Ag2MnO2 is studied as a possible candidate compound for an antiferromagnetic XY spin model on a triangular lattice. In spite of the large Curie-Weiss temperature of -430 K found in magnetic susceptibi-lity, Mn3+ spins with S = 2 do not undergo a conventional long-range order down to 2 K probably owing to the geometrical frustration and two dimensionality in the system. Instead, a unique phase transition is found at 80 K, where specific heat exhibits a clear sign of a second-order phase transition, while magnetic susceptibility changes smoothly without a distinct anomaly. We think that this transition is related to the chirality degree of freedom associated with a short-range order, which has been expected for the classical XY spin model on a triangular lattice. On further cooling, spin-glass-like behavior is observed below 22 K, possibly corresponding to a quasi-long-range order.Comment: to appear in J. Phys. Soc. Jpn, Vol. 77, No.

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The Ag 13 OsO 6 complex contains isolated Ag 13 4+ icosahedral clusters in which the central silver ion has the distinction of being bonded solely to other silver ions. This unique character, and the electronic structure of the complex, is described by M. Jansen et al. on the following pages.

Preparative exploration of the system AgϪMnϪO under an elevated oxygen pressure yielded so far unknown Ag 4 Mn 3 O 8 . Single crystal X-ray investigations have revealed a trigonal crystal system, space group P3 1 21 with lattice parameters a ϭ 12.5919(1) and c ϭ 15.4978(1) Å . Due to fourfold twinning a large cubic unit cell with a ഠ 26 Å is simulated. The structure, which was refined

ZuschriftenDer Ag 13 OsO 6 -Komplex enthält isolierte ikosaedrische Ag 13 4+ -Cluster, in denen das zentrale Silberion ausschließlich mit anderen Silberionen verknüpft ist. Die elektronische Struktur dieses einzigartigen Komplextyps beschreiben M. Jansen et al. auf den folgenden Seiten.