High pressure solid phases of benzene. I. Raman and x-ray studies of C6H6 at 294 K up to 25 GPa

Abstract

Crystalline benzene has been investigated at room temperature as a function of pressure up to 25 GPa in diamond anvil cells by Raman scattering and powder x-ray diffraction techniques. The concomitant spectroscopic and crystallographic results show the existence of numerous pressure-induced phases. Changes in the profiles of the Raman spectra and in the x-ray diffraction patterns, as well as changes in the variations of the Raman frequencies and the cell parameters with pressure indicate two first-order phase transitions at 1.4±0.1 and 4±1 GPa and a second-order one at 11±1 GPa. At 24 GPa the x-ray diffraction pattern seems to indicate the existence of a new phase. Two monoclinic structures are proposed for the phases above 1.5 GPa, in addition to the already known one. From these data, molar volume has been determined as a function of pressure and the Grüneisen parameters have been inferred in the different phases. Their pressure dependences are analyzed in the light of theoretical predictions. Arguments are given for a phase transformation at normal pressure and below 140 K or at room temperature below 1 GPa. A schematic P–T phase diagram is suggested and a controversy on the nature of the triple points located on the melting curve is clarified.

Capturing the Role of Temperature and Entropy in Crystal Polymorphs Through Molecular Simulations

“…This is consistent with experimental observations of benzene I at ambient temperature and pressure [32,76]. The metastable benzene II crystal has a higher entropy than benzene I as evidenced by the decrease in relative free energies at higher temperatures.…”

“…Properties than can change between polymorphs include density [11,20,31], compressibility [32], hardness [33,34], gel strength [35], flexibility [36], conductivity [26][27][28][29][30][37][38][39], explosivity [20,[22][23][24], catalytic activity [40,41], transparency [42,43], weather resistance [25,44], heat capacity [20], melting point [32,45,46], vapor pressure [46], stability [25,29], solubility [14,[47][48][49], bioavailability [14,33,48,[50][51][52][53][54], and dissolution rate [47,55,56].…”

Simulation of structural phase transitions by metadynamics

“…At room temperature and ambient pressure benzene crystallizes in an orthorhombic structure, Pbca, which is stable up to 1.4 GPa [32]. Beyond this pressure a sluggish transition to benzene II is observed [32,33,34]. Determining the crystalline structure of this phase was a big challenge and only 10 years after its first observation theoretical calculations could determine that it is orthorhombic and belongs to the P4 3 2 1 2 space group [35].…”

“…In this paper we follow the notation used by Thiéry and Léger in Ref. [32]. At room temperature and ambient pressure benzene crystallizes in an orthorhombic structure, Pbca, which is stable up to 1.4 GPa [32].…”

“…Determining the crystalline structure of this phase was a big challenge and only 10 years after its first observation theoretical calculations could determine that it is orthorhombic and belongs to the P4 3 2 1 2 space group [35]. Benzene II is stable up to 4 GPa and then it transforms into benzene III, which is monoclinic and belongs to the P2 1 /c space group [32,33,34]. Upon increasing pressure two more phases are observed: benzene III', which is stable between 11 and 24 GPa, and benzene IV, which is stable at even higher pressure [32,34].…”