Eight-Coordinated Arsenic in the Zintl Phases RbCd4As3 and RbZn4As3: Synthesis and Structural Characterization

Reported are two new series of Zintl phases, ACd4Pn3 and AZn4Pn3 (A = Na, K, Rb, Cs; Pn = As, P), whose structures feature complex atomic arrangements based on four- and eight-coordinated arsenic and phosphorus. A total of 12 compounds have been synthesized from the corresponding elements via high temperature reactions, and their structures have been established by X-ray diffraction. RbCd4As3, KCd4As3, NaCd4As3, NaZn4As3, KCd4P3, and KZn4P3 crystallize with a new rhombohedral structure (space group R3̅m, Z = 3, Pearson symbol hR24), while the isoelectronic RbZn4As3, CsCd4As3, CsZn4As3, KZn4As3, CsZn4P3, and RbZn4P3 adopt the tetragonal KCu4S3-type structure (space group P4/mmm, Z = 1, Pearson symbol tP8). Both structures are very closely related to the ubiquitous CaAl2Si2 and ThCr2Si2 structure types, and the corresponding relationships are discussed. The experimental results have been complemented by linear muffin-tin orbital (LMTO) tight-binding band structure calculations. Preliminary transport properties measurements on polycrystalline samples suggest that the compounds of these families could be promising thermoelectric materials.