Low-temperature anomaly in heat capacity due to overlapping the spectrums in molecular crystals

الأصل · EN

Investigations of dynamic and thermodynamic properties for a molecular crystal tris-hexafluoroacetylacetonate-iron Fe(O₂C₅HF₆)₃ are presented. Heat capacity Cₚ(T) has been measured by adiabatic calorimetry method in the temperature range 4.8-321 K. An anomaly with a maximum at Tc = 44.6 K has been discovered. Intermolecular vibrations spectrum was calculated by lattice dynamics method in quasiharmonic approximation. Intramolecular frequencies are found by solving the Schrödinger equation in approach of small harmonic oscillations. In the frequency interval ≈ 30-70 cm⁻¹ overlapping the spectrums intra- and intermolecular oscillations has been found. The good agreement for calculated and experimental Cₚ(T) occurred to be possible for two sets of the force constants. These sets describe two phases above and below Tc. The difference between phases is connected with freezing of rotation CF₃ groups. It has been concluded that the interaction between different modes leads to phase transition and anomaly in heat capacity.