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

Abstract

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 onedimensionally 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.