Geometric magnetic frustration has attracted substantial interest due to the exotic physics and rich phase diagrams revealed by the cancellation of normally dominant magnetic interactions, giving impetus for the search for novel frustrated systems, most often based on antiferromagnetic correlations between magnetic ions decorating triangular or tetrahedral lattices. We report here low-temperature magnetic susceptibility and muon spin relaxation results on ${\mathrm{Li}}_4{\mathrm{MgOsO}}_6$ and ${\mathrm{Li}}_3{\mathrm{Mg}}_2{\mathrm{OsO}}_6$, members of the $A_5{B\text{O}}_6$ rock-salt ordered family of frustrated materials. In ${\mathrm{Li}}_3{\mathrm{Mg}}_2{\mathrm{OsO}}_6$ we find spin freezing below 12 K. In ${\mathrm{Li}}_4{\mathrm{MgOsO}}_6$, which can crystallize into either orthorhombic $Fddd$ or monoclinic $C2/m$ crystal symmetries depending on synthesis conditions, we find magnetism consistent with glassylike behavior dominating below 2 K, with partial ordering and evidence for dynamics at somewhat higher temperatures.

• Received 12 September 2018

DOI:https://doi.org/10.1103/PhysRevB.98.184401