Raman Scattering Studies of the Carbon Atom Vibrations in Superconducting Rare-Earth Carbide Halides RE2C2(X,X′)2 (RE = Y, Gd; X, X′ = Br, I)

Abstract

The Raman spectra of the layered superconductors RE2C2(X,X′)2 (RE = Y, Gd; X, X′ = Br, I) were measured on single-crystal and polycrystalline specimens using a micro-Raman technique. Low energy modes (ω < 300 cm—1) correspond to mixed vibrations of rare-earth and halogen atom sublattices. Two high energy modes at ≈︂ 400 and ≈︂ 1600 cmm1) can be assigned to the tilting and stretching vibrations of the carbon C2 units. These carbon-related phonons were identified by analyzing the isotope shifts of the Raman lines in samples in which the natural carbon was replaced by isotope-enriched 13C. The energy of the stretching vibration is independent of the particular host compound while the energy of the tilting vibration correlates with the size of the rare-earth atom cage that surrounds the carbon atoms.