A combined experimental and computational study of the pressure dependence of the vibrational spectrum of solid picene C_22H_14

TL;DR

This study combines experimental optical data and density functional calculations to analyze how pressure affects the vibrational spectrum of solid picene, revealing its molecular nature persists up to 8 GPa and insights into electron-phonon interactions.

Contribution

It provides a detailed first-principles analysis of pressure effects on picene's vibrational modes, including spectral fitting and quantification of molecular character loss under pressure.

Findings

Picene remains a molecular solid up to 8 GPa with smooth phonon hardening.

First-principles calculations accurately reproduce experimental pressure effects.

Phonon softening in K-doped picene is mainly due to charge transfer and electron-phonon coupling.

Abstract

We present high-quality optical data and density functional perturbation theory calculations for the vibrational spectrum of solid picene (C$_{22}$H$_{14}$) under pressure up to 8 GPa. First-principles calculations reproduce with a remarkable accuracy the pressure effects on both frequency and intensities of the phonon peaks experimentally observed . Through a detailed analysis of the phonon eigenvectors, We use the projection on molecular eigenmodes to unambiguously fit the experimental spectra, resolving complicated spectral structures, in a system with hundreds of phonon modes. With these projections, we can also quantify the loss of molecular character under pressure. Our results indicate that picene, despite a \sim 20 % compression of the unit cell, remains substantially a molecular solid up to 8 GPa, with phonon modes displaying a smooth and uniform hardening with pressure. The Grueneisen parameter of the 1380 cm^{-1} a_1 Raman peak ($\gamma_p=0.1$) is much lower than the effective value ($\gamma_d=0.8$) due to K doping. This is an indication that the phonon softening in K doped samples is mainly due to charge transfer and electron-phonon coupling.