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Specificmolar heat capacity and thermodynamic properies of hf(so4)2(h2o)4

BaikushevaG.Dimitrova, S.Genieva, R.Yankova, N.alachev

Using hydrothermal synthesis, Hf(SO4)2(H2O)4 was obtained and studied by X-ray diffraction, thermogravimetric analysis and infrared spectroscopy. It was found that the compound had orthorhombic symmetry (Fddd) and unit cell parameters a=25.8700 Å, b=5.5300 ú, c=11.5900 ú, á=â=ã = 90.00°. Absorption bands characteristic for the valence and deformation vibrations of the water molecules, as well as for the valence vibrations of the bidentate sulfate anion coordinated with the hafnium atom were registered in the IR spectra. The thermogravometric analysis carried out up to 850°Ñ did not end with formation of solid HfO2 residue but with formation of dihafnyl sulfate Hf2O3(SO4). Using the experimental data obtained from the differential scanning calorimetry (DSC), the specific heat capacities (Cp,m) and the thermodynamic values ( 0 ' m T T S , 0 ' m T T  H , – 0 ' m T T  G /T) of all the intermediate phases by the dehydration and decomposition of Hf(SO4)2(H2O)4 were calculated. The temperature dependences of their molar heat capacities were determined by regression analysis.