Spin-glass behaviour in the double perovskite “Sr 2 FeTeO 6 ” due to mis-site disorder and cation vacancy formation

The double perovskite of nominal composition Sr2FeTeO6 has been prepared by the freeze-drying method. Simultaneous Rietveld refinement of X-ray and high resolution D2B neutron diffraction data has shown it to have space group I12/m1 but with a significant degree of pseudo-tetragonal symmetry with a = 5.6080(3) Å, b = 5.5952(3) Å, c = 7.9081(5) Å and β = 90°, and a far more complex cation distribution over the B- and B′-sites. Refinement of Fe and Te fractions over these sites yielded the composition (Fe0.84□0.16)B(Te0.87Fe0.13)B′. This, and the absence of any significant formation of oxygen vacancies, yields an oxidation state for Fe close to +3, confirmed by EPR measurements (g ∼ 2.00). The disorder found over the B- and B′-sites implies that each Fe3+O6 octahedron is coordinated to six species, each of which can be Te, □ or another Fe, producing a statistical distribution of exchange pathways. The presence of several signals in the room temperature 57Fe Mössbauer spectrum and the broadening of the ESR signals can be related to this statistical distribution of the coordination environments around Fe3+. Magnetic DC susceptibility measurements indicate that the most important interactions are of antiferromagnetic nature. Absence of magnetic peaks in the 4 K neutron diffraction data, divergence below 40 K of field-cooled and zero-field-cooled DC susceptibilities, the frequency dependence of χ′ and χ″ magnetic AC susceptibilities and the lack of a three-dimensional λ-type peak in the specific heat measurements are all consistent with spin-glass type behaviour, as expected from the variety of frustrated magnetic interactions arising from the observed mis-site disorder and vacancies.