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The magnetic and electronic properties of Ba4Ru3O10 and Ba3CoRu2O9 were investigated by the ab initio and model calculations.
It is shown that nonmagnetic ground state of the one-third Ru4+ ions in Ba4Ru3O10 is not due to the correlation effects. It is rather caused by the charge disproportionation between crystallographically different Ru ions and the molecular orbital formation in the Ru's trimer. An unusual orbital state was recently proposed to explain the magnetic and transport properties of Ba3CoRu2O9 [Phys. Rev. B. 85, 041201 (2012)]. We show that this state contradicts to the first Hund's rule and does not realize in the system under consideration because of a too small crystal-field splitting in the t shell. Two mechanisms which may explain the suppression of the local magnetic moment on the Ru are proposed. According to the first one, "band mechanism", the suppresion is attributed to a strong hybridization between the Ru 4d and O 2p states. On another hand there are two different sets of the 4d orbitals due to a special geometry of the system (dimers consisted of the face-sharing RuO6 octahedra). We argue that only a part of the electrons occupying the egπ states should be considered as localized and hence providing the local magnetic moments, while other electrons form the spin singlet state on the orbitals of the a symmetry. |
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