Preparation, Magnetic Properties, and Pressure-Induced Transitions of Some MIIMIVF6 (MII=Ni, Pd, Cu; MIV=Pd, Pt, Sn) Complex Fluorides

MIIMIVF6 (MII=Ni, Pd, Cu; MIV=Pd, Pt) and PdSnF6 complex fluorides have been synthesized via different preparative methods using either BrF3 as oxidizer and solvent, or solid state reactions. For MII=Ni, Pd, the phases crystallize in the rhombohedral space group R3 (LiSbF6 type). Cationic ordering has been studied by X-ray diffraction and 119Sn Mössbauer resonance for PdSnF6. A lowering of symmetry has been observed when the involved divalent cation presents a Jahn–Teller configuration (CuII). Except for PdSnF6, which is paramagnetic down to 4 K, all compounds are Pd2F6-type ferromagnets at low temperature. This behavior has been related to the ordering between half-filled eg orbitals of the divalent cation and empty eg orbitals of the tetravalent cation. A drastic increase in conductivity has been observed under high pressures. In particular the insulator–semiconductor transition induced under pressure (up to 80 kbar) in Pd2F6 corresponds to a decrease of the electrical resistivity by six orders of magnitude. The assumption of an electronic transition induced under pressure from mixed oxidation states (MII+MIV) to an unique trivalent MIII oxidation state has been proposed.