Mode Gruneisen parameters of boron carbide

IR- and Raman-active phonons of boron carbide and the mode Gruneisen parameters gamma related are studied concerning their dependence on chemical composition, temperatures between 30 and 800 K and pressures up to similar to 70 GPa. Most bulk phonons yield gamma between +1.5 and - 1.5: those related to icosahedra yield gamma = 0.8(3). Surface phonons are distinguished by considerably higher gamma. Negative gamma of chain bending modes supports the assumption that the chain center buckles out under pressure. Some striking specific mode Gruneisen parameters are explained. Pressure-dependent bond lengths suggest the reversible high-pressure phase transition to be second order. (C) 2017 Elsevier Masson SAS. All rights reserved.