Magnetic properties of the S = 3 2 geometrically frustrated double perovskites La 2 LiRuO 6 and Ba 2 YRuO 6

Two $B$-site ordered double perovskites, ${\text{Ba}}_{2}{\text{YRuO}}_{6}$ and ${\text{La}}_{2}{\text{LiRuO}}_{6}$, have been reinvestigated as part of a systematic study of geometric magnetic frustration in this class of oxide materials. Both involve ${\text{Ru}}^{5+}(4{d}^{3},S=3/2)$ as the magnetic ion residing on a face-centered-cubic lattice---one of the canonical frustrated lattices. Results from dc susceptibility, neutron-diffraction, heat-capacity, $^{7}\text{L}\text{i}$ and $^{89}\text{Y}$ NMR studies are presented. ${\text{La}}_{2}{\text{LiRuO}}_{6}$ $(P{2}_{1}/n)$ shows long-range antiferromagnetic order below 24 K from $^{7}\text{L}\text{i}$ NMR, heat-capacity and magnetic-susceptibility (Fisher's heat-capacity) data which is well below the susceptibility maximum at 30 K. Analysis of the entropy loss and the $^{7}\text{L}\text{i}$ data indicates the importance of short-range spin correlations at higher temperatures, consistent with a frustrated system. ${\text{Ba}}_{2}{\text{YRuO}}_{6}$ retains $Fm3m$ symmetry found at room temperature down to 2.8 K with cell constants, $a=8.33559(9)$ and $a=8.3239(5)\text{ }\text{\AA{}}$, respectively. However, $^{89}\text{Y}$ magic-angle-spinning NMR detects a very low $\ensuremath{\sim}1%$ site mixing between Y and Ru ions. Magnetic-susceptibility data are more complex than reported previously with two broad peaks around 37 and 47 K. The transition temperature is 36 K from heat capacity and variable-temperature neutron-diffraction data. The Weiss temperatures and frustration indices, $|\ensuremath{\theta}|/{T}_{\text{N}}$, for ${\text{Ba}}_{2}{\text{YRuO}}_{6}$ are $\ensuremath{-}522$ K and 16 while much smaller values are observed for ${\text{La}}_{2}{\text{LiRuO}}_{6}$, $\ensuremath{-}184$ K and 8, which can be attributed to the monoclinic structural distortion in the latter which weakens the superexchange interactions.