On a TiNiSi‐Type Superstructure: Synthesis, Crystal and Electronic Structures of CaAgGe and Its Mn‐Substituted Derivative

Abstract

AbstractThe compound CaAgGe and its Mn‐substituted derivative CaMn0.07Ag0.93Ge were synthesized by reaction of the element mixtures at high temperature. Their structures were refined from single‐crystal X‐ray diffraction data. CaAgGe crystallizes in the isomorphic (i3) superstructure of the TiNiSi type (CaCuGe type). The LMTO band structure calculations predicted the CaAgGe phase to be metallic. In addition, it appeared that the valence electron concentration is critical for the atomic ordering and the resulting superstructure. Thus, CaAgGe is one electron short per asymmetric unit, but a drastic narrowing of the electron shortage is achieved through the more complex atomic ordering in the supercell, relative to the simple TiNiSi type subcell. This results in the formation of {Ge2} dumbbells in the supercell, which ascribe greater electronic flexibility to the structure. Despite the fact that CaAgGe is not a Zintl phase in the strict definition as valence balanced intrinsic semiconductor, its structural aspects and transport properties can still be understood within the Zintl concept. The formal Zintl phase Ca3MnAg2Ge3 could be predicted from the theoretical calculations, but no significant phase width was observed experimentally in the substituted derivative Ca3Mn0.21Ag2.79Ge3.