Isothermal Compressibility and Pressure Dependence of the Crystal Structures of the Superconducting Charge-Transfer Salt κ-(BEDT-TTF)₂Cu(NCS)₂ [BEDT-TTF = Bis(ethylenedithio)tetrathiafulvalene]

Abstract

From the pressure dependence of the lattice parameters measured by single-crystal X-ray diffraction at room temperature, the amplitudes and directions of the principal components of the isothermal compressibility tensor of κ-(BEDT-TTF)2Cu(NCS)2, bis(ethylenedithio)tetrathiafulvalene copper dithiocyanate, have been determined from 1 × 105 Pa to 12.9 × 105 kPa. The crystal structure has been determined at 1 × 105 Pa and 7.5 × 105 kPa. From the latter, the pressure dependence of the intermolecular electron-transfer integrals and the band structure have been calculated and compared with those at 1 × 105 Pa and 15 K.

Effective Coulomb interactions within BEDT-TTF dimers

“…47 Nuclear positions for C and S atoms were obtained from x-ray crystallography. 24,25,[48][49][50][51][52][53] H atoms, which are not observed in x-ray scattering experiments, were relaxed by the conjugate-gradient method. Total DFT energy differences between the relevant charge states ͓E͑1͒ − E͑0͔͒ and ͓E͑2͒ − E͑0͔͒ were equated with the corresponding analytical expressions in Eq.…”

“…Previous calculations of U d ͑v͒ based on both the Hückel method [24][25][26][27][28][29][30][31]55 ͑v͒ between the two polymorphs and when the anion is changed is interesting in the context of theories of these organic charge-transfer salts based on the Hubbard model. These theories require U d / W to vary significantly as the anion is changed ͑chemical pressure͒ and under hydrostatic pressure.…”

“…Previously, the on-site Coulomb repulsion term in the Hubbard model, U d , has been estimated from both Hückel [24][25][26][27][28][29][30][31] and density-functional 14,15 calculations under the assumptions that the intramonomer Coulomb repulsion U m → ϱ, and the intermonomer Coulomb repulsion V m → 0. We will show below that this assumption is incorrect and leads to a systematic underestimate of U d .…”

Toward the parametrization of the Hubbard model for salts of bis(ethylenedithio)tetrathiafulvalene: A density functional study of isolated molecules

“…Figures 4-6 show the calculated values of, respectively, U m ͑v͒ , ⑀ m , and m for experimental geometries taken from x-ray scattering experiments on crystals with a wide range of anions and several different polymorphs. The crystallographic data sets [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51] used to determine the nuclear positions include data taken at a range of temperatures and under different pressures. Details of the calculated Hubbard parameters and the crystallographic measurements on which the calculations are based are given in the supplementary information.…”

“…[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51] In Sec. IV we will discuss various schemes for relaxing these atomic positions and for calculating U m ͑v͒ , therefore we delay defining these methods to that section.…”