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) angstrom, b = 10.2229(7) angstrom, c = 11.6160(5) angstrom 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+[Si-0](6)center dot 2e(-). 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, EuSi6 is a bad metallic conductor (approximate to 1 m Omega cm at 300 K). Magnetization measurements demonstrate that europium is stable divalent with the 4f(7) 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. (c) 2006 Elsevier SAS. All rights reserved.