Structural and vibrational properties of phenanthrene under pressure

TL;DR

This study investigates how phenanthrene's structure and vibrations change under high pressure, revealing phase transitions, amorphization, and coexistence of disorder and crystallinity up to 30 GPa.

Contribution

It provides detailed high-pressure structural and vibrational data of phenanthrene, identifying multiple phase transitions and the amorphization process with specific structural models.

Findings

Two phase transitions at 2.3 GPa and 5.4 GPa observed in Raman spectra.

Phenanthrene transforms into amorphous hydrogenated carbon above 20 GPa.

Crystalline characteristics persist in amorphous phase as shown by x-ray diffraction.

Abstract

The structural and vibrational properties of phenanthrene are measured at high pressures up to 30.2 GPa by Raman spectroscopy and synchrotron x-ray diffraction techniques. Two phase transitions are observed in raman spectra under pressures of 2.3 GPa and 5.4 GPa with significant changes of intermolecular and intramolecular vibrational modes, and all the raman peaks disappeared in the luminesce background above 10.2 GPa. Upon further compression above 20.0 GPa, three broad bands are observed respectively around 1600, 2993 and 3181 cm$^{-1}$ in Raman spectra, strongly indicating that phenanthrene has transformed into amorphous hydrogenated carbon. Based on x-ray diffraction, the structures of corresponding phases observed in Raman spectra are identified with space group of $P_{\rm}2_{1}$ for phase I at 0-2.2 GPa, $P_{\rm}$2/\emph{m} for phase II at 2.2-5.6 GPa and $P_{\rm}$2/\emph{m}+\emph{Pmmm} for phase III at 5.6-11.4 GPa, and the structure above 11.4 GPa is identified with space group of \emph{Pmmm}. Although phenanthrene has amorphized above 20.0 GPa, the amorphous hydrogenated carbon clusters still remain the crystalline characteristic based on x-ray diffraction patterns. Our results suggest that the long-range periodicity and the local disorder state coexist in phenanthrene at high pressures.