2020 年 93 巻 2 号 p. 260-272
The ET•+ molecules in a charge-transfer salt (ET)Ag4(CN)5 form a three-dimensional diamond spin-lattice with S = 1/2 (ET: bis(ethylenedithio)tetrathiafulvalene), where a geometrical spin-frustration is expected when an appropriate spin interaction is realized. A metallic nature has been proposed for this salt based on both band calculation and electron paramagnetic resonance measurements. We studied the crystal and band structures, optical spectra, resistivity, magnetic, and NMR measurements and found the salt to be a three-dimensional monomer Mott insulator with a resistivity of 1.8 × 102 Ω cm at room temperature (// c), though the calculated band structure showed a Dirac-like semimetallic dispersion. 1H NMR and magnetic susceptibility measurements reveal an antiferromagnetic spin ordering at TN = 102 K, above which characteristic temperature insensitive behaviors of T1−1 and spin susceptibility are observed. A weak ferromagnetism is detected below TN with a spin canting angle of ∼0.01°, possibly arising from a Dzyaloshinskii-Moriya interaction due to a lowering of the crystal symmetry. This is the first example of a weak ferromagnetic three-dimensional diamond spin-lattice among the organic charge-transfer solids.