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

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.

Ultrasonic study of 1-X adamantane (X = F, Cl, Br) compounds at high pressure and at order–disorder transitions

“…The disordered phase is most clearly observed in the dependences of both elastic moduli above T = 260 K. The LT-MT phase transition is not accompanied by a volume jump, but the MT-HT phase transition is manifested in a collapse of the volume less than 1%. Some discrepancy between our data on the phase transition temperatures and data from works 36,37 is explained by the presence of a low pressure of 0.05 GPa in our experimental chamber, which is necessary for acoustic contact with the sample.…”

“…The volume jump (Fig. 1c) indicates that the HT -MT phase transition is a first-order transition, whereas the MT -LT transition is classified in works 36,37 as a second-order transition. It is interesting that the shear modulus of 1-fluoroadamantane increases stepwise successively in the HT -MT -LT transitions, whereas the bulk modulus in these transitions decreases successively.…”

Enantiotropy of Simvastatin as a Result of Weakened Interactions in the Crystal Lattice: Entropy-Driven Double Transitions and the Transient Modulated Phase as Seen by Solid-State NMR Spectroscopy

“…It is worth emphasizing that a significant decrease in the heat flow and heat capacity was observed above the transition temperature T 1 (specific heat capacity, c p , is reduced by about 0.102 J·g −1 ·K −1 , Figure 2 ), which is consistent with the findings of Simões et al [ 16 ]. Such a decrease in heat capacity is usually explained by the increase in molecular order, typically occurring upon the crystallization of liquids or glasses.…”

“…The lack of temperature hysteresis and the specific λ shape are signatures of second-order phase transitions, which usually accompany the order–disorder type of transitions [ 15 ], although a λ-transition has also been observed recently for a disorder–disorder transition in molecular crystals [ 16 ]. It is interesting to see that the measured Δ H values associated with both these transitions are rather low (Δ H = 1.13 and 0.46 kJ.mol −1 , at T 1 and T 2 , respectively), which is consistent with the exceptionally small changes of the unit–cell parameters upon both these phase transitions.…”

Polymorphism and solid–solid phase transitions of hydrogen bonded 2-adamantanol and 2-methyl-2-adamantanol compounds

“…[2][3][4][5] Adamantane derivatives, conformationally rigid molecules, are examples of organic compounds displaying a rich polymorphism. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] The pristine adamantane compound (C 10 H 16 ) is one of the most studied examples of organic molecular crystals which undergoes a first-order orderdisorder phase transition, from an ordered low-temperature phase (P4 ¯21 c, Z = 2) 21 to an orientationally disordered (OD) phase (Fm3 ¯m, Z = 4), which displays a translational facecentered cubic lattice with the center of masses of the molecules sitting at the lattice nodes but orientationally disordered. 22,23 Adamantane derivatives, with simple single 3692 | CrystEngComm, 2022, 24, 3692-3700 This journal is © The Royal Society of Chemistry 2022 a Grup de Caracteritzacio de Materials, Departament de Fisica and Barcelona Research Center in Multiscale Science and Engineering, Universitat Politecnica de Catalunya, EEBE, Campus Diagonal-Besos, Av.…”