Metallic Re–Re bond formation in different MRe 2O 6 ( M[dbnd]Fe, Co, Ni) rutile-like polymorphs: The role of temperature in high-pressure synthesis

Abstract

Different polymorphs of MRe 2O 6 ( M Fe, 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 ( P4 2/ 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 Re 2O 10-pairs were found for CoRe 2O 6 and NiRe 2O 6 at a synthesis temperature of 1473 K. According to the XPS measurements, a mixture of Re +4/Re +6 and M 2+/ M 3+ is present in both structural modifications of CoRe 2O 6 and NiRe 2O 6. The low-temperature forms contain more Re +4 and M 3+ than the high-temperature forms. Tetragonal and monoclinic modifications of NiRe 2O 6 order with a ferromagnetic component at ∼24 K, whereas tetragonal and orthorhombic CoRe 2O 6 show two magnetic transitions: below ∼17.5 and 27 K for the tetragonal and below 18 and 67 K for the orthorhombic phase. Tetragonal FeRe 2O 6 is antiferromagnetic below 123 K.