Szewczyk Daria, Gebbia Jonathan F, Jeżowski Andrzej, Krivchikov Alexander I, Guidi Tatiana, Cazorla Claudio, Tamarit Josep-Lluís
Institute of Low Temperature and Structure Research PAS, Okólna 2, 50-422, Wrocław, Poland.
Grup de Caracterització de Materials, Departament de Física, EEBE and Research Center in Multiscale Science and Engineering, Universitat Politècnica de Catalunya, Eduard Maristany, 10-14, 08019, Barcelona, Catalonia, Spain.
Sci Rep. 2021 Sep 20;11(1):18640. doi: 10.1038/s41598-021-97973-2.
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 (CHF), 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.
无序-无序相变在自然界中很少见。在此，我们对1-氟代金刚烷（CHF）的热容和态密度进行了全面的低温实验和理论研究，1-氟代金刚烷是一种有趣的分子晶体，在T = 180 K时呈现连续的无序-无序相变，并且在低温下呈现出四方相，其中氟的占有率为分数。结果表明，1-氟代金刚烷低温相中的氟占有率无序导致相应比热（即“玻色子峰”-BP-）和态密度中出现低温玻璃态特征。我们确定低能光学模式的膨胀是导致此类玻璃态热容特征出现的主要原因，并提出了无序分子晶体（无论是占有率还是取向）中第一个局域光学模式与最大BP温度之间的直接关联。因此，本研究为无序材料中出现的BP的可能起源提供了新的物理见解，并扩展了已观察到“类玻璃态”热容特征的分子晶体集合。
