The Zintl Phases Eu₃Mg₅Si₅ and Eu₃Mg₅Ge₅

Abstract

Abstract The Zintl phases Eu 3 Mg 5 Si 5 and Eu 3 Mg 5 Ge 5 were synthesized by high‐frequency melting of the elements in sealed tantalum tubes. Both of their structures have been refined from X‐ray single crystal diffractometer data: Sr 12 Mg 17.8 Li 2.2 Si 20 type, Pnma , a = 1424.3(1), b = 450.5(2), c = 1821.4(2) pm, wR 2 = 0.0544, 1995 F 2 values, 80 variables for Eu 3 Mg 5 Si 5 and a = 1445.8(2), b = 452.9(1), c = 1845.0(2) pm, wR 2 = 0.0375, 2053 F 2 values, 80 variables for Eu 3 Mg 5 Ge 5 . The electron precise Zintl phases [Eu 2+ ] 3 [Mg 2+ ] 5 [Si 4– ] 2 [Si 3 8– ] and [Eu 2+ ] 3 [Mg 2+ ] 5 [Ge 4– ] 2 [Ge 3 8– ] contain isolated tetrelide ions and bent Si 3 ( d (Si–Si): 239–241 pm) and Ge 3 units ( d (Ge–Ge): 251–256 pm) units respectively which have single bond character and are isoelectronic to Cl 2 O, Cl 2 S, and [P 3 5– ]. The formal chemical bonding is confirmed by ab‐initio calculations. The ecliptic stacking of the anions along b causes a strong dispersion of the dedicated bands. Eu 3 Mg 5 Si 5 is predicted to be a semi‐metal; i.e., a semi‐conductor with zero energy gap between bands derived from anti‐bonding silicon states. Eu 3 Mg 5 Ge 5 , however, appears to be a metal, due to the participation of germanium‐ d orbitals to states at the Fermi level. The divalent europium has been manifested through magnetic susceptibility measurements. At high temperatures, Eu 3 Mg 5 Si 5 and Eu 3 Mg 5 Ge 5 are Curie–Weiss paramagnets with experimental magnetic moments of 7.96(1) and 7.94(1) μ B /Eu atoms, respectively. Magnetic ordering, hinting at frustrated interactions, is observed at 19.1 (Eu 3 Mg 5 Si 5 ) and 14.2 (Eu 3 Mg 5 Ge 5 ) K. Temperature dependent 151 Eu Mössbauer spectra are indicative of purely divalent europium and complex magnetic hyperfine field splitting at low temperatures.