Raman modes and Grüneisen parameters of graphite under compressive biaxial stress

We study the behaviour of the most characteristic Raman contributions of highly oriented pyrolytic graphite at biaxial stresses up to 5 GPa. We use moissanite anvils to compress the samples, which allowed us to observe for the first time the evolution with stress of the disorder-induced D band along with all the overtones and combination bands in the Raman spectrum of graphite. Theoretical calculations at different pressures, performed within the density functional theory, are provided to confirm that the D band process involves iTO phonons along the K–Γ direction. A complete set of Grüneisen parameters is reported for all the features of the Raman spectrum of graphite; the analysis of the ratios of such parameters provides direct information on the intrinsic coupling and stress response of each phonon branch.