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 10.07 Å at 300 K. 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 0.10-0.15 eV. The coexistence of metallic and antiferromagnetic nonmetallic 2D layers was observed in 1 above 200-230 K. It was assumed that the nonmetallic layers undergo a transition to the metallic state below 200 K 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 10.177 Å at 250 K). 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 -27 K) and no magnetic ordering down to 1.9 K. 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.