The Highly Frustrated 5d2 Double Perovskite Doppelgangers, SrLaMgReO6 and SrLaLiOsO6. A Comparison including Isostructural La2LiReO6

The synthesis and characterization of the double perovskite SrLaLiOsO6 is presented. It is isostructural (P2(1)/n) and isoelectronic (5d(2)) with SrLaMgReO6, which has been reported previously. The cell volumes are the same to within 1.4%: i.e., these perovskites are doppelgangers. 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 (Theta) for this material is -161 K, an enormous frustration index, f approximate to 100, is implied ( f = vertical bar Theta vertical bar/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 mu(eff) = 1.96 mu(B), Theta = -102 K, and TIP = 1.01 x 10(-3) emu/mol. A clear zero-field-cooled (ZFC), field-cooled (FC) divergence in the dc data occurs at similar to 10 K, suggesting a much reduced frustration index, f approximate to 10, relative to SrLaMgReO6. The real part of the ac susceptibility data, chi'(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 lambda 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 chi' data due to a much weaker signal. Nonetheless, the 10-fold difference in f between these doppelganger 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 approximate to 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.

Automated summary

The synthesis and characterization of the double perovskite SrLaLiOsO6, which is isostructural and isoelectronic with SrLaMgReO6, show different magnetic behaviors at low temperatures. SrLaLiOsO6 has a significantly reduced frustration index compared to SrLaMgReO6.