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The polymorphism of 1,3-dimethyladamantane (13DMA), 1,3-adamantanediol (13DOHA) and 1,3-dibromoadamantane (13DBrA) has been studied by X-ray powder diffraction, density measurements and differential scanning calorimetry at normal and high-pressure. 13DMA exhibits a low-temperature ordered monoclinic phase (space group P21/c, Z = 4, Z' = 1) which transforms at 222.6 ± 0.5 K into a high-temperature hexagonal plastic phase (P63/mmc, Z = 26), stable up to the melting point at 246.8 ± 0.5 K at normal pressure. 13DOHA exhibits a low-temperature hexagonal phase (P63/mcm, Z = 6, Z' = 0.25) which transforms into a face-centered cubic plastic phase (Fm[3 with combining macron]m). 13DBrA exhibits a unique ordered orthorhombic phase (Pnma, Z = 4, Z' = 0.5) which remains till the melting point at 382.1 ± 0.5 K. The stability temperature domain of the plastic phases increases with pressure for both 13DMA and 13DOHA. The ability to form the plastic phase for these compounds as well as a series of adamantanes derivatives is analysed through the globularity and asphericity parameters derived from the Hirshfeld surface analysis., Peer Reviewed, Postprint (author's final draft)

Phase transitions of 1-fluoro-adamantane have been thoroughly investigated by temperature-resolved second harmonic generation (TR-SHG) and X-ray powder diffraction (XRPD). A new polymorph - an intermediate centrosymmetric phase (MT) - between the known orientationally disordered high temperature phase (HT, Fm3-m, Z = 4) and the low temperature phase (LT, P4-21c, Z = 2) was unveiled by TR-SHG. The crystal structure of MT was resolved by XRPD in the P42/nmc (Z = 2) space group, and it is related to the LT phase in a group-subgroup relation. No evidence of any solid-solid transition between these two phases by differential scanning calorimetry (DSC) or cold-stage microscopy could be obtained. Therefore, combing TR-SHG, XRPD, DSC, and cold-stage microscopy results, a second-order transition mechanism is proposed for the MT ¿ LT transition. Moreover, the critical exponent (ß) of the order parameter was calculated by fitting TR-SHG data to a critical power law. The obtained ß value (0.26) is close to the value from XRPD data (0.25)., Postprint (author's final draft)

The polymorphism and the dynamics of a simple rigid molecule (1-fluoro-adamantane) have been studied by means of X-ray powder diffraction and broadband dielectric spectroscopy. At temperatures below the melting point, the molecule forms an orientationally disordered Phase I with a cubic-centered structure (Phase I, Fm (3) over barm, Z = 4). This phase possesses eight equilibrium positions for the fluorine atom, with equal occupancy factors of 1/8. A solid-solid phase transition to a low-temperature tetragonal phase (Phase II, P (4) over bar2(1)c, Z = 2) reduces the statistical disorder to only four possible equivalent sites for the fluorine atom, with fractional occupancies of 1/4. The dynamics has been rationalized under the constraints imposed by the space group of the crystal structure determined by powder X-ray diffraction. The dielectric spectroscopy study reveals that the statistical disorder in Phase II is dynamic in character and is associated with reorientational jumps along the two-and three-fold axes. In the dielectric loss spectra, the cooperative (alpha) relaxation exhibits a shoulder on the high-frequency side. This remarkable finding clearly reveals the existence of two intrinsic reorientational processes associated with the exchange of the F atom along the four sites. In addition to such, Peer Reviewed, Postprint (published version)

The polymorphism of 1-adarnantane-methanol C11H18O has been investigated by differential thermal analysis and single-crystal and powder X-ray diffraction. Below the melting temperature (389.5 +/- 0.4 K), this compound exhibits an orthorhombic phase (phase I, Pnnm, Z = 12, Z' = 1.5). The melting enthalpy was determined to be 20.5 +/- 0.4 kJ mol(-1), i.e., with an entropy change of (6.34 +/- 0.13)R, which is much higher than the quoted value from Timmermans for the melting orientationally disordered phases (2.5R), thus supporting the orientationally ordered character of phase I. This orthorhombic phase I exhibits a statistical disorder of the hydrogen atom related to the oxygen atom, due to the position of one independent molecule on the mirror. At ca. 272 K, phase I transforms continuously through an order disorder transition to a low-temperature monoclinic phase II (P2(1)/n, Z = 12, Z' = 3). The monoclinic and orthorhombic phases are related by a group subgroup relationship, which perfectly agrees with the continuous character of the II to I transition. Moreover, by a convenient choice of an order parameter related to the continuous tilt of the c-axis, the critical exponent for this transition is found to be close to the theoretical prediction of the three-dimensional Ising model (with a critical exponent of ca. 0.27)., Postprint (published version)