Cation Dependence of Crystal Structure and Band Parameters in a Series of Molecular Conductors, β'-(Cation)[Pd(dmit)2]2 (dmit = 1,3-dithiole-2-thione-4,5-dithiolate)

Abstract

An isostructural series of anion radical salts, β'-(Et x Me 4−x Z) [Pd(dmit) 2 ] 2 (x = 0-2, Z = P, As, Sb), with a quasi-triangular lattice comprising the dimer unit [Pd(dmit) 2 ] 2 − belong to a strongly correlated electron system with geometrical frustration. Intra and interdimer transfer integrals between the frontier molecular orbitals, which characterize the strength of electron correlation and degree of frustration, can be tuned by selection of the counter cation. We have systematically analyzed the crystal structure with X-ray diffraction method and intermolecular transfer integrals using extended Hückel molecular orbital calculations based on structural data. The variation in the cation affects the unit cell in a manner equivalent to an anisotropic pressure. Increasing the covalent radius of the central atom Z and the number of ethyl groups (x) in the cation leads to slight arching of the Pd(dmit) 2 molecule. This arch-shaped distortion of the Pd(dmit) 2 molecule modifies the interdimer transfer integrals in formation of the regular triangular dimer lattice. On the other hand, the intradimer transfer integral, which is correlated with the effective on-site Coulomb interaction of the dimer, is weakly dependent on the type of cation.

Temperature Dependence of Crystal Structures and Band Parameters in Quantum Spin Liquid β′-EtMe3Sb[Pd(dmit)2]2 and Related Materials

“…The electronic structure of molecular conductors is simple and can be explained by tight-binding approximation [1,[3][4][5][6]. We can describe the band structure of the β -Pd(dmit) 2 salts using a dimer model where electrons are considered to move from dimer to dimer [1,3,4].…”

“…We can describe the band structure of the β -Pd(dmit) 2 salts using a dimer model where electrons are considered to move from dimer to dimer [1,3,4]. The metal dithiolene complex is characterized by a small energy gap between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO; ∆ε in Figure 1b).…”

“…The CO phase transition in the Et2Me2Sb salt is accompanied by a large structural transition [15], where the coupling between the lattice and the electron plays an important role. The difference in the ground state can be understood by the anisotropy of the isosceles triangular lattice t′/t that can be tuned by the choice of counter cation [1][2][3][4]. The QSL state, where spin fluctuation prevents the spin ordering/freezing and provides liquidlike properties among the spins even at zero temperature, is realized in the EtMe3Sb salt with t′/t = Temperature dependence of the magnetic susceptibility for the β -Pd(dmit) 2 salts can be described by the model of a spin-1/2 Heisenberg antiferromagnet on the triangular lattice with J = 200-300 K [8].…”

“…Therefore, most of the β′-Pd(dmit)2 salts are Mott insulators at ambient pressure. The interdimer transfer integrals, tB, tS, and tr ( Figure 1f) were obtained by the following equations using the intermolecular transfer integrals, TB, Tr, Tp, and Tq:tB = TB/2, ts = (Tp + 2Tq)/2, and tr = Tr/2 [3]. In the β′-Pd(dmit)2 salts, the tB and ts values are nearly the same and the tr is smaller, thus, the dimer units form an isosceles triangular lattice characterized by t′/t where t = (tB + ts)/2 and t′ = tr.…”

Theories on Frustrated Electrons in Two-Dimensional Organic Solids

“…In order to perform comparative discussions, ab-initio calculations on other materials such as κ-ET 2 Cu[N(CN) 2 ]Cl, EtMe 3 Sb[Pd(dmit) 2 ] 2 , and also the ones on the molecular-based models (not in unit of dimers) are awaited. At present, the semi-empirical evaluation of the transfer integrals at hand [14,106,107] summarized in Figure 23 provides a hint to understand the difference between materials; the degree of dimerization against other transfer integral yields t d /t B ∼ 2, 2.6, and 7 for κ-ET 2 Cu 2 (CN) 3 , κ-ET 2 Cu[N(CN) 2 ]Cl and EtMe 3 Sb[Pd(dmit) 2 ] 2 , respectively. For example, if one assumes U/t B = 10 and V d /t B = 6, the dimer approximation in Equation (5) gives U eff /t eff = 5 ∼ 8 for κ-type and 12 for the dmit salt.…”