Low temperature thermal properties of cobalt(II) bromate hexahydrate

The heat capacity of cobalt(II) bromate hexahydrate Co(H2O)(6)(BrO3)(2) was measured at temperatures between 13 and 310 K. Infrared and Raman spectra of the crystal were recorded in the frequency range of 400-4000 cm(-1) for the former and 50-4000 cm(-1) for the latter. The heat capacity data were analyzed in harmonic approximation using Debye and Einstein heat capacity functions. Some of the characteristic frequencies of the heat capacity functions were taken from the vibrational spectra and others determined by non-linear least squares fitting of the heat capacity function to the experimental heat capacity data. One Debye temperature (92 +/- 1 K) and three Einstein temperatures (123 +/- 2 K, 231 +/- 1 K, 614 +/- 50 K) were determined as functions of the temperature interval of the heat capacity data used for the fitting. The best-fit characteristic temperatures were constant, except for the third Einstein temperature, against variation of the upper limit temperature of the fitting interval from 16 to 280 K with the lower limit temperature being kept at 13 K. The Debye temperature agreed with a value of 92.4 K derived from the sound velocity. The best-fit Einstein temperatures have counterparts in the Raman spectrum in the lattice vibration region. This is the first detailed analysis of heat capacity data on a structurally complex crystal in which acoustic phonon frequencies from thermal and elastic properties are favorably compared.