Low-temperature heat capacity of benzene derivatives crystals
Institute of Low Temperature and Structure Research, Wroclaw, Poland, July 23–28
Blogpost by Oleksandr Kryvchikov, Prof., Ph.D

The STSM was planned for a three-week period, during which experimental studies were conducted on the heat capacity of specially selected chloromethyl-substituted benzene crystals. The temperature range covered was from 2 to 300 K. The heat capacity of the following materials was investigated over the three weeks: 1,2,3,4,5-pentachloro-6-methylbenzene; 1-chloro-2,3,4,5,6- pentamethylbenzene; 1,2,3-trichloro-4,5,6-trimethylbenzene; and 1,2,3,5-tetrachloro-4,6- dimethylbenzene.

Four samples were prepared to measure the heat capacity using PPMS. For each sample, heat capacity measurements were taken with both the calorimetric insert and the insert in place with the sample. Eight temperature-dependent heat capacity measurements were performed for each of the four samples. The measurements were conducted after the samples were preliminarily cooled to a minimum temperature. For two of the materials, the samples were cooled at a rate of 3 K/min to achieve a metastable state with orientational disorder.

The contribution of the sample’s heat capacity was determined, and new experimental data on temperature dependence were analysed in the context of existing theoretical models.

The temperature dependence of C_p provides new insights into the order-disorder phenomena in aromatic compounds composed of planar molecules with controlled substitution of chlorine atoms, excluding the methyl group. A significant substitution effect is observed in the low-temperature heat capacity, where it notably influences both the position and amplitude of the calorimetric boson peak at temperatures below 10 K. The phonon parameters and linear term are influenced by the number of chlorine atoms in the benzene molecule. This is particularly relevant for understanding the interaction between acoustic and optical vibration excitations. This research contributes to a deeper understanding of the thermodynamic properties of chloromethylbenzene crystals and their behaviour under various conditions, offering valuable insights into the fundamental principles governing the thermal properties of aromatic compounds.