Deciphering the Atomic Size Factor in Quasi-ternary Zintl phases RAg0.5Al0.5Ge (R = Ca, Eu) with the TiNiSi type Structure

The targeted synthesis of two analogous quasi-ternary Zintl phases CaAg0.5Al0.5Ge and EuAg0.5Al0.5Ge were conducted by direct reaction of stoichiometric mixtures of the elements at high temperature and, their crystal structures refined from single-crystal X-ray diffraction data. These pseudo-ternary structures are intended to probe the generalization of the pseudo-element concept to the small polarizing cation component of the ternary isovalent Zintl phases CaMgGe and EuMgGe, with the complete replacement of the divalent Mg position by an equiatomic mixture of monovalent Ag and trivalent Al atoms, while the TiNiSi type structure (space group Pnma) is retained. The two title phases are almost line compounds without significant experimental phase width. However, the calculated band structure of the hypothetical ordered model of Ca2AgAlGe2, based on first principles DFT method (LMTO code), predicted the compound to be metallic. The chemical bonding analysis with the help of crystal orbital Hamilton population (COHP) could confirmed that, due to the difference in atomic size between Ag and Al, the Al–Ge contacts are localized 2c-2e bonds, in contrast to the more polar and non-localized Ag–Ge bonds. Hence, the ionic formulation should be (Ca2+)2Ag+(4b-Al–)(2b-Ge2–)2, according to the Zintl-Klemm concept (2b- and 4b- indicated two bonded and four bonded atoms). This is also in agreement with an electron localization function (ELF) topology analysis, revealing monosynaptic valence basins on Ge atoms (lone pairs) and only on the Ge–Al bonds (two-center, two-electron bond). Thus, due to the similar atomic size between Al and Ge, an accumulation of charge density on the severely distorted AlGe4 tetrahedra is observed leading to strong Pauli repulsion between the localized electron pairs, while Ag–Ge bonds are depleted. These findings are interesting regarding the influence of the geometric factor on the stability and physical properties of the TiNiSi type structural family in general.