Heat capacity anomalies of the molecular crystal 1-fluoro-adamantane at low temperatures

Daria Szewczyk,Tatiana Guidi,Jonathan F Gebbia,Claudio Cazorla,Andrzej Jeżowski,Josep-Lluís Tamarit,Alexander I Krivchikov

doi:10.1038/s41598-021-97973-2

Abstract

Disorder–disorder phase transitions are rare in nature. Here, we present a comprehensive low-temperature experimental and theoretical study of the heat capacity and vibrational density of states of 1-fluoro-adamantane (C10H15F), an intriguing molecular crystal that presents a continuous disorder–disorder phase transition at T = 180 K and a low-temperature tetragonal phase that exhibits fractional fluorine occupancy. It is shown that fluorine occupancy disorder in the low-T phase of 1-fluoro-adamantane gives rise to the appearance of low-temperature glassy features in the corresponding specific heat (i.e., “boson peak” -BP-) and vibrational density of states. We identify the inflation of low-energy optical modes as the main responsible for the appearance of such glassy heat-capacity features and propose a straightforward correlation between the first localized optical mode and maximum BP temperature for disordered molecular crystals (either occupational or orientational). Thus, the present study provides new physical insights into the possible origins of the BP appearing in disordered materials and expands the set of molecular crystals in which “glassy-like” heat-capacity features have been observed.

Highlights

Phase transitions between stable phases occur as a result of changes in the external conditions[1]
In addition to the first-order MT → HT phase transition occurring at 227 K and that corresponds to the Cp maximum, several other transformations are identified
Despite that the experimental vibrational density of states (VDOS) have been measured within a reduced energy range, the calculation of the specific heat capacity from such g(ω), by adjusting the Debye level, matches very well the corresponding Cp experimental values. Such an agreement illustrates that, as far as for the low-temperature contributions to the specific heat capacity, only the lowest-energy modes are relevant to explain the appearance of the boson peak (BP)

Summary

Introduction

Phase transitions between stable (or metastable) phases occur as a result of changes in the external conditions (e.g., temperature, pressure, magnetic or electric field, and/or mechanical stress)[1]. A critical exponent (β) can be estimated for the temperature variation of the order-parameter, ∝ (T − Tc)−β , around the LT to MT transition point, defined either as the angle φ (see inset in Fig. 1) formed by the C–F bond of the four F equilibrium positions and a (or b) tetragonal axis, or as the intensity of the second harmonic signal (which vanishes at the centrosymmetric MT phase)[15].

Results

Conclusion