Concerning the first-order transition in the κ\kappa-phase (BEDT-TTF)4_4PtCl6_6 ⋅\cdot C6_6H5_5CN

(BEDT-TTF)$_4$PtCl$_6$ $\cdot$ C$_6$H$_5$CN is a $\kappa$-phase exhibiting a first-order transition at 250 K. The crystal structure of this salt contains two different donor layers before the transition but only one after the transition. Every layer contains two different BEDT-TTF donor molecules. Our tight binding band structure study suggests that this is the first BEDT-TTF salt containing BEDT-TTF in three different oxidation states (0, + 1/2 and + 1). One of the layers (${\cal A}$) contains [ BEDT-TTF$^{+1}$] $_2$ and [ BEDT-TTF$^{0}$] $_2$ dimers whereas the other layer (${\cal B}$) contains two different [ BEDT-TTF$^{+1/2}$] $_2$ dimers. We suggest that the first-order transition is associated with the disproportionation of the two [ BEDT-TTF$^{+1/2}$] $_2$ dimers of layer ${\cal B}$ in [ BEDT-TTF$^{+1}$] $_2$ and [ BEDT-TTF$^{0}$] $_2$ dimers. This process is associated with a conformational change in one of the dimers which apparently optimizes the donor-acceptor interactions in this $\kappa$-phase. According to the present results it would be worthwhile to reexamine the transport properties of this salt

Comparison of the electronic structures of isostructural (BEDT-TTF)3_3(HSO4_4)2_2 and [ Ni(dddt)2_2] 3_3(HSO4_4)2_2 molecular metals

The isostructural charge transfer salts (BEDT-TTF)$_3$(HSO$_4$)$_2$ and [ Ni(dddt)$_2$] $_3$(HSO$_4$)$_2$ are metallic at room temperature and exhibit metal to insulator transitions at 130 K and 25 K, respectively. X-ray diffuse scattering measurements for (BEDT-TTF)$_3$(HSO$_4$)$_2$ and tight-binding band structure calculations for both salts suggest that these transitions are most likely due to slight structural modifications, probably donor displacements, which suppress the semi-metallic overlap in these 3:2 salts. Although the Fermi surface of (BEDT-TTF)$_3$(HSO$_4$)$_2$ is made of closed loops, our study suggests that this salt has remarkable pseudo-1D properties. We show that even when they are isostructural, the BEDT-TTF and Ni(dddt)$_2$ salts can be electronically quite different, because the LUMO of the latter donor is not too far in energy from the HOMO. Consequently, the LUMO of Ni(dddt)$_2$ can mix into the partially filled bands of their charge transfer salts, which are based on the HOMO of the donor. This feature can change noticeably the topology of the band structure and the Fermi surface. We show that this actually happens in [ Ni(dddt)$_2$] $_3$(HSO$_4$)$_2$ and that it can be the clue to understand the differences in physical properties of the (BEDT-TTF)$_2$(HSO$_4$)$_2$ and [ Ni(dddt)$_2$] $_3$(HSO$_4$), salts

Metal-insulator interplays rendered by lattice transformations and structural disorder in DOEO salts

Three salts, β-DOEONO(HO) (I), β″-DOEONO(HO) (II), and β″-DOEOHSO(HO) (III), have been synthesized and characterized by means of four-probe conductivity measurements from room temperature down to 4.2 K and X-ray single crystal analysis at room temperature and 100 K. Salt I shows dielectric properties below room temperature, and salts II and III are stable metals. The DOEO molecules in I-III are packed into organic (conductive) layers expanding along the ab plane. The layer packing in II and III is the same in the projection along the long molecular axis and is of the β″ type, but differs noticeably in the perpendicular direction. Salts I and II are not isostructural, but their structures are very similar. However, the electronic structures of II and III are very similar, which leads to quantitatively comparable conductivities for II and III. The upper bands of II and III are 1/4-filled, whereas strong dimerization and band splitting leads to effective 1/2-filling of the upper band in I, which predetermines stable metallic properties for the former and renders strong electronic correlations in the latter. A superstructure of I is formed at room temperature. The Fermi surface obtained by means of extended Hückel tight-binding (EHTB) calculations is essentially altered by the superstructure: Two parallel 1D chains instead of a set of 1D chains and closed 2D pockets visible on the substructure are realized. The Fermi surface of I consists of flattened sections that could be a source of nesting instability. In contrast, the appearance of a superstructure of II at 100 K does not alter the Fermi surface that significantly, leaving the 2D conduction system intact and the Fermi surface with enough curvature to be stable against nesting effects. The DOEO terminal ethylene groups are strongly disordered in I with position occupancies (PO) of 0.6/0.4, less disordered in III (PO: 0.8/0.2), and fully ordered in II at room temperature. All these factors together with the very large value of the dimensionless ratio U/W = 1.66, where U is an on-site Coulomb repulsion and W is a bandwidth, indicate that I most likely is a Mott insulator. Two polymorphs were observed for DOEONO(HO): β and β″.The β polymorph is a Mott insulator with U/W = 1.66, where U is an on-site Coulomb repulsion and W is a bandwidth, whereas β″ is a stable metal with a wide 1/4-filled (W = 1 eV) upper band. DOEOHSO(HO) also shows β″ packing. Electronic structures and conductivities of both β″ compounds are nearly the same. Their X-ray structures are similar, however not isostructural

Specific Structural Disorder in an Anion Layer and Its Influence on Conducting Properties of New Crystals of the (BEDT-TTF)4A+[M3+(ox)3]G Family, Where G Is 2-Halopyridine; M Is Cr, Ga; A+ Is [K0.8(H3O)0.2]+

New crystals (1–4) of organic conductors based on the radical cation salts of the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) with paramagnetic and diamagnetic tris(oxalato)metallate anions {A+[M3+(ox)3]3−G}2−, where M is Cr, Ga; G is 2-chloropyridine, 2-bromopyridine; and A+ is [K0.8(H3O)0.2]+ have been prepared and their crystal structure and transport properties were studied. All crystals belong to the monoclinic group of the (BEDT-TTF)4A+[M3+(ox)3]G family with β″-packing type of conducting BEDT-TTF layers. In contrast to the known superconducting crystals with M3+ = Fe3+ and G = 2-chloro- or 2-bromopyridine (Tc = 4.0–4.3 K), crystals with Cr3+ and Ga3+ ions exhibit metallic properties down to 0.5 K without superconducting transition. Upon cooling these crystals, the incommensurate superstructure appears, which has never been observed before in the numerous β″-salts of the family. In addition, orthorhombic (sp. group Pbca) semiconducting crystals α″-(BEDT-TTF)5[Ga(ox)3]·3.4·H2O·0.6 EtOH (5) were obtained. It is a new compound in the family of BEDT-TTF crystals with tris(oxalato)metallate anions