Electronic properties and Raman spectra of rare-earth carbide halides investigated from first principles - art. no. 024519

Y2C2X2 (X=Br,I) are layered metals, where superconductivity up to 11.6 K ha s been found. We have investigated these materials by means of first-princi ples calculations within the full-potential linearized augmented plane wave method based on density functional theory. In particular, we report electr onic densities of states (DOS) and electronic band structures for the exper imental geometry as well as for the geometry optimized structures. We discu ss the peak in the DOS at the Fermi energy that has been suggested to be re sponsible for the T-e variation in the Y2C2(Br,I)(2) system when applying p ressure or changing the bromine to iodine ratio. Moreover, we study in deta il all Raman-active modes of Y2C2Br2 and Y2C2I2. where the phonon frequenci es are determined within the frozen-phonon approach by carrying out total-e nergy and atomic-force calculations. The excellent agreement of theoretical and experimental vibrational frequencies together with the calculated Rama n intensities allow for a clear assignment of all measured Raman-active mod es to A(g) and B-g symmetry.