Breaking the Zintl rule: High-pressure synthesis of binary EuSi 6 and its ternary derivative EuSi 6− xGa x ( [formula omitted

Abstract

The new metastable binary europium silicide EuSi 6 and its ternary derivative EuSi 6− x Ga x ( x = 0 – 0.60 ) were prepared by applying a pressure of 8 GPa and a temperature of 1520 K. Single crystal and powder diffraction data refinements reveal that the compounds crystallize orthorhombic (space group Cmcm, a = 4.4489 ( 3 ) Å , b = 10.2229 ( 7 ) Å , c = 11.6160 ( 5 ) Å for EuSi 6) and are isotypic to EuGa 2Ge 4. Analysis of the chemical bonding with the electron localization function reveals a covalently-bonded polyanion of four-bonded silicon atoms with embedded europium cations according to the electron balance Eu 2+[Si 0] 6 ⋅ 2 e − . The excess electrons are found in the basins of Si Si bonds. In the electronic density of states they are assigned to a filling of the antibonding states. In agreement with the picture of the chemical bonding, EuSi 6 is a bad metallic conductor ( ≈ 1 m Ω cm at 300 K). Magnetization measurements demonstrate that europium is stable divalent with the 4 f 7 configuration. EuSi 6 shows an antiferromagnetic ordering at 11.2 K and two further transitions between magnetically ordered phases below that temperature. Specific heat data confirm the magnetic ground state and the phase transitions.