Metallic Re–Re bond formation in different MRe2O6 (Mdouble bond; length as m-dashFe, Co, Ni) rutile-like polymorphs: The role of temperature in high-pressure synthesis

Different polymorphs of MRe2O6 (Mdouble bond; length as m-dashFe, Co, Ni) with rutile-like structures were prepared using high-pressure high-temperature synthesis. For syntheses temperatures higher than ∼1573 K, tetragonal rutile-type structures (P42/mnm) with a statistical distribution of M- and Re-atoms on the metal position in the structure were observed for all three compounds, whereas rutile-like structures with orthorhombic or monoclinic symmetry, partially ordered M- and Re-ions on different sites and metallic Re–Re-bonds within Re2O10-pairs were found for CoRe2O6 and NiRe2O6 at a synthesis temperature of 1473 K. According to the XPS measurements, a mixture of Re+4/Re+6 and M2+/M3+ is present in both structural modifications of CoRe2O6 and NiRe2O6. The low-temperature forms contain more Re+4 and M3+ than the high-temperature forms. Tetragonal and monoclinic modifications of NiRe2O6 order with a ferromagnetic component at ∼24 K, whereas tetragonal and orthorhombic CoRe2O6 show two magnetic transitions: below ∼17.5 and 27 K for the tetragonal and below 18 and 67 K for the orthorhombic phase. Tetragonal FeRe2O6 is antiferromagnetic below 123 K.