Mg(HSeO3)(2)center dot 3H(2)O: crystal structure, rotational and orientational disorder of 'free' HSeO3- ions and low temperature phase transition

Mg(HSeO3)2 . 3H(2)O was obtained from a concentrated aqueous solution of Mg (HSeO3)(2) and characterized by means of single crystal X-ray diffraction, Raman spectroscopy and thermoanalytical measurements. The crystal structure was found to contain hexagonally close-packed [Mg-2(SeO2OH)(3)(H2O)(6)](+) cations of symmetry 3/m, which are built up from two MgO3(H2O)(3) octahedr a connected via Se atoms of three trigonal pyramidal SeO2OH units. The octa hedral interstices are occupied by rotationally-orientationally disordered 'free' [SeO2OH](-) anions. The cations of the resulting NiAs-like structure are connected among themselves and with the anions by ordered and disorder ed hydrogen bonds, respectively. DSC and Raman measurements indicate a reve rsible phase transition between 230 and 250 K. (C) 1999 Elsevier Science B. V. All rights reserved.