Eu₉Cd_4-xCM_2+x-y□_ySb₉: Ca₉Mn₄Bi₉-type structure stuffed with coinage metals (Cu, Ag, and Au) and the challenges with classical valence theory in describing these possible zintl phases

The synthesis, crystal structure, magnetic properties, and europium Mössbauer spectroscopy of the new members of the 9-4-9 Zintl family of Eu₉Cd_4-xCM_2+x-y□_ySb₉ (CM = coinage metal: Au, Ag, and Cu) are reported. These compounds crystallize in the Ca₉Mn₄Bi₉ structure type (9-4-9) with the 4g interstitial site almost half-occupied by coinage metals; these are the first members in the 9-4-9 family where the interstitial positions are occupied by a monovalent metal. All previously known compounds with this structure type include divalent interstitials where these interstitials are typically the same as the transition metals in the anionic framework. Single-crystal magnetic susceptibility data indicate paramagnetic behavior for all three compounds with antiferromagnetic ordering below 10 K (at 100 Oe) that shifts to lower temperature (<7 K) by applying a 3 T magnetic field. ¹⁵¹Eu Mössbauer spectra were collected on polycrystalline powder samples of Eu₉Cd_4-xCM_2+x-y□_ySb₉ at 50 and 6.5 K in order to evaluate the valence of Eu cations. Although the Zintl formalism states that the five crystallographically distinct Eu sites in Eu₉Cd_4-xCM_2+x-y□_ySb₉ should bear Eu²⁺, the Mössbauer spectral isomer shifts are clearly indicative of both 2+ and 3+ valence of the Eu cations with the Cu- and Au-containing compounds showing higher amounts of Eu³⁺. This electronic configuration leads to an excess of negative charge in these compounds that contradicts the expected valence-precise requirement of Zintl phases. The spectra obtained at 6.5 K reveal magnetic ordering for both Eu²⁺ and Eu³⁺. The field dependence of Eu²⁺ indicates two distinct magnetic sublattices, with higher and lower fields, and of a small field for Eu³⁺. The site symmetry of the five Eu sites is not distinguishable from the Mössbauer data.