The crystal structures of the ternary selenogallates Cs 6 Ga 2 Se 6 , Cs 8 Ga 4 Se 10 , €8x003686,4 and CsGaSe 2 -I are characterized by the existence of linear oligimeric and polymeric anions [Ga n Se 2n+2 ] (,I+4)_ (« = 2,4,6, oo). These anions consist of edge sharing GaSe 4 -tetrahedra and show systematic geometrical variations of distances and angles which correlate with the degree of oligomerisation. In connection with the geometrical variations remarkable space filling changes are observed. Preliminary transmission measurements with small single crystals of the air sensitive compounds show an optical band gap of about 3 eV. A significant shift of the optical band gap to higher energies is observed and can be correlated with the length of the oligomeric anions and qualitative arguments about the chemical bonding in them.
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