Polymorphism of 1,3-X-adamantanes (X=Br, OH, CH3) and the crystal plastic phase formation ability

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 displays a low-temperature ordered monoclinic phase (space group P21/c, Z=4, Z’=1) which transforms at 222.6±0.5 K to 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 displays a low-temperature hexagonal phase (P63/mcm, Z=6, Z’=0.25) which transforms to a face-centered cubic plastic phase (Fm¯3m). 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 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