Vibrational Modes in LiBC: Theory Compared with Experiment

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The search for other superconductors in the MgB2 class currently is focussed on Li1-xBC, which when hole-doped (concentration x) should be a metal with the potential to be a better superconductor than MgB2. Here we present the calculated phonon spectrum of the parent semiconductor LiBC. The calculated Raman-active modes are in excellent agreement with a recent observation, and comparison of calculated IR-active modes with a recent report provides a prediction of the LO--TO splitting for these four modes, which is small for the B-C bond stretching mode at 1200 cm⁻¹, but large for clearly resolved modes at 540 cm⁻¹ and 620 cm⁻¹.