Phase transitions in M(ClO4)2.6H2O (M = magnesium, zinc). Investigations by adiabatic calorimetry and infrared spectroscopy.

Abstract

IR spectra of Mg(ClO4)2.6H2O at 10 K indicate a complex structure contg. at least 16 distinct types of H2O protons. At 107.9 K this structure transforms sharply to a much simpler disordered form (?Strans = 7.754 J K-1 mol-1), and at 273.2 K it transforms more gradually to a form in which all H2O protons are equiv. (?Strans = 8.25 J K-1 mol-1). There is also a phase transition at 325.36 K (?Strans = 21.9 J K-1 mol-1). IR spectra of Zn(ClO4)2.6H2O at 10 K indicate a disordered structure, with a continuum of proton environments, probably related to the structure of the Mg compd. above 107.9 K. The phase transitions in the Zn compd. at 70.1, 209.8, 290.5, and 349.0 K (?Strans = 3.70, 0.08, 9.50, and 21.6 J K-1 mol-1, resp.) occur without a major change in the IR spectrum. The measured entropy changes in the Mg, Zn, and Cd compds. is probably related to the no. of distinguishable types of H2O protons that already exist at 10 K.