Thermodynamics and vibrational spectrums for molecular crystals of β-diketonate of metals: Modeling in frameworks of the lattice dynamics method

Computer modeling and low temperature calorimetry investigations of dynamic and thermodynamic properties for a molecular crystal tris-hexafluoroacetylacetonate iron Fe(O2C5HF6)3 are presented. A heat capacity Cp(T) has been measured by adiabatic calorimeter method in the temperature range 4.8–321 K. An anomaly passing through the maximum at temperature Tc = 44.6 K has been discovered. Intermolecular vibrational spectrum was calculated by lattice dynamics method in quasiharmonic approximation. Intramolecular frequencies are found by solving the Schrödinger equation in harmonic approximation. In the frequency interval ≈30–70 cm−1 overlapping the spectrums intra- and intermolecular oscillations has been found. Calculation of intramolecular spectrums has been carried out on a basis of best coincidence calculated and experimental heat capacities. The good agreement for calculated and experimental Cp(T) was found for two sets of the force constants. Obtained results allow us to assume that heat capacity anomaly mark on a change in intra- or intermolecular vibrational modes or possible resonance interaction between these modes near Tc.