Raman Scattering Spectroscopy of Phase Transition in n-Pentadecane under High Temperature and High Pressure

The Raman spectroscopy of n-pentadecane is investigated in a moissanite anvil cell at normal temperatures and a diamond anvil cell under pressure to about 3000MPa and at temperature from 298 to 573K. Result indicates that at room temperature the vibration modes, assigned to the symmetric and asymmetric stretching of CH₃ and CH₂ stretching, shift to higher frequency and display a pressure dependent quasi-linear curve. A liquid--solid phase transition appears at a pressure of 150MPa. The high temperature solidus line of n-pentadecane follows a quadratic function of P=0.02369T²-9.117T+725.58, in agreement with previous conclusion derived from studies of other hydrocarbons. Upon phase transition, fitting the experimental data obtained in a temperature range of 283-553K to the Clausius--Clapeyron equation allows one to define the thermodynamic parameters of n-pentadecane of dP/dT=0.04738T -9.117.