High pressure Raman study of LiBC

The effects of pressure on the zone-center optical phonon modes of LiBC, which is structurally and electronically similar to the superconductor MgB2, have been investigated by Raman spectroscopy. At ambient pressure, four main lattice modes have been revealed with frequencies at 1173, 831, 552, and 174 cm(-1), respectively, while only lines at 1173 and 174 cm(-1) correspond to the expected Raman active E-2g modes. The longitudinal-optical phonon modes involving the Li sheets which can provide a coupling between neighboring B-C planes are expected to couple with the plasmon mode and are suggested to be related with the appearance of the forbidden lines. With increasing pressures, it is found that LiBC remains stable up to 15.5 GPa. All the observed modes harden and exhibit broadening effects under high pressure, especially for the B-C bond stretching mode (at 1173 cm(-1)) which shows a nonlinear pressure dependence. It indicates that the broadening may be caused by complex coupling interactions, and the pressure-enhanced coupling should give rise to a strongly anharmonic behavior of these modes. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim