In an isostructural series of anion radical salts β -(Me 4-x Et x Z)[Pd(dmit) 2 ] 2 (Z = P, As, Sb; x = 0, 1, 2), [Pd(dmit) 2 ] 2 − units form a two-dimensional Mott insulator layer with a quasi-isosceles triangular lattice. The anisotropy of the triangular lattice is characterized by a ratio of interdimer transfer integrals, t /t. The crystal structures of EtMe 3 Sb, Me 4 Sb, Me 4 As, and Et 2 Me 2 As salts were determined in the range of 5-295 K by the single crystal X-ray diffraction

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... e. Interdimer transfer integrals, Fermi surface, and band structures at low temperatures were calculated by the tight binding method and the first-principles density-functional theory (DFT) method based on experimentally obtained crystal structures. Interdimer transfer integrals increased with lowering temperature. At 5 K, the ratio t /t decreased by about 15% from the room temperature value in every salt. The relationship between the transfer integrals and interdimer S· · · S distances indicated that the change of the t /t value with temperature was due to a thermal contraction, rather than the arch-shaped molecular distortion of the Pd(dmit) 2 molecule associated with the cation dependence of t /t. Crystals 2018, 8, 138 2 of 15 HOMO and LUMO in the monomer forms a bonding pair and an anti-bonding pair with a large energy gap (dimerization gap, ∆ε 1 and ∆ε 2 ). The dimerization gap is governed by the intradimer transfer integral T A (Figure 1e ). When the dimerization gaps are large enough, the energy level of the anti-bonding HOMO pair can be located higher than that of the bonding LUMO pair (Figure 1b) [7]. Since the dimer unit has one negative charge, the anti-bonding HOMO pair has one unpaired electron. When each energy level forms an energy band in the crystal, the conduction band originates from the HOMO of the monomer. The conduction band is two-dimensional, half-filled, and narrow. Therefore, most of the β -Pd(dmit) 2 salts are Mott insulators at ambient pressure. The interdimer transfer integrals, t B , t S , and t r (Figure 1f) were obtained by the following equations using the intermolecular transfer integrals, T B , T r , T p , and T q :t B = T B /2, t s = (T p + 2T q )/2, and t r = T r /2 [3]. In the β -Pd(dmit) 2 salts, the t B and t s values are nearly the same and the t r is smaller, thus, the dimer units form an isosceles triangular lattice characterized by t /t where t = (t B + t s )/2 and t = t r . Crystals 2018, 8, x FOR PEER REVIEW 2 of 15