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.

After pressurization to 30 GPA, in a diamond anvil cell, benzene transforms, at room temperature, to a white solid which is stable at ambient pressure. We report here the infrared spectroscopy analysis performed under pressure and at ambient conditions. These preliminary results show that the transformation involves an opening of the benzene rings leading to a highly cross-linked polymer.

With a computer simulated mechanical model for molecular packing analysis, the reconstitution and identification of the intermediate pressure-induced phase II of solid benzene C6H6 at 293 K, has been undertaken. The atom-atom intermolecular potential of the Buckingham type was generalized to account for short interatomic distances, especially under pressure. The model includes thermal motion and molecular deformation effects. Various crystal structures calculated in the pressure range of phase II and checked by their reticular distances and structure factors, are compared with the structure IIo proposed for this phase. Among them two possible monoclinic structures IIc and IIc′ have been evidenced by the calculation. Structure IIc has energy and enthalpy levels lower than that of phases Ic and IIIc, in the pressure range 0.5<P<1.0 GPa. This allows to suggest this stable monoclinic structure IIc for the real structure of the experimentally observed phase II. This structure IIc corresponds to the structure previously determined as metastable by Dzyabchenko and Bazilevskii [J. Struct. Chem. 26, 553 (1985)].

The complete vibrational spectra of crystalline C6H6 and C6D6 have been calculated for the different pressure-induced solid phases recently determined at 293 K up to 25 GPa, and compared to Raman scattering data. The normal coordinate analysis has been carried out by using intermolecular Buckingham-type atom–atom interactions and the intramolecular force field of the free molecule. Results of such frequency calculations are compared to experimental values at ambient pressure. The variation of the relevant crystalline parameters is discussed to construct a model and calculate the vibrational frequencies under pressure. The quantitative fit of the frequency shift of the Raman active modes under pressure demonstrates the necessity of including different C–C and C–H (C–D) bond compressibilities within the benzene molecule. Such intramolecular distance variations which allow to estimate the frequency corrections for the totally symmetric (a1g) breathing modes, have been determined from the observed pressure-frequency dependence of these internal modes. The behavior of other nonsymmetric (e2g) internal modes which become comparatively weak under pressure, suggests a charge delocalization within—and possibly out of—the benzene ring, eventually leading to irreversible opening of the hexagonal cycle. This can be directly related to the irreversible transformation of benzene to a polymer which is observed after pressurization above 20 GPa.

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Crystal and molecular structures of hexachlorobenzene C6Cl6 were investigated by molecular-packing analysis with a computed dynamical model. This model includes thermal motions and molecular deformations. Several crystalline structures with monoclinic symmetry P21/c have been found by the calculation. Factors which influence the stability of these structures are investigated. The lowest-energy structure, labeled I, agrees with the experimental one. Calculated crystal and molecular parameters agree with data determined by x-ray diffraction at 296 K. While no temperature-induced phase transition has been found from 296 to 124 K at ambient pressure, a phase transformation has been evidenced numerically at 0.43 GPa and 296 K. The monoclinic pressure-induced phase, stable above 0.43 GPa, is labeled II.

We have studied the Raman spectra of solid CH4 at 4.2 K below 9 kbar. The vibration-libration structures of the intramolecular ν1–ν4 spectra were essentially the same as the previous data of 1.5 kbar, so that the three-site (S1, S2, and W) model of phase III proposed in paper I was utilized to analyze the observed data. It is noteworthy that one of the three bands in the ν4 spectrum exhibits a decrease in frequency with pressure. This ’’softening’’ is interpreted as a result of distortion of the local potential fields of methanes with pressure. On the other hand, the lattice Raman spectrum exhibits a marked change in the II–III phase transition at 0.5 kbar, and furthermore shows changes in the splittings of certain bands at 1.9 and 4.9 kbar. In the ν-P plots of both intramolecular and lattice vibrational bands, prominent discontinuities are observed at 1.9 and 4.9 kbar, indicative of the existence of phase II (0–0.5 kbar), phase III (0.5–1.9 kbar), and new phases IV (1.9–4.9 kbar) and V (4.9 kbar), below 9 kbar. Mode Grüneisen parameters γ have been estimated. It is found that the γ’s of phase III are much larger than those of phases IV and V. Assignments of the lattice Raman bands are made using the observed ν3 Raman spectrum, the ν-P plots and the existing far-infrared spectrum. It is concluded that the lattice Raman spectrum mainly consists of two translational bands and three librational bands arising from S1, S2, and W sites. Finally, we have estimated from the observed data the magnitude of the A-T tunneling rotational splittings of methane as a function of pressure. Results imply that the splitting at the W (weak field) sites is as large as ∼3.6 cm−1, even at P = 8 kbar. On the basis of this result, we discuss the tunneling splittings in solid CD4 phase III at normal pressure.

Theory of phase transitions in solid methanes. XI. Infrared and Raman spectra of the ν3 and ν4 modes in phase II of solid CH4 Raman spectra of the intramolecular vibrations in solid CH. have been observed at 4.2 K under pressure.The observed VIv) vibration-rotation band structures in phase II agree well with previous experimental and theoretical data. The v. band was detected in both phases II and III. A remarkable change of each spectrum was found at 0.4 kbar, indicating the II-III phase transition. The rotational states of methanes in phase III have been deduced from the observed data. It is concluded that in phase III there are at least three kinds of sites with different site symmetries.