The Highly Frustrated 5d2 Double Perovskite Doppelgängers, SrLaMgReO6 and SrLaLiOsO6. A Comparison including Isostructural La2LiReO6Journal Articles
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The synthesis and characterization of the double perovskite SrLaLiOsO6 is presented. It is isostructural (P21/n) and isoelectronic (5d2) with SrLaMgReO6, which has been reported previously. The cell volumes are the same to within 1.4%: i.e., these perovskites are doppelgängers. In a previous study SrLaMgReO6 showed no sign of spin order to 2 K. New data at lower temperatures disclose a maximum in the dc susceptibility near 1.5 K. As the Curie-Weiss (C-W) temperature (Θ) for this material is -161 K, an enormous frustration index, f ≈ 100, is implied (f = |Θ|/Tord). On the other hand, SrLaLiOsO6 does not follow the C-W law over the investigated susceptibility range, 2-300 K. Fitting with an added temperature independent term (TIP) gives μeff = 1.96 μB, Θ = -102 K, and TIP = 1.01 × 10-3 emu/mol. A clear zero-field-cooled (ZFC), field-cooled (FC) divergence in the dc data occurs at ∼10 K, suggesting a much reduced frustration index, f ≈ 10, relative to SrLaMgReO6. The real part of the ac susceptibility data, χ'max, shows a frequency shift that is consistent with a spin glass ground state according to the Mydosh criterion. Heat capacity data for SrLaLiOsO6 show no sign of a λ peak at 10 K and a linear dependence on temperature below 10 K, also supporting a spin glass ground state. A spin frozen ground state for SrLaMgReO6 could not be established from χ' data due to a much weaker signal. Nonetheless, the 10-fold difference in f between these doppelgänger materials is remarkable. It is possible that the enhanced covalency with the oxide ligands for Os6+ relative to Re5+ plays a major role here. Finally, a comparison with isostructural La2LiReO6 (with a much smaller f ≈ 4) is made and a correlation between the frustration level and the sense of the local distortion of the Re(Os)-O octahedron is pointed out.
