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

In an isostructural series of anion radical salts beta'-(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 EtMe3Sb, Me4Sb, Me4As, and Et2Me2As salts were determined in the range of 5-295 K by the single crystal X-ray diffraction technique. 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 tilt 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.