Theoretical Raman spectrum of boron carbide B4.3C under pressure

The most striking features of the Raman spectrum of boron carbide under pressure are explained theoreticallyby computing the Raman tensor using density functional perturbation theory and the second-order response.While the observed pressure-induced changes in frequencies and intensities of all of the peaks above 450 cm−1are convincingly explained by the vibrations of (B11C) icosahedra and C-B-C chains – that have been identifiedfor long as the two main components of the atomic structure of pristine B4C –, the puzzling non-monotonicbehavior of a broad Raman band at low frequency, whose intensity increases under pressure up to 44 GPa,decreases and then vanishes, was so far unexplained. We find that the behavior under pressure of both thefrequency and intensity of this band turns out to be remarkably accounted for, in the calculations, by theactivation of the chain bending mode in atypically flexured chains. We show that the flexion of the chainoccurs at high pressure in presence of interstitial B atoms that, at ambient pressure, sit in the prolongationof standard C-B-C chains. We propose the ambient-P mode observed at 270 cm−1 as a fingerprint for theidentification of both such B-C-B-C chain-defects in boron carbide and local deviations from the rhombohedralsymmetry.