Metallic and Mott insulating spin-frustrated antiferromagnetic states in ionic fullerene complexes with a two-dimensional hexagonal C60.- packing motif

Abstract

(MDABCO+)(C60.-)(TPC) (1), in which MDABCO+ is N-methyldiazabicyclooctanium, TPC is triptycene, and both have threefold symmetry, is a rare example of a fullerene-based quasi-2D metal and contains closely packed hexagonal fullerene layers with interfullerene center-to-center distances of 10.07Å at 300K. Evidence for the metallic nature of 1 was obtained by optical and microwave conductivity measurements on single crystals. The metal is characterized by a nontypical Drude response and relatively large optical mass (m*/m0=6.7). The latter indicates a narrow-band nature, which is consistent with the calculated bandwidth of 0.10-0.15eV. The coexistence of metallic and antiferromagnetic nonmetallic 2D layers was observed in 1 above 200-230K. It was assumed that the nonmetallic layers undergo a transition to the metallic state below 200K due to ordering of the fullerene and cationic sublattices. New layered complex (MQ +)(C60.-)(TPC) (2) with a hexagonal arrangement of C60.- was obtained by increasing the interfullerene distance with the bulkier N-methylquinuclidinium cations (MQ+) having threefold symmetry. The structure of 2 is characterized by increased interfullerene center-to-center distances in the layers (10.124, 10.155, and 10.177Å at 250K). Unit-cell doubling parallel to the 2D layer (along the b axis) was observed at low temperatures. In contrast to metallic 1, 2 exhibits a nonmetallic spin-frustrated state with an antiferromagnetic interaction of spins (the Weiss temperature is -27K) and no magnetic ordering down to 1.9K. It was supposed that the expanded interfullerene distances in the triangular arrangement decrease the bandwidth and suppress metallic conductivity in 2, and thus a Mott-Hubbard insulating state with antiferromagnetically frustrated spins results. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.