A. M. Flakina

Phase transition at 320 K in a new layered organic metal conductor (BEDT-TTF)4CoBr4(C6H4Cl2)

A new layered organic conductor θ-(BEDT-TTF)4CoBr4(1,2-C6H4Cl2) with different resistivity behaviour that has metallic conducting layers and is semiconducting perpendicular to the layers has been prepared. A phase transition near 320 K was found in this salt by temperature variable resistivity and X-ray diffraction measurements. Precise crystal structure data, below transition (triclinic phase) and above transition (tetragonal phase), was determined. The crystal structures of both phases are composed of θ-type radical cation layers alternating with layers involving [CoBr4]2− anions and 1,2-dichlorobenzene molecules. The lower symmetry in the crystal structure of the triclinic phase is due to a non-symmetrical arrangement of the solvent molecules within the anion layers. In each structure, the neighbouring conducting layers of the unit cell differ in stacking directions. In the tetragonal phase, all conducting layers are similar in structure, and in the low-temperature triclinic phase, the neighbouring conducting layers differ in periods along the stacks (4.703 A and 4.993 A) and in dihedral angles θ between the BEDT-TTF radical cations of the adjacent stacks. The finding of compounds of the θ-(BEDT-TTF)4CoBr4(1,2-C6H4Cl2) type with phase transitions above room temperature provides a wide possibility of investigating magnetoresistance, quantum oscillations, and the mechanism of interlayer transport in layered conductors.

Synthesis and studies of new organic conductors based on ET and EDT-TTF with the [ZnBr4]2− anions

New layer organic conductors based on bis(ethylenedithio)tetrathiafulvalene (ET)4ZnBr4Solv (Solv stands for solvent) were synthesized in various halobenzenes as solvents (C6H5X, X = Cl, Br, F and C6H4Y2, Y = Cl, Br), as well as based on ethylenedithiotetrathiafulvalene (EDT-TTF)4MBr4(1,1,2-C2H3Cl3) (M = Zn, Mn) and (EDT-TTF)9(ZnBr4)2. The crystal structure of (ET)4ZnBr4(C6H4Cl2) at room temperature was established. It was found to be composed of alternating conducting layers with various structure of stacks formed of the ET radical cations. Their conductivity and ESR spectra were measured. The ET compounds obtained are organic metals up to the temperatures of 4.2, 72, 80, or 183 K (depending on the solvent: C6H4Cl2, C6H5Cl, C6H5Br, or C6H5F, respectively); the replacement of the solvent with more bulky 1,2-dibromobenzene led to the formation of a semiconductor. The compounds (EDT-TTF)4MBr4(C2H3Cl3) with M = Zn, Mn and (EDT-TTF)9(ZnBr4)2 retain metallic character of conductivity up to the temperatures of 260, 280, and 210 K.

Phase transitions in θ-(ET)4MnBr4(C6H6−nCln) (n = 1, 2) driven by ordering in anion and/or cation layers

Newly synthesized layered organic conductors θ-(ET)4MnBr4(1,2-C6H4Cl2) (1) and θ-(ET)4MnBr4(C6H5Cl) (2) show metal-like conductivity along conducting layers down to 4.3 and 60 K, respectively, while showing semiconducting properties in the perpendicular direction. Phase transitions driven by ordering in anion layers in 1 and ordering in a half of cation layers in 2 were found to occur at 297 and ~240 K, respectively. Crystal structures above and below the phase transitions were determined for 1 at 260 and 333 K and for 2 at 260 and 220 K. In the above-transition phases of both compounds, attributed to the tetragonal crystal system, all ET layers are similar in structure. In the below-transition phases, attributed to triclinic and monoclinic crystal systems, the neighboring ET layers of each phase differ in periods along the stacks and in dihedral angles θ between radical cations of the adjacent stacks.

Shubnikov-de Haas oscillations in a new dual-layered quasi-two-dimensional organic metal (BETS)4CoBr4(C6H4Cl2)

The interlayer and intralayer resistances and Shubnikov-de Haas oscillations in a new dual-layered quasi-two-dimensional organic metal (BETS)4CoBr4(C6H4Cl2) with a periodically varying structure of cation layers have been studied. It has been shown that the interlayer resistivity corresponds to an incoherent or weakly incoherent transport regime. The oscillations of the magnetoresistance have been described by a model of a chain of coherent magnetic breakdown orbits taking into account the quantum interference effect. The behavior of the interlayer transport, as well as quantum oscillations, is in good agreement with the theoretical calculations of the band structure.

Tetrathiafulvalene-based organic conductors with Pb-containing anions

Hybrid compounds containing organic layers composed of tetrathiafulvalenes (BEDT-TTF, BETS, BEDO-TTF) and inorganic layers consisting of anions based on lead bromide were synthesized. The crystal structure of (BEDT-TTF)6Pb3Br10(PhBr) was determined. The temperature dependences of electrical conductivity for the synthesized compounds were measured and the EPR spectra of these compounds were recorded. In the structure of (BEDT-TTF)6Pb3Br10(PhBr), the organic conducting layers of BEDT-TTF molecules alternate with non-conducting layers composed of infinite chains of bromoplumbate anions [Pb3Br9]3–, solvent molecules, and Br– anions. The newly synthesized BEDT-TTF bromoplumbates have similar linewidths of EPR signals, which indicate that their conducting layers have similar structures. The BEDT-TTF bromoplumbates are semiconductors, while temperature-dependent electrical resistivity measurements show the metallic behavior for BEDO-TTF and BETS bromoplumbates.

Effect of the quality of crystalline samples on the interlayer electron transport in the organic quasi-two-dimensional metal (ET)8Hg4Cl12(C6H5Cl)2

The behavior of magnetoresistance is studied in two samples of the quasi-two-dimensional organic metal (ET)8Hg4Cl12(C6H5Cl)2 having different quality for various magnetic field directions. The interlayer transport in the high-quality sample is shown to be close to a coherent mode, and the interlayer transport in the low-quality sample is closer to an incoherent mode.

Organic conductor θ-(BEDT-TTF)Cd 1.38 I 3 with layered perovskite-like structure

The organic conductor θ-(BEDT-TTF)Cd1.38I3 (1) (BEDT-TTF is bis(ethylenedithio)- tetrathiafulvalene) with layered perovskite-like structure was synthesized. In the crystal structure of 1, θ-type organic conducting layers, which are built up from BEDT-TTF stacks, alternate with inorganic insulating layers. The latter are composed of CdI6 octahedra and CdI4 tetrahedra linked together to form a two-dimensional anionic network. Measurements of the temperature dependence of the electrical resistance demonstrated that compound 1 is a semiconductor.

Quantum oscillations in dual-layered quasi-two-dimensional organic metal (ET)4HgBr4(C6H4Cl2)

The behavior of de Haas–van Alfven (dHvA) and Shubnikov–de Haas (ShdH) quantum oscillations in dual-layered quasi-two-dimensional organic metal (ET)4HgBr4(C6H4Cl2) is investigated. The oscillation spectra qualitatively agree with theoretical calculations of the bandgap structure. The angular dependence of the oscillation amplitude of magnetoresistance contains “spin zeros”; the analysis of the location of these zeros allows one to evaluate the electron–phonon interaction constant: λ ≈ 0.2.