Deciphering the Atomic Size Factor in the TiNiSi type Zintl phase CaAg0.5Al0.5Ge

The electron precise Zintl phase CaAg0.5Al0.5Ge (≡ Ca2AgAlGe2) was rationally obtained using the pseudo-element concept whereby the divalent Mg position in the prototype CaMgGe (8 ve/f.u) is completely replaced by an equiatomic mixture of monovalent Ag and trivalent Al atoms, while the TiNiSi type structure is retained. According to the Zintl-Klemm concept, formal valence electron transfer from the active metal Ca to the polyanionic AgAlGe2 sublattice is assumed, indicating a charge balance phase showing no significant phase width despite randomized Ag/Al mixing. However, the calculated band structure obtained based on first principles DFT method (LMTO code) and the chemical bonding analysis with the help of crystal orbital Hamilton population (COHP) and electron localisation function (ELF) could confirmed that, due to the difference in atomic size between Ag and Al, the Al–Ge chemical bond is essentially covalent, in contrast to the rather polar Ag–Ge bond. These indicate the ionic formulation for Ca2AgAlGe2 to be (Ca2+)2Ag+(4b-Al–)(2b-Ge2–)2, since the bond-ing electron pairs are shared between Al and Ge, while Ag+ is essentially cationic. This is also in agreement with an elec-tron 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). Hence, due to relatively similar atomic size between Al and Ge, an accumulation of charge density on the AlGe4 tetrahedra is observed leading to strong Pauli repulsion, 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.