Breaking the Zintl rule: High-pressure synthesis of binary EuSi6 and its ternary derivative EuSi6−xGax (0⩽x⩽0.60⩽x⩽0.6)

The new metastable binary europium silicide EuSi6 and its ternary derivative EuSi6−xGax (x=0–0.60x=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) Åa=4.4489(3) Å, b=10.2229(7) Åb=10.2229(7) Å, c=11.6160(5) Åc=11.6160(5) Å for EuSi6) and are isotypic to EuGa2Ge4. 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 Eu2+[Si0]6⋅2e−⋅2e−. The excess electrons are found in the basins of SiSi 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, EuSi6 is a bad metallic conductor (≈1 mΩcm at 300 K). Magnetization measurements demonstrate that europium is stable divalent with the 4f74f7 configuration. EuSi6 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.

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