LiBC, a semiconducting ternary borocarbide constituted of the lightest elements only, has been synthesized and characterized by x-ray powder diffraction, dielectric spectroscopy, and conductivity measurements. Utilizing an infrared microscope the phonon spectrum has been investigated in a quasi-single-crystalline sample. The in-plane B-C stretching mode has been detected at 150 meV, noticeably higher than in ${\mathrm{AlB}}_2,$ a nonsuperconducting isostructural analog of ${\mathrm{MgB}}_2.$ It is this stretching mode, which reveals a strong electron-phonon coupling in ${\mathrm{MgB}}_2,$ driving it into a superconducting state below 40 K, and is believed to mediate high-temperature superconductivity predicted in hole-doped LiBC [H. Rosner, A. Kitaigorodsky, and W. E. Pickett, Phys. Rev. Lett. 88, 127001 (2002)].

• Received 12 July 2002

DOI:https://doi.org/10.1103/PhysRevB.67.132502