Vibrational Spectra of the Cluster Anions [E4]4- in themetallic Sodium and Barium Compounds Na4E4 and Ba2E4 (E = Si, Ge)

Abstract

The vibrational spectra of the cluster anions [E4]4– (E = Si, Ge) in the metallic compounds Ba2E4 and Na4E4 have been measured and assigned based on the Td symmetry of the discrete tetrahedranide anion. Due to the lower site-symmetries in the respective crystals all degenerate modes are split, but to different extends. The characteristic breathing frequency ν(E–E) of the [E4]4– cluster appears exclusively in the Raman spectrum and is almost unaffected by the nature of counterions: ν(E–E) = 486 cm–1 (E = Si) and 276 and 278 (Ge), respectively. The calculated valence force constants fd (Si–Si) = 1.17 Ncm–1 (Na); 1.15 Ncm–1 (Ba) and fd (Ge–Ge) = 0.98 Ncm–1 (Na); 0.94 Ncm–1 (Ba) are in good agreement with those previously reported.