Thermodynamic Studies of Rod- and Spindle-Shaped β-FeOOH Crystals

Different morphologies of β-FeOOH including rod- and spindle-shaped crystals were synthesized via a hydrothermal reaction at low temperature. The molar heat capacities of the obtained samples were determined by a precision automated adiabatic calorimeter over the temperature range of (78 to 390) K. The observed results demonstrated that the change of the molar heat capacity with thermodynamic temperature was different for the rod and spindle-shaped β-FeOOH crystals. Polynomial equations of the molar heat capacities as a function of temperature were fitted by a least-squares method for the rod- and spindle-shaped β-FeOOH crystals. Smoothed heat capacities and thermodynamic functions of the obtained samples, such as H(T/K) − H(298.15) and S(T/K) − S(298.15), were calculated on the basis of the fitted polynomials and the relationships of the thermodynamic functions. In addition, the as-prepared samples were also characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and thermal gravimetric analysis (TGA).