Pressure-induced phase transitions in trans-stilbene crystals

Pressure effects on the phase transitions in [H-1(12)] and [H-2(12)]trans-stilbene crystals were studied. Pressure-frequency plots and pressure-half band width plots for the Raman bands due to the inter- and intramolecular vibrations of [H-1(12)] and [H-2(12)]trans-stilbene crystals showed inflectional variations of the slopes of the plots at about 2 and 4 GPa. The fact that no clear discontinuous changes of the slopes of the plots were observed indicates that the second-order phase transitions take place at about 2 and 4 GPa. The low-frequency Raman bands of [H-1(12)] and [H-2(12)]trans-stilbene crystals were reinvestigated and assignments to the intermolecular rotational vibrations about the x, y, and z axes were made on the basis of the isotopic effect on the vibrational frequency.