A high pressure Raman study of 9-hydroxyphenalenone and pressure-induced phase transitions

Abstract

The pressure effect on 9‐hydroxyphenalenone (9‐HPLN) has been investigated using Raman scattering and optical absorption techniques in a diamond anvil cell up to 70 kbar hydrostatic pressure. A pressure‐induced first‐order phase transition occurs near 6 kbar and the Raman data shows that the transition is to the same phase that is obtained on cooling 9‐HPLN below 255 K at ambient pressure. The phase boundary has a positive slope with dT/dP=7 K/kbar. The Raman data indicate that there may be another phase change near 50 kbar. A strong and sharp Raman frequency near 12 cm⁻¹ is observed in all the phases up to the highest pressure investigated. The weak pressure dependence of this frequency and its presence up to the highest pressure suggests that it may be a molecular mode rather than an external mode frequency. The optical absorption associated with π–π∗ excitation exhibits a large red shift, as expected of an aromatic molecular crystal.