Quaternary Arsenides AM1.5Tt0.5As2 (A = Na, K, Rb; M = Zn, Cd; Tt = Si, Ge, Sn): Size Effects in CaAl2Si2- and ThCr2Si2‑Type Structures

Ten quaternary arsenides AM1.5Tt0.5As2 (A = Na, K, Rb; M = Zn, Cd; Tt = Si, Ge, Sn) have been prepared by stoichiometric reactions of the elements at 600–650 °C. Seven of them (NaZn1.5Si0.5As2, NaZn1.5Ge0.5As2, NaZn1.5Sn0.5As2, NaCd1.5Sn0.5As2, KZn1.5Sn0.5As2, KCd1.5Sn0.5As2, RbCd1.5Sn0.5As2) adopt the trigonal CaAl2Si2-type structure (Pearson symbol hP5, space group P3̅m1, Z = 1, a = 4.0662(3)–4.4263(7) Å, c = 7.4120(5)–8.4586(14) Å), whereas the remaining three (KZn1.5Si0.5As2, KZn1.5Ge0.5As2, RbZn1.5Ge0.5As2) adopt the tetragonal ThCr2Si2-type structure (Pearson symbol tI10, space group I4/mmm, Z = 2, a = 4.0613(10)–4.1157(5) Å, c = 14.258(3)–14.662(2) Å). Both structure types contain anionic [M1.5Tt0.5As2] slabs that are built from edge-sharing tetrahedra and that stack alternately with nets of A cations. A structure map delineates the formation of these structure types for AM1.5Tt0.5As2 as a function of simple radius ratios. Although these arsenides have charge-balanced formulations, band structure calculations on NaZn1.5Tt0.5As2 (Tt = Si, Ge, Sn) indicate that semimetallic behavior is predicted as a result of overlap of the valence and conduction bands.