Frustrated spin- 12 molecular magnetism in the mixed-valence antiferromagnets Ba3MRu2O9

Abstract

We have performed magnetic susceptibility, heat capacity, muon spin relaxation, and neutron scattering measurements on three members of the family Ba3M Ru2O9, where M = In, Y and Lu. These systems consist of mixed-valence Ru dimers on a triangular lattice with antiferromagnetic interdimer exchange. Although previous work has argued that charge order within the dimers or intradimer double exchange plays an important role in determining the magnetic properties, our results suggest that the dimers are better described as molecular units due to significant orbital hybridization, resulting in one spin-1/2 moment distributed equally over the two Ru sites. These molecular building blocks form a frustrated, quasi-two-dimensional triangular lattice. Our zero and longitudinal field µSR results indicate that the molecular moments develop a collective, static magnetic ground state, with oscillations of the zero field muon spin polarization indicative of longrange magnetic order in the Lu sample. The static magnetism is much more disordered in the Y and In samples, but they do not appear to be conventional spin glasses.

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“…5. This type of structure can be found in materials, such as Ba 5 AlIr 2 O 11 [51,52] and the 6H-hexagonal oxides Ba 3 AB 2 O 9 (A=In, Y, Lu, Na and B=Ru, Ir) [53,54,55]. Such face sharing structure leads to strong hopping between these two Ir sites, and dimer excitations have been observed in experimental RIXS spectra and confirmed by a two-site model simulation [56].…”

Persistent low-temperature spin dynamics in the mixed-valence iridate Ba3InIr2O9

“…where N A is Avogadro's number, g 0 is electronic g-factor for the ground state, and g 1 and g 2 are g-factors for the excited states, which are separated from the ground state by energy gaps of ∆ 1 and ∆ 2 , respectively, yields decent description of the magnetic susceptibility up to at least 650 K. This fitting function can be understood as follows. In the absence of electronic correlations and spin-orbit coupling, 9 electrons occupying 6 t 2g orbitals of two Ir atoms give rise to S = 1 2 and S = 3 2 states depending on the filling of the molecular orbitals of the dimer [37]. The effect of spin-orbit coupling is taken into account by introducing electronic g-factors as fitting parameters.…”

Stacking Faults and Short‐Range Magnetic Correlations in Single Crystal Y5Ru2O12: A Structure with Ru+4.5 One‐Dimensional Chains

“…The average oxidation state of Ru in the octahedra in Ba 3 MRu 2 O 9 ( M = In, Lu, Y) is 4.5+ and the RuRu distance is ≈2.5 Å, compared to 2.755(3) and 3.057(3) Å in Y 5 Ru 2 O 12 . [ 14,20 ] The significantly longer RuRu distance in Y 5 Ru 2 O 12 suggests that we could eliminate the presence of RuRu metallic bonds, and therefore the S = ½ state, in Y 5 Ru 2 O 12 . Further, the shorter RuRu distance of Y 5 Ru 2 O 12 (2.755(3) Å) is comparable with the RuRu distances of the 6H perovskites Ba 3 LiRu 2 O 9 and Ba 3 NaRu 2 O 9 , which are 2.7482(9) and 2.768(1) Å, respectively.…”

“…As one illustration of the potential of these materials, Ru compounds in the 6H perovskite type structures with a common formula of Ba 3 ARu 2 O 9 ( A = In, La–Lu, Y, Zn, Sr, Na) are good examples of the rich physical properties that can be observed in ruthenates. [ 14–22 ] The general structure contains 2D triangular layers of face‐sharing octahedra forming Ru 2 O 9 dimers, and can range from relatively high ( P 6 3 / mmc ) to lower (monoclinic) symmetry, or exhibit structural transitions (Ba 3 NaRu 2 O 9 : P 6 3 / mmc to P 2/ c at 210 K) [ 17,18 ] or distortions (Ba 3 BiRu 2 O 9 : structural dimerization at 176 K [ 21 ] ) with temperature. The oxidation state of Ru in 6H compounds depends on the nature of the A‐site cation, so the magnetic behavior varies widely.…”

“…The synthesis of ruthenates typically uses high‐temperature solid state ( T > 1200 °C) reactions that most often yield products in the form of powders. [ 14–19 ] This approach has some limitations, in that the high temperatures can volatilize components and potentially lead to nonstoichiometry, lattice defects, and site occupancy disorder. These problems can cause difficulty in the accurate determination of physical properties.…”