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Abstract

Vibrational spectra of the compounds M4E4 (M = K, Rb, Cs; E = Ge, Sn)
and of beta-Na4Sn4 with the cluster anions [E-4](4-) were analysed
based on the point group (4) over bar 3m-T-d of isolated tetrahedranide
units. The lower individual (4) over bar 2m-D-2d symmetry of the anions
in the real structure being more patterned and complex primarily
affects the spectra of the tetrahedro-tetragermanides. nu(3)(F-2)
clearly splits both in Raman and IR and in the case of K4Sn4 only in
JR. Rb4Sn4 and Cs4Sn4 exhibit very simple spectra with three bands in
Raman and one band in JR. The breathing mode nu(1)(A(1)) for the quasi
isolated [E-4](4-) cluster appears only in the Raman spectrum and is
hardly influenced by the structural environment and by the nature of
the alkali metal cations: nu(1)(A(1)) = 274 cm(-1) ([Ge-4](4-)) and
183-187 cm(-1) ([Sn-4](4-)), respectively. The calculated valence force
constants fd(E-E) are: [Ge-4](4-): f(d) = 0.89 Ncm(-1) (K), 0.87
Ncm(-1) (Rb), 0.86 Ncm(-1) (Cs) and [Sn-4](4-): 0.67 Ncm(-1) (Na), 0.66
Ncm(-1) (K), 0.67 Ncm(-1) (Rb), 0.68 Ncm(-1) (Cs). Both, the
frequencies and the force constants fit well into the range previously
reported.