Abstract
The usual classical behavior of $S=3/2$, $B$-site ordered double perovskites generally results in simple, commensurate magnetic ground states. In contrast, combined magnetic susceptibility, heat capacity, and neutron powder diffraction measurements for the $S=3/2$ systems La${}_{2}$Na${B}^{\ensuremath{'}}$O${}_{6}$ (${B}^{\ensuremath{'}}=\text{Ru}$, Os) reveal an incommensurate magnetic ground state for La${}_{2}$NaRuO${}_{6}$ and a drastically suppressed ordered moment for La${}_{2}$NaOsO${}_{6}$. This behavior is attributed to the large monoclinic structural distortions of these double perovskites. The distortions have the effects of creating inequivalent nearest neighbor (NN) superexchange interactions and weakening them on average, possibly to an energy scale that is comparable with the average next nearest neighbor (NNN) superexchange. The exotic ground states in these materials can then arise from a competition between some combination of inequivalent NN and NNN exchange interactions, providing an unusual mechanism for achieving frustration in the double perovskite family.
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