Simple rigid molecules, of 1-Cl-, 1-Br-, and 1-I-adamantane, exhibit remarkable polymorphism. These compounds undergo phase transitions between a high-temperature orientationally disordered state and a low-temperature ordered phase (P21/c, Z=4). In this ordered state, the molecules exhibit rotational dynamics around the C3 molecular axis, which aligns with the C-X bond (X = Cl, Br, I)—a phenomenon confirmed by 13C NMR spin-lattice relaxation measurements.
Through heat capacity experiments conducted in the temperature range of 100 mK to 25 K, we identify a Boson peak (BP)-like anomaly in the low-temperature ordered phase of both materials. This anomaly originates from strong interactions between propagating (acoustic) phonons and low-energy quasi-localized (optical) phonons, as described through phonon dispersion relations and vibrational density of states obtained via first-principles calculations based on density functional theory.
Moreover, our experimental results reveal a linear temperature dependence, commonly associated with quantum tunneling between nearly equivalent yet distinct system configurations—referred to as two-level systems (TLS). For chosen materials, using the same first-principles simulation approach, we analyze the intermolecular potential governing molecular rotation along the three-fold axis within the crystal lattice. Interestingly, this potential exhibits three equivalent energy minima, directly contradicting the conventional TLS model, which is often employed to explain the “universal” anomaly observed in glasses and is considered a deviation from Debye’s theory of classical crystalline solids.

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