The Highly Frustrated 5d² Double Perovskite Doppelgängers, SrLaMgReO₆ and SrLaLiOsO₆. A Comparison including...

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|/T_ord). 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 μ_eff = 1.96 μ_B, 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.