Studies of aromatic and heteroaromatic compounds under high pressure in recent years have attracted great attention to the possibility of the synthesis of amorphous hydrogenated materials with attractive technological properties. In this context, pyridine is of particular interest in terms of its rich polymorphism [1,2]. With an increase in pressure, pyridine crystallizes at room temperature into a disordered phase I, which, with a further increase in pressure, transfers into an ordered phase II. The same phase I obtained by cooling the liquid at a temperature of 231 K at atmospheric pressure. According to the literature, melting occurs at an ambient temperature of 0.55 GPa [2], but reports indicate 1 GPa as the onset of crystallization. In addition it was reported that the glassy phase is formed at atmospheric pressure and low temperature.
We present the high pressure ultrasonic study and dielectric spectroscopy of liquid and solid pyridine. The measurements were performed by the pulsed ultrasonic method using LiNO3 plates as piezoelectric sensors with carrier frequencies of 5-10 MHz [3]. Pyridine phase diagram was determined at 77-295 K in the pressure range 0<4.2 GPa and 150K<T<350K. During isobaric cooling in the range from 0 to 0.6 GPa, pyridine goes into a mixed crystalline + glassy state, as evidenced by the presence of a characteristic relaxation process. The glassy state is unstable, and upon subsequent heating crystallization occurs, accompanied by followed by a decrease in the amplitude of the relaxation process to zero and a transition from the orientationally ordered phase, characterized by low dielectric constant values, to the orientationally disordered phase. Relaxation and elastic properties of pyridine can be qualitatively described using the soft sphere or the Lennard-Jones model. The value of the Poisson coefficient for glassy pyridine indicates a significant contribution of non-central forces to the intermolecular potential.
Acknowledgments: This work was supported by Russian Science Foundation, Grant 19-12-00111.
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3. E.L. Gromnitskaya, I.V. Danilov, A.G. Lyapin and V.V. Brazhkin, Phys.Chem.Chem.Phys.2019, 21, 2665.