High-Pressure–High-Temperature Study of Benzene: Refined Crystal Structure and New Phase Diagram up to 8 GPa and 923 K

The high-temperature structural properties of solid benzene were studied at 1.5–8.2 GPa up to melting or decomposition using multianvil apparatus and <i>in situ</i> neutron and X-ray diffraction. The crystal structure of deuterated benzene phase II (<i>P</i>2₁/<i>c</i> unit cell) was refined at 3.6–8.2 GPa and 473–873 K. Our data show a minor temperature effect on the change in the unit cell parameters of deuterated benzene at 7.8–8.2 GPa. At 3.6–4.0 GPa, we observed the deviation of deuterium atoms from the benzene ring plane and minor zigzag deformation of the benzene ring, enhancing with the temperature increase caused by the displacement of benzene molecules and decrease of van der Waals bond length between the π-conjuncted carbon skeleton and the deuterium atom of adjacent molecule. Deformation of benzene molecule at 723–773 K and 3.9–4.0 GPa could be related to the benzene oligomerization at the same conditions. In the pressure range of 1.5–8.2 GPa, benzene decomposition was defined between 773–923 K. Melting was identified at 2.2 GPa and 573 K. Quenched products analyzed by Raman spectroscopy consist of carbonaceous material. The defined benzene phase diagram appears to be consistent with those of naphthalene, pyrene, and coronene at 1.5–8 GPa