The Zintl Phases Eu3Mg5Si5 and Eu3Mg5Ge5

Abstract

AbstractThe Zintl phases Eu3Mg5Si5 and Eu3Mg5Ge5 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: Sr12Mg17.8Li2.2Si20 type, Pnma, a = 1424.3(1), b = 450.5(2), c = 1821.4(2) pm, wR2 = 0.0544, 1995 F2 values, 80 variables for Eu3Mg5Si5 and a = 1445.8(2), b = 452.9(1), c = 1845.0(2) pm, wR2 = 0.0375, 2053 F2 values, 80 variables for Eu3Mg5Ge5. The electron precise Zintl phases [Eu2+]3[Mg2+]5[Si4–]2[Si38–] and [Eu2+]3[Mg2+]5[Ge4–]2[Ge38–] contain isolated tetrelide ions and bent Si3 (d(Si–Si): 239–241 pm) and Ge3 units (d(Ge–Ge): 251–256 pm) units respectively which have single bond character and are isoelectronic to Cl2O, Cl2S, and [P35–]. 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. Eu3Mg5Si5 is predicted to be a semi‐metal; i.e., a semi‐conductor with zero energy gap between bands derived from anti‐bonding silicon states. Eu3Mg5Ge5, 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, Eu3Mg5Si5 and Eu3Mg5Ge5 are Curie–Weiss paramagnets with experimental magnetic moments of 7.96(1) and 7.94(1) μB/Eu atoms, respectively. Magnetic ordering, hinting at frustrated interactions, is observed at 19.1 (Eu3Mg5Si5) and 14.2 (Eu3Mg5Ge5) K. Temperature dependent 151Eu Mössbauer spectra are indicative of purely divalent europium and complex magnetic hyperfine field splitting at low temperatures.