Spin-glass behaviour in the double perovskite "sr ₂FeTeO₆" due to mis-site disorder and cation vacancy formation

The double perovskite of nominal composition Sr₂FeTeO ₆ 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 (Fe_0.840.16) _B(Te_0.87Fe_0.13)_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.