Metallic Re–Re bond formation in different MRe2O6 (MFe, Co, Ni) rutile-like polymorphs: The role of temperature in high-pressure synthesis

Abstract

Keywords: 3d-metal rhenium oxide MRe 2 O 6 Rutile-like structure Polymorphism High-pressure synthesis Re-Re bond Magnetism XPS spectra a b s t r a c t Different polymorphs of MRe 2 O 6 (MQFe, Co, Ni) with rutile-like structures were prepared using highpressure 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 2 O 10 -pairs were found for CoRe 2 O 6 and NiRe 2 O 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 2 O 6 and NiRe 2 O 6 . The low-temperature forms contain more Re +4 and M 3+ than the high-temperature forms. Tetragonal and monoclinic modifications of NiRe 2 O 6 order with a ferromagnetic component at $24 K, whereas tetragonal and orthorhombic CoRe 2 O 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 2 O 6 is antiferromagnetic below 123 K.