Studies on the nature and pressure evolution of phase transitions in 1-adamantylamine and 1-adamantanol.

[Display omitted] • H-bond strength influences the nature of phase transitions in 1-NH 2 -OH and 1-OH-ADM. • There are three transitions between various plastic crystal (PC) phases in 1-NH 2 -OH. • Only one transition between PC and ordinary crystal phases occurs in 1-OH-ADM. • Compression has a strong impact on phase transitions in both examined compounds. In this paper, several experimental techniques, i.e., differential scanning calorimetry, X-ray diffraction, Fourier transform infrared, Raman, and broadband dielectric spectroscopy were applied to study the nature of the phase transitions in 1-adamantylamine (1-NH 2 -ADM, C 10 H 17 N) and 1-adamantanol (1-OH-ADM, C 10 H 16 O). Calorimetric measurements showed one and three endothermic peaks in thermograms for the latter and the former substance, respectively. Indeed, results of spectroscopic investigations indicated that the observed thermal events in 1-NH 2 -ADM correspond to transitions between various plastic crystal (PC) phases (I, II, III, IV), while the endothermic process in 1-OH-ADM can be assigned to a phase transition between the PC and the ordinary crystal (OC). Especially interesting were the outcomes of dielectric studies carried out both at ambient and high-pressure conditions, during heating and cooling cycles. They showed: i) noticeable changes in the frequency dependencies of the imaginary ( ε ' ' ) and real ( ε ') parts of the complex dielectric permittivity that occurred around temperatures of the characteristic endothermic events detected by the calorimetry, and ii) significant fluctuations of ε ' ' and ε ' at pressures attributed to the respective phase transitions. Moreover, the pressure coefficients of the phase transition temperatures were estimated to be approximately equal to 0.2 K/MPa for both compounds. In turn, volume variation (Δ V) at the PC (II)-PC (III) and PC (III)-PC (IV) transition temperatures for 1-NH 2 -ADM was essentially different than Δ V for the PC-OC transition in 1-OH-ADM. [ABSTRACT FROM AUTHOR]