institut für Geowissenschaften der Albert-Ludwigs-Universität Freiburg, Hermann-Herder-Str. 5, 79104 Freiburg, Germany
Reprint requests to Dr. Egbert Keller. Fax: 0049 761 6434. E-mail:
egbert.keller@krist.uni-freiburg.de
Received August 23, 2012 / published online November 30, 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 Å 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.
Key words: Chromium Alums, Double Salt, Crystal Growth, Crystal Structure