Non-isovalent substitution in a Zintl phase with the TiNiSi type structure, CaMg_1–xAg_xGe [x= 0.13 (3)]

Abstract

Single crystals of the title Ag-substituted calcium magnesium germanide, CaMg1–xAgxGe [x = 0.13 (3)] were obtained from the reaction of the corresponding elements at high temperature. The compound crystallizes with the TiNiSi structure type (Pearson code oP12) and represents an Ag-substituted derivative of the Zintl phase CaMgGe in which a small fraction of the divalent Mg atoms have been replaced by monovalent Ag atoms. All three atoms in the asymmetric unit (Ca, Mg/Ag, Ge) occupy special positions with the same site symmetry (.m.). Although the end member CaAgGe has been reported in an isomorphic superstructure of the same TiNiSi type, higher Ag content in solid solutions could not be achieved due to competitive formation of other, perhaps more stable, phases.

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

“…[31] Previous endeavours to prepare the hypothetical Mg-substituted charge-balanced phase Ca 3 MgAg 2 Ge 3 were also elusive, and always produced phases with higher Mg content, CaMg 1-x Ag x Ge (x Ͻ 0.15) in the simple TiNiSi type subcell. [32] …”

Valence State Driven Site Preference in the Quaternary Compound Ca5MgAgGe5: An Electron-Deficient Phase with Optimized Bonding

“…Hence, alio -valent partial substitutions of monovalent Ag by divalent Mg or trivalent Al in CaAgGe with a TiNiSi-type superstructure − were conducted under the hypothesis that the superstructure will be suppressed only when the valence electron (ve) count corresponds to the limit of the Zintl concept (8 ve/formula unit). Surprisingly, in the case of Mg, the reaction yielded the Ag-poor compound CaMg 1– x ­Ag x Ge with the Ag content limited to x = 0.13(3) . Finally, the missing member of the structure series Ca 2+ n M 2+ x ­Ge 2– x + n , with n = 3, was serendipitously obtained during similar Ag substitution by Mg metal in the Zintl phase Ca 3 Ag 1+ x ­Ge 3– x ( x = 1/3) .…”

“…Finally, the oven was switched off to allow the product to cool down to room temperature. Routine analysis by powder X-ray diffraction on a Stoe diffractometer (Ge(111) monochromator for Cu–Kα 1 radiation: λ = 1.54056 Å) equipped with a linear position sensitive (PSD) detector indicated that the resulting air- and moisture-sensitive products are multiphasic with a larger amount of the title compound in the form of highly reflective black crystals with bulky shapes and some undesired phases, including CaMg 1– x ­Ag x Ge (TiNiSi-type, x = 0.13) and other unidentified phases, that are generally in the form of microcrystalline powder (no single crystal of these side products was found in the sample). In addition, single crystals from the reaction of the mixture Ca:Mg:Ag:Ge = 6:1:2:4, with a lower amount of Mg, were refined, yielding the composition Ca 5 Mg 0.88 ­Ag 1.12(1) Ge 5 .…”

“…Recently, we embarked on assessing the experimental limit of the electronic flexibility of the TiNiSi-type structure. Hence, alio -valent partial substitutions of monovalent Ag by divalent Mg or trivalent Al in CaAgGe with a TiNiSi-type superstructure − were conducted under the hypothesis that the superstructure will be suppressed only when the valence electron (ve) count corresponds to the limit of the Zintl concept (8 ve/formula unit). Surprisingly, in the case of Mg, the reaction yielded the Ag-poor compound CaMg 1– x ­Ag x Ge with the Ag content limited to x = 0.13(3) .…”

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

“…However, only a modest 13% Ag could be inserted in the Mg position of the CaMg1-xAgxGe system, while retaining the TiNiSi type structure. 7 The experimentally limited miscibility range of the pseudo-ternary CaMg1-xAgxGe system may be ascribed to the atomic size mismatch between Mg (1. 60 Å) and Ag (1.44 Å).…”