Molecular Simulation of the Pressure-Induced Crystallographic Phase Transition of p-Terphenyl

Abstract

The pressure- and temperature-induced polymorphic crystal phase transitions of p-terphenyl (PTP) have been modeled using a modified PCFF interaction force field. Modifications of the interaction potential were necessary to simultaneously model both the temperature-induced phase transition at ambient pressure and the pressure-induced phase transition at low temperature. Although the high-temperature and high-pressure phases are both characterized by flattening of the PTP molecule, the mechanisms of the temperature- and pressure-induced phase transitions are different. At high temperature thermal energy exceeds the torsional barrier, resulting in a bimodal phenyl ring twist angle distribution that averages to zero. In contrast, compression of PTP at high pressure results in a static planar structure. At high pressure the compression of the unit cell is also characterized by large compression of the a lattice parameter and weak compression of c, but some expansion of the b lattice parameter. The expansion of the b lattice parameter is likely associated with pressure-induced soft mode behavior of some lattice vibrations as well as soft mode behavior of pseudolocal phonons associated with impurities in PTP. The crystallographic angles α, β, and γ also indicate a triclinic crystal phase above the critical phase transition pressure of P(c) ~ 0.5 GPa at low temperature, suggesting a distinct phase separate from the monoclinic high-pressure phase at high temperature.

High-Throughput Pressure-Dependent Density Functional Theory Investigation of Herringbone Polycyclic Aromatic Hydrocarbons: Part 1. Pressure-Dependent Structure Trends

“…The structural analysis of 40 HB-PAHs under hydrostatic pressure is based on methods previously used by us to investigate the property−structure relationships of crystalline PAHs and other small OMCs under both ambient and high-pressure conditions. 1,6,10,15,19,22 Here, as well as in the previous investigations, PBE + vdW provides excellent agreement between the calculated and experimental structural descriptors (e.g., lattice parameters, internal molecular geometries, intermolecular close contacts, and density). The agreement between the calculated and experimental unit cell parameters and densities is within ∼2% for structures at nearly all pressures.…”

A new parameter for classification of polycyclic aromatic hydrocarbon crystalline motifs: a Hirshfeld surface investigation

“…The shortest cell axis then transitions into a new motif bracket while q remains uncharacteristic of that motif. 3,6,9,10 Table 3 lists several high pressure and low temperature PAH structures accompanied by their intermolecular interactions, motif assignment and q. It was observed that naphthalene undergoes no observable motif variation upon pressurization.…”

“…5 It has been shown that when crystalline PAHs are placed under pressure three mechanisms of energy minimization exist: (1) any twists in the molecule become flattened, (2) all unit cell axes become compressed and (3) the interplanar angle (q) and molecular set angle (c) adjust to allow for more efficient packing. [6][7][8] Upon pressurization, the motif assignment of a PAH can change as its crystal defining axis (usually b) and q values adjust. In studies of several PAHs under pressure, 3,9,10 the shortening of b moves the PAH into a new motif bracket, but the pressurized crystal does not exhibit the motif's typical q distribution.…”

“…16 Further proof of value for the evaluation of crystal systems via Hirshfeld surfaces is shown through the description of the crystalline motifs in heterocyclic and PAH structures under both ambient and high pressures. 3,5,6 While previous Hirshfeld surface investigations of PAH motifs examined intermolecular interaction trends, 5 a sweeping quantitative analysis of intermolecular close contact percentages for the varying motif types has never been performed.…”

Exploration of the transition temperatures and crystal structure of highly crystalline poly(1,3-cyclohexadiene): An experimental and computational investigation

“…Crystallinity in polymeric systems has enormous significance in chemistry, biology, and materials science. [1][2][3] It profoundly affects the physical properties and thermal stabilities 4-6 of polymeric materials, and is influenced by factors such as stereoregularity, 7 pi-pi stacking, 8 mainchain rigidity, 9 end-group dependence, 10 and hydrogen bonding. 11 An illustrative example of crystallinity's influence on polymer properties resides in stereoregular polyolefins; where high crystallinity improves solvent resistance, high-temperature performance, and degradation time.…”