New Lithium-Containing Pnictides with 1-D Infinite Chains of Supertetrahedral Clusters: Synthesis, Crystal and Electronic Structure of Ba4Li2Cd3Pn6 (Pn = P, As and Sb)

The novel complex pnictides Ba(4)Li(2)Cd(3)Pn(6) (Pn = P, As and Sb) have been synthesized by direct combination of the respective elements at high temperature, and structurally characterized by single-crystal X-ray diffraction. The three isostructural compounds crystallize with their own structure type in the centrosymmetric orthorhombic space group Cmcm (Pearson code oC60). The crystal structure is based on one-dimensional infinite chains of supertetrahedral clusters, [Cd(4)Pn(10)], running parallel the a-axis. These chains are connected through Pn(2)-type dumbbells. Tight-binding electronic structure calculations show that the electronic stability of these compounds requires strong covalent Pn-Pn and Cd-Pn bonds. The interactions within the polyanionic sub-structure are complimented by weaker Ba-Pn and Li-Pn bonds, which also show a substantial degree of covalency, and the strength of all interactions correlates very well with the corresponding interatomic distances. The precise satisfaction of the valence rules and the Zintl-Klemm concept is not essential though as structural vacancies on Cd and Li sites bring about an interplay between ionicity and covalency among the electronegative components. Electronic structure calculations show that Ba4Li2Cd3P6 is expected to be a semiconductor with a band gap of ca. 0.5 eV, while the gap decreases and vanishes altogether for the As- and Sb-analogs, respectively. Solid solutions between arsenides and antimonides appear possible, which could be an effective way to fine-tune transport properties. Since the latter two compounds can be considered as moderately-to-heavily doped intrinsic semiconductors, these materials might be suitable candidates for thermoelectric applications.