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

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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.