The dynamics of a simple rigid pseudoglobular molecule (2-adamantanone) has been studied by means of dielectric spectroscopy and examined under the constraints imposed by the space group of the crystal structure determined by x-ray powder diffraction. The low-temperature monoclinic structure of 2-adamantanone, with one molecule per asymmetric unit (Z = 1), displays a statistical intrinsic disorder, concerning the site occupancy of the oxygen atom along three different sites. Such a physically identifiable disorder gives rise to large-angle molecular rotations which inherently lead to time-average fluctuations of the molecular dipole, thus contributing to the dielectric susceptibility. The dielectric spectra for the low-temperature "ordered" phase displays a universal feature of glassy-like materials, i.e., coexistence of α-and β-relaxation processes. The former is clearly identified with the strongly restricted reorientational motions within the long-range "ordered" crystalline lattice. The latter, never observed before in fully translationally and highly orientationally ordered phases, displays all the properties of an original Johari-Goldstein β-relaxation, in spite of the strong character of this glass-like phase. These findings can be explained according to the coupling model, applied to such "ordered" phases.

The polymorphism and the dynamics of a simple rigid molecule (1-fluoro-adamantane) have been studied by means of xray powder diffraction and broadband dielectric spectroscopy. At temperatures below the melting, the molecule forms an orientationally disordered phase I with cubic-centered structure (Phase I, Fm 3 m, Z=4). This phase possesses eight equilibrium positions for the fluorine atom, with equal occupancies factors of 1/8. A solid-solid phase transition to a lowtemperature tetragonal phase (Phase II, P 4 21c, 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 (α) 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 "bimodal" relaxation, a secondary Johari-Goldstein relaxation is detected at lower temperatures.

The crystal structures of [NH3(CH2)5NH3]CdCl4 are determined by X‐ray diffraction at 293 K (phase I) and 353 K (phase II) in order to obtain a structural description of the second‐order phase transition occurring at 337 K. This continuous transition is mainly of order‐disorder type and can be associated to the reorientational dynamics of the alkylenediammonium chains, with all trans conformation between two distinct positions. A model of “twisted” chains, coexisting with the trans ones is proposed in the high temperature phase to account for the observed electron density distribution due to the carbon atoms.

Halogenoethane derivatives are known to exhibit different polymorphs involving a different translational, orientational, and conformational order. The 1,1,2,2-tetrachloroethane (Cl2HC–CHCl2) exhibits a normal pressure orthorhombic phase β (space group P212121 with Z = 8 and Z′ = 2), formed by molecules with one of the two gauche conformations. At high pressure, the stable polymorph is known to be monoclinic (space group P21/c, with Z = 2 and Z′ = 0.5), phase α, in which only the trans conformer appears. In this work, we demonstrate the existence of a normal pressure metastable polymorph, phase γ, for which the two gauche conformers show up in the asymmetric unit of a monoclinic (space group P21/c with Z = 8 and Z′ = 2) structure. The new phase γ is obtained by recrystallization upon heating the glass obtained after quench of the melt. It displays shorter type II Cl···Cl contacts than the high-pressure phase α due to attractive and directional interactions.