HEAT CAPACITY AND THERMODYNAMIC PROPERTIES OF α- AND β -Zn(ClO_4)_2

摘要

The heat capacity of α- and β-Zn(ClO_4)_2 was measured as a function of temperature by adiabatic calorimetry. The results were used to calculate the thermodynamic properties of the two polymorphs under standard conditions: For α- Zn(ClO_4)_2,C_p~0(298.15 K)=188.7± 0.3 J/(mol K),S~0(298.15 K)=219.2±0.2 J/(mol K),Φ~0(298.15 K)=108.3 ±0.2 J/(mol K),and H~0(298.15 K)-H~0(0)=33080±30 J/mol.For β -Zn(ClO_4)_2,C_p~0(298.15 K)=185.6±0.3 J/(mol K),S~0(298.15 K)=221.6 ±0.2 J/(mol K),Φ~0(298.15 K)=111.0±0.2 J/(mol K),and H~0(298.15 K)-H~0(0)=32960±30 J/mol. At elevated temperatures, the thermal behavior and heat capacity of α- and β-Zn(ClO_4)_2 were studied by differential scanning calorimetry, The heating curves showed no thermal events that might be attributed to the α→β phase transition reported to occur above 370 K.Gibbs energy calculations also demonstrate that this phase transition is unlikely. The lattice dynamics of the two polymorphs was analyzed by evaluating the contributions of various vibrational modes to the heat capacity.