Influence of Element Substitution on the Cluster Arrangement in the Novel Structures Ca3Tl5, Sr3Tl5, and Sr3Sn5−xTlx (x = 1.8 and 2.2)
Résumé
The compounds Sr3Sn5–xTlx (x = 1.78(4), I; 2.16(5), II), Ca3Tl5 (III), and Sr3Tl5 (IV) have been obtained by direct fusion of the elements in sealed Nb ampoules at high temperature. Their structures were determined by single crystal X–ray diffraction studies and the bonding was investigated by TB–LMTO–ASA calculations. The ternary phases I and II crystallize in the Tm3Ga5 structure type; space group Pnma (No. 62); Z = 4 with a = 13.422(1), b = 10.897(1), c = 6.897(1) Å for I and a = 13.482(2), b = 10.938(2), c = 6.850(1) Å for II. The binary compounds III and IV crystallize in the Pu3Pd5 structure type; space group Cmcm (No. 63); Z = 4 with a = 10.231(2), b = 8.362(2), c = 10.623(2) Å for III and a = 10.592(5), b = 8.667(5), c = 10.973(6) Å for IV. Both structure types consist of zigzag chains of distorted square pyramidal {E5} (E = Tl, Sn/Tl) clusters. In the lower symmetric structures I/II the clusters are reoriented with respect to each other and have only two exo–contacts each, whereas in III/IV the clusters are aligned parallel and show a considerably higher degree of polymerization. The DOS for Sr3Sn5−xTlx calculated for x = 0 reveals a pseudo gap for x = 2 within the rigid band model, whereas the electronic band structure for Sr3Tl5 and Ca3Tl5 show metallic properties and strong participation of the cations in the bonding.
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