4d3 Ru5+ triangular lattice antiferromagnets: layered rock-salt β-Li3Mg2RuO6 and ion-exchanged delafossite Ag3Mg2RuO6

Abstract The crystal structure and magnetic properties of Ru5+ triangular lattice antiferromagnets β-Li3Mg2RuO6 and Ag3Mg2RuO6 are reported. We successfully synthesized a pseudohexagonal layered rock-salt type phase β-Li3Mg2RuO6 with 2:1 ordering of Mg and Ru, which is a polymorph of an orthorhombic partial ordered rock-salt phase α-Li3Mg2RuO6. Shortening a calcinating time and adding excessive amounts of Li can prevent a polymorphic transformation from β-Li3Mg2RuO6 to α-Li3Mg2RuO6. An ordered delafossite compound Ag3Mg2RuO6 has been obtained by topochemical molten salt ionic exchange reaction to β-Li3Mg2RuO6. In β-Li3Mg2RuO6 and Ag3Mg2RuO6, multiple superexchange pathways Ru5+–O2−–O2−–Ru5+ leads to large antiferromagnetic interactions of θW ​= ​−113.9 and −108.1 ​K, respectively, despite a long distance between the magnetic Ru5+ ions. In addition, β-Li3Mg2RuO6 and Ag3Mg2RuO6 exhibit antiferromagnetic orderings with a trace of spin glasslike contribution at TN ​= ​13 and 24 ​K, respectively, which is evidenced by the heat capacity measurement. While β-Li3Mg2RuO6 exhibits a magnetic behavior that follows the Curie-Weiss curve up to just above TN, Ag3Mg2RuO6 exhibits a magnetic behavior that deviates from the Curie-Weiss curve below 100 ​K, which is considerably higher than TN. The different magnetic behaviors suggest a difference in magnetic dimensionality, indicating that the difference in the coordination environment around interlayer between Li+ and Ag+ ions gives rise to considerably different interlayer superexchange pathways in both systems.