The Highly Frustrated 5d Double Perovskite Doppelgängers, SrLaMgReO and SrLaLiOsO. A Comparison including Isostructural LaLiReO.

The synthesis and characterization of the double perovskite SrLaLiOsO is presented. It is isostructural (2/) and isoelectronic (5d) with SrLaMgReO, 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 SrLaMgReO 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, ≈ 100, is implied ( = |Θ|/). On the other hand, SrLaLiOsO 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 = 1.96 μ, Θ = -102 K, and TIP = 1.01 × 10 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, ≈ 10, relative to SrLaMgReO. The real part of the ac susceptibility data, χ', shows a frequency shift that is consistent with a spin glass ground state according to the Mydosh criterion. Heat capacity data for SrLaLiOsO 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 SrLaMgReO could not be established from χ' data due to a much weaker signal. Nonetheless, the 10-fold difference in between these doppelgänger materials is remarkable. It is possible that the enhanced covalency with the oxide ligands for Os relative to Re plays a major role here. Finally, a comparison with isostructural LaLiReO (with a much smaller ≈ 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.