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

The new metastable binary europium silicide EuSi6 and its ternary derivative EuSi6-xGax (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)A, b=10.2229(7)A, c=11.6160(5)A 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+[SiO]62e-. 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 ( at 300 K). Magnetization measurements demonstrate that europium is stable divalent with the 4f7 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.