Phase Transition of Heptane under Pressure

Abstract

A solid-to-solid phase transition was discovered in nheptane with an onset at a pressure of 1.7 ± 0.1 GPa, evidenced by a discontinuity in the pressure−volume equation of state obtained from synchrotron X-ray diffraction experiments conducted at room temperature. The phase transition also resulted in discontinuous shifts of vibrational peaks in the infrared spectra. No additional phase transitions were detected up to 10 GPa, contradicting an earlier claim of a phase transition at 3 GPa. The hydrostatic limit of heptane in a diamond anvil cell was estimated to be 3.5 GPa. Above that pressure, the stress state is not well-defined, which may explain prior reports of a phase transition at 7.5 GPa.