Magnetic properties of the S= 32 geometrically frustrated double perovskites La2LiRuO6 and Ba2YRuO6

Two B-site ordered double perovskites, Ba2YRuO6 and La2LiRuO6, have been reinvestigated as part of a systematic study of geometric magnetic frustration in this class of oxide materials. Both involve Ru5+(4d3,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, L7i and Y 89 NMR studies are presented. La2LiRuO6 (P21/n) shows long-range antiferromagnetic order below 24 K from L7i 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 L7i data indicates the importance of short-range spin correlations at higher temperatures, consistent with a frustrated system. Ba2YRuO6 retains Fm3m symmetry found at room temperature down to 2.8 K with cell constants, a=8.33559(9) and a=8.3239(5) Å, respectively. However, Y 89 magic-angle-spinning NMR detects a very low ∼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, |θ|/TN, for Ba2YRuO6 are −522 K and 16 while much smaller values are observed for La2LiRuO6, −184 K and 8, which can be attributed to the monoclinic structural distortion in the latter which weakens the superexchange interactions.