Structural Diversity of Ternary and Quaternary Alkali-Metal Pnictides

Abstract

To evaluate the applicability and expand the limit of the Zintl-Klemm concept, many new alkali-metal pnictides were synthesized by high temperature solid reactions in A-M-Pn or A-Tt-Pn ternary systems and A-M-Tt-Pn quaternary systems (A = Na, K, Rb; M = Mn, Zn, Cd; Tt = Si, Ge, Sn; Pn = As, Sb). Their crystal structures were characterized by X-ray diffraction methods. Their electronic structures and bonding interactions were determined using TB-LMTO calculations. Ternary pnictides adopt their own-type structures, having unique structural patterns: corrugated chains (Rb4Zn7As7- and Rb7Mn12Sb12-type structure) and layers (KSn3As3-type structure), GeAs-type layers (NaGe6As6), and 3D-networks (K2Zn5As4). The quaternary AM1.5Tt0.5As2 pnictides prefer to crystallize either in the CaAl2Si2- or ThCr2Si2-type structure, depending on the relative sizes of atoms, as expressed through a structure map. The structure map also suggested that ACd1.5Ge0.5As2 (A = K, Rb) phases would not form; indeed, attempts to prepare them gave new triclinic phases ACdGeAs2 instead. Band structure calculations revealed electronic structures in good agreement with expectations: K2Zn5As4 (0.4 eV), KGe3As3 (0.71 eV), and KCdGeAs2 (0.80 eV) are small band-gap semiconductors; NaZn1.5Si0.5As2 (small overlap at Fermi level) is a semimetal; Rb4Zn7As7 (small gap above Fermi level), Rb7Mn12Sb12 (pseudogap above Fermi level), and NaGe6As6 (small gap below Fermi level) are metallic. Pnictides that have A-to-Pn ratio of 1:2 to 1:3 follow the Zintl concept. Rb4Zn7As7 and Rb7Mn12Sb12 are electron-deficient Zintl phases having an A-to-Pn ratio of ˂2 whereas NaGe6As6 is an electron-rich Zintl phase having an A-to-Pn ratio of ˃3. This work opens a new window to study substitutional chemistry.