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

TL;DR

This study investigates a low-temperature heat capacity anomaly in a molecular crystal caused by overlapping vibrational spectra, revealing a phase transition linked to molecular rotation freezing.

Contribution

It introduces a combined experimental and theoretical analysis of vibrational spectrum overlap and its role in the heat capacity anomaly in a specific molecular crystal.

Findings

Discovered a heat capacity anomaly at 44.6 K.

Identified spectrum overlap between intra- and intermolecular vibrations.

Linked phase transition to freezing of CF3 group rotation.

Abstract

Investigations of dynamic and thermodynamic properties for a molecular crystal tris-hexafluoroacetylacetonate-iron $Fe(O_2C_5HF_6)_3$ are presented. Heat capacity $C_p(T)$ has been measured by adiabatic calorimetry method in the temperature range $4.8-321 K$. An anomaly with a maximum at $T_c = 44.6 K$ has been discovered. Intermolecular vibrations spectrum was calculated by lattice dynamics method in quasiharmonic approximation. Intramolecular frequencies are found by solving the Schr\"odinger equation in approach of small harmonic oscillations. In the frequency interval $\approx 30-70 cm^{-1}$ overlapping the spectrums intra- and intermolecular oscillations has been found. The good agreement for calculated and experimental $C_p(T)$ occurred to be possible for two sets of the force constants. These sets describe two phases above and below $T_c$. The difference between phases is connected with freezing of rotation $CF_3$ groups. It has been concluded that the interaction between different modes leads to phase transition and anomaly in heat capacity.