Large Positive Zero-Field Splitting in the Cluster Magnet Ba₃CeRu₂O₉

Abstract

We present the synthesis and magnetic characterization of a polycrystalline sample of the 6Hperovskite Ba 3 CeRu 2 O 9 , which consists of Ru dimers based on face-sharing RuO 6 octahedra. Our low-temperature magnetic susceptibility, magnetization, and neutron powder diffraction results reveal a nonmagnetic singlet ground state for the dimers. Inelastic neutron scattering, infrared spectroscopy, and the magnetic susceptibility over a wide temperature range are best explained by a molecular orbital model with a zero-field splitting parameter D = 85 meV for the S tot = 1 electronic ground-state multiplet. This large value is likely due to strong mixing between this ground-state multiplet and low-lying excited multiplets, arising from a sizable spin molecular orbital coupling combined with an axial distortion of the Ru 2 O 9 units. Although the positive sign for the splitting ensures that Ba 3 CeRu 2 O 9 is not a single molecule magnet, our work suggests that the search for these interesting materials should be extended beyond Ba 3 CeRu 2 O 9 to other molecular magnets based on metal−metal bonding.

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.…”

“…[ 24 ] In contrast, when A 2+ contains open shell d electrons (e.g., Ba 3 CoRu 2 O 9 and Ba 3 NiRu 2 O 9 ), antiferromagnetic ordering occurs at lower temperatures. [ 16 ] When the A‐site cation is trivalent, ferromagnetic Ru‐spin dimers were observed in Ba 3 ARu 2 O 9 (A = La, Nd, and Y) using neutron powder diffraction, [ 15 ] while Ba 3 CeRu 2 O 9 displays no evidence of long‐range magnetic ordering down to 0.3 K. [ 20 ] This extensive list of different properties justifies further development of the chemistry of metal ion ruthenates.…”

Realization of the orbital-selective Mott state at the molecular level in Ba$_3$LaRu$_2$O$_9$

“…1 and they have already been shown to host a variety of interesting electronic ground states at the molecular level. For example, Ba 3 NaRu 2 O 9 exhibits interdimer charge order below 210 K [28], Ba 3 (Y,In,Lu)Ru 2 O 9 are quantum magnets with the S tot = 1/2 degree of freedom delocalized over the Ru dimers [24], and Ba 3 CeRu 2 O 9 has a non-magnetic ground state that arises from quasi-molecular orbital formation combined with a large zero field splitting [29]. The rich molecular behavior in this family likely arises from the ability of this structure to accommodate heavy transition metal dimers based on face-sharing octahedra, which feature metal-metal distances shorter than the nearest neighbor distance in the corresponding elemental metal [29] in some cases.…”

“…For example, Ba 3 NaRu 2 O 9 exhibits interdimer charge order below 210 K [28], Ba 3 (Y,In,Lu)Ru 2 O 9 are quantum magnets with the S tot = 1/2 degree of freedom delocalized over the Ru dimers [24], and Ba 3 CeRu 2 O 9 has a non-magnetic ground state that arises from quasi-molecular orbital formation combined with a large zero field splitting [29]. The rich molecular behavior in this family likely arises from the ability of this structure to accommodate heavy transition metal dimers based on face-sharing octahedra, which feature metal-metal distances shorter than the nearest neighbor distance in the corresponding elemental metal [29] in some cases. In fact, quasimolecular orbital formation has been argued to give rise to the quantum magnetism in Ba 3 (Y,In,Lu)Ru 2 O 9 [24] rather than the large spin state of S tot = 5/2 per dimer that would be realized by a double exchange mechanism [30].…”