On the microscopic mechanism behind the purely orientational disorder–disorder transition in the plastic phase of 1-chloroadamantane

Unraveling the Complex Solid-State Phase Transition Behavior of 1-Iodoadamantane, a Material for Which Ostensibly Identical Crystals Undergo Different Transformation Pathways

Phase transitions in crystalline molecular solids have important implications in the fundamental understanding of materials properties and in the development of materials applications. Herein, we report the solid-state phase transition behavior of 1-iodoadamantane (1-IA) investigated using a multi-technique strategy [synchrotron powder X-ray diffraction (XRD), single-crystal XRD, solid-state NMR, and differential scanning calorimetry (DSC)], which reveals complex phase transition behavior on cooling from ambient temperature to ca. 123 K and on subsequent heating to the melting temperature (348 K). Starting from the known phase of 1-IA at ambient temperature (phase A), three low-temperature phases are identified (phases B, C, and D); the crystal structures of phases B and C are reported, together with a re-determination of the structure of phase A. Remarkably, single-crystal XRD shows that some individual crystals of phase A transform to phase B, while other crystals of phase A transform instead to phase C. Results (from powder XRD and DSC) on cooling a powder sample of phase A are fully consistent with this behavior while also revealing an additional transformation pathway from phase A to phase D. Thus, on cooling, a powder sample of phase A transforms partially to phase C (at 229 K), partially to phase D (at 226 K) and partially to phase B (at 211 K). During the cooling process, each of the phases B, C, and D is formed directly from phase A, and no transformations are observed between phases B, C, and D. On heating the resulting triphasic powder sample of phases B, C, and D from 123 K, phase B transforms to phase D (at 211 K), followed by the transformation of phase D to phase C (at 255 K), and finally, phase C transforms to phase A (at 284 K). From these observations, it is apparent that different crystals of phase A, which are ostensibly identical at the level of information revealed by XRD, must actually differ in other aspects that significantly influence their low-temperature phase transition pathways. This unusual behavior will stimulate future studies to gain deeper insights into the specific properties that control the phase transition pathways in individual crystals of this material.

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

We present an ultrasonic study of the elastic properties of 1-X adamantane (X = F, Cl, Br) during order-disorder and order-quasi-order transitions at various temperatures (77-305 K) and high pressures (up to 1 GPa). On the basis of our ultrasonic experiments, we studied for the first time the high-temperature (HT) Fm3m, medium-temperature (MT) P4₂/nmc, and low-temperature (LT) P42₁c phases of 1-fluoroadamantane at high pressures. The elastic properties of these phases at pressures up to 1 GPa at T = 293 and 77 K, as well as at isobaric heating from 77 to 293 K, have been determined. The boundaries of the HT → MT → LT phase transitions have been evaluated, which makes it possible to extend the phase diagram of 1-fluoroadamantane to higher pressures. We have confirmed that the MT → LT transition is a second-order phase transition because it is not accompanied by volume jumps but is manifested in anomalies of the elastic properties and ultrasound transmission both in high-pressure experiments and under isobaric heating. The comparison of the elastic properties of 1-X adamantanes (X = H, F, Cl, Br) has indicated a monotonic dependence at low pressures: the bulk modulus is the highest for adamantane and decreases with an increase of the atomic number of the halogen substitute (from fluorine to bromine).

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

The polymorphic behavior at normal pressure as a function of temperature of 2-adamantanol (C10H16O) and 2-methyl-2-adamantanol (C11H18O) has been investigated by single-crystal and powder X-ray diffraction as well as differential scanning calorimetry.

Phase transitions in 1-bromoadamantane compared to 1-chloroadamantane: similarities and unique features

The elastic properties of 1-chloroadamantane and 1-Bromoadamantane in order-disorder and order-quasi-order phase transitions at temperatures in the range of 77-305 K and high pressures up to 1.1 GPa are studied by the ultrasonic method. The elastic moduli of halogenated adamantanes clearly indicate these transitions, demonstrating high capabilities of the ultrasonic method. Our ultrasonic studies have detected for the first time the λ-anomaly of the elastic properties and, thereby, have confirmed that the phase transition from the orientationally ordered to quasi-ordered phase (M → O) in 1-bromoadamantane is a weak first-order phase transition having some properties of a second-order phase transition. The pressure derivatives of the elastic moduli of 1-chloroadamantane and 1-bromoadamantane in the orientationally ordered phase at 77 K are dB/dP ≈ 8, which allows using the Lennard-Jones potential to theoretically describe this phase.

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

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.

Comparative study of the elastic properties of adamantane and 1-chloroadamantane at high pressure and different temperatures and at order-disorder transitions

We present a comparative ultrasonic study of the elastic properties of adamantane and 1-chloroadamantane at high pressure (up to 1.4 GPa) and different temperatures (77-293 K) and at order-disorder transitions. The ultrasonic method provides complementary pictures of the order-disorder transitions in diamondoids under pressure. The equation of state of adamantane and 1-chloroadamantane was determined up to 1.4 GPa from ultrasonic measurements of bulk modulus and is in good accordance with the previous equations developed from volumetric data. We measured the bulk and shear moduli and Poisson's ratio of adamantane and 1-chloroadamantane up to 1.4 GPa. The behaviors of elastic moduli are different for adamantane and 1-chloroadamantane. This indicates that the substitution of one hydrogen atom for chlorine significantly reduces both elastic moduli, particularly the shear modulus (≈30%). Although the pressure dependences of the bulk modulus B are almost linear and its pressure derivatives for adamantane and 1-chloroadamantane are close to each other (B' ≈ 10-12), a jump is hardly observed on the pressure dependence B(P) for adamantane at the transition from the plastic to ordered phase, whereas the pressure dependence of the bulk modulus for 1-chloroadamantane exhibits a jump of almost 17%. The experimental dependences of the bulk modulus and relative changes in the volume for both materials clearly demonstrate that the compressibility of 1-chloroadamantane is much higher for both phases. The Poisson coefficient calculated from our experimental data is larger for 1-chloroadamantane, having lower both bulk and shear moduli.

Polymorphism of 1,3-cyclohexanediols: molecular structure and plastic crystal formation of cyclohexanediol isomers

Different plastic crystal formation abilities of cyclohexanediol isomers interpreted with Hirshfeld surfaces.