Tatiana G. Prokhorova

Effect of electrocrystallization medium on quality, structural features, and conducting properties of single crystals of the (BEDT-TTF)4AI[FeIII(C2O4)3]·G family

A modified procedure for the electrocrystallization of organic conductors with paramagnetic anions of the (BEDT-TTF)4AI[MIII(C2O4)3]·G family has been proposed. It is found that single crystals of different phases of the family can be prepared if the electrocrystallization medium is represented by the mixture of 1,2,4-trichlorobenzene (or 1,3-dibromobenzene), 96% ethanol and different solvents (G), only the latter being included into the composition of the resulting salts as neutral guest molecules (G = benzonitrile, fluorobenzene, chlorobenzene, 1,2-dichlorobenzene, bromobenzene, nitrobenzene). Using this approach, a number of known and new BEDT-TTF salts with the tris(oxalato)ferrate anion have been synthesized. Among them, there are superconducting crystals of monoclinic β′′-series with different guest solvents (G) and their mixtures. For the first time, crystals of a triclinic phase (G = 1,2-dibromobenzene), with alternating α- and ‘pseudo-κ’ BEDT-TTF layers and metallic behaviour down to 1.5 K, were obtained. Additionally, monoclinic crystals having another stoichiometry and α-type donor packing were prepared.

Coexistence of two donor packing motifs in the stable molecular metal α-‘pseudo-κ’-(BEDT-TTF)4(H3O)[Fe(C2O4)3]·C6H4Br2

The crystal and electronic structure of a new radical cation salt α-‘pseudo-κ’-(BEDT-TTF)4H3O[Fe(C2O4)3]·C6H4Br2 have been studied. The new triclinic crystals contain two conducting organic layers which are characterized by different BEDT-TTF packing motifs: a ‘pseudo-κ’-layer which is composed of charged dimers and neutral monomers of BEDT-TTF orthogonal to each other and an α-layer which consists of inclined, uniformly charged BEDT-TTF stacks. According to electronic band structure calculations, the ‘pseudo-κ’ layer has a large gap between the HOMO bands at the Fermi level and should be associated with an activated conductivity. In contrast, the α-layer is a strongly two-dimensional electronic system with uniform intermolecular interactions. The absence of any nesting in the Fermi surface of the α-layer suggests that this salt should be a stable metal down to low temperatures. Metallic properties have been observed in the crystals in the 300–0.4 K temperature range. Besides, well pronounced Shubnikov–de Haas oscillations of the magnetoresistance have been revealed at B > 8 T. The salt investigated is a new phase in the (BEDT-TTF)4AI[MIII(C2O4)3]G family of organic molecular conductors with paramagnetic anions and different guest solvent molecules G in the anion layer. Structural features of the new α-‘pseudo-κ’-crystals and other known phases of the family (β″, ‘pseudo-κ’ and α-β″) have been compared.

Structure and magnetotransport properties of the new quasi-two-dimensional molecular metal β″-(BEDT-TTF)4H3O[Fe(C2O4)3] · C6H4Cl2

The β″-(BEDT-TTF)4AI[MIII(C2O4)3] · G(AI=NH4+, H3O+, K+, Rb+; MIII=Fe, Cr; G = “guest” solvent molecule) family of layered molecular conductors with magnetic metal oxalate anions exhibits a pronounced dependence of the conducting properties on the type of neutral solvent molecules introduced into the complex anion layer. A new organic dichlorobenzene (C6H4Cl2)-containing conductor of this family, namely, β″-(BEDT-TTF)4H3O[Fe(C2O4)3] · C6H4Cl2, is synthesized. The structure of the synthesized single crystals studied by X-ray diffraction is characterized by the following parameters: a = 10.421(1) Å, b= 19.991(2) Å, c= 35.441(3) Å, β = 92.87(1)°, V= 7374(1) Å3, space groupC2/c, and Z = 4. In the temperature range 0.5&2-300 K, the conductivity of the crystals is metallic without changing into a superconducting state. The magnetotransport properties of the crystals are examined in magnetic fields up to 17 T at T = 0.5 K. In fields higher than 10 T, Shubnikov-de Haas oscillations are detected, and the Fourier spectrum of these oscillations contains two frequencies with maximum amplitudes of about 80 and 375 T. The experimental results are compared with the related data obtained for other phases of this family. The possible structural mechanisms of the effect of a guest solvent molecule on the transport properties of the β″-(BEDT-TTF)4AI[MIII(C2O4)3] · G crystals are analyzed.

Structural phase transition in the β′′-(BEDT-TTF)4H3O[Fe(C2O4)3]·G crystals (where G is a guest solvent molecule)

A structural phase transition from monoclinic C2/c to triclinic P symmetry has been found by X-ray diffraction in a number of single crystals of the known family of organic metals and superconductors β′′-(BEDT-TTF)4H3O[Fe(C2O4)3]·G where G stands for halogenated benzene derivatives and their mixtures with benzonitrile. The transition occurs upon lowering the temperature at 180–230 K. Comparison of the crystal and electronic structure of the monoclinic and triclinic phases reveals details of the structural transformations in the (PhCl + PhCN)-containing superconducting β′′-crystal, as an example. It is shown that the transition concerns mainly the anion layer and has a weak influence on the structure of the BEDT-TTF layer and, consequently, on the conducting properties of the single crystals.

Structure and magnetotransport properties of the new quasi-two-dimensional molecular metal {beta} Double-Prime -(BEDT-TTF){sub 4}H{sub 3}O[Fe(C{sub 2}O{sub 4}){sub 3}] {center_dot} C{sub 6}H{sub 4}Cl{sub 2}

The β″-(BEDT-TTF)4AI[MIII(C2O4)3] · G(AI=NH4+, H3O+, K+, Rb+; MIII=Fe, Cr; G = “guest” solvent molecule) family of layered molecular conductors with magnetic metal oxalate anions exhibits a pronounced dependence of the conducting properties on the type of neutral solvent molecules introduced into the complex anion layer. A new organic dichlorobenzene (C6H4Cl2)-containing conductor of this family, namely, β″-(BEDT-TTF)4H3O[Fe(C2O4)3] · C6H4Cl2, is synthesized. The structure of the synthesized single crystals studied by X-ray diffraction is characterized by the following parameters: a = 10.421(1) Å, b= 19.991(2) Å, c= 35.441(3) Å, β = 92.87(1)°, V= 7374(1) Å3, space groupC2/c, and Z = 4. In the temperature range 0.5&2-300 K, the conductivity of the crystals is metallic without changing into a superconducting state. The magnetotransport properties of the crystals are examined in magnetic fields up to 17 T at T = 0.5 K. In fields higher than 10 T, Shubnikov-de Haas oscillations are detected, and the Fourier spectrum of these oscillations contains two frequencies with maximum amplitudes of about 80 and 375 T. The experimental results are compared with the related data obtained for other phases of this family. The possible structural mechanisms of the effect of a guest solvent molecule on the transport properties of the β″-(BEDT-TTF)4AI[MIII(C2O4)3] · G crystals are analyzed.

The first molecular superconductor based on BEDT-TTF radical cation salt with paramagnetic tris(oxalato)ruthenate anion

The first molecular superconductor based on BEDT-TTF radical cation salt with the paramagnetic tris(oxalato)ruthenate anion, β′′-(BEDT-TTF)4Kx(H3O)1−x[RuIII(ox)3]C6H5Br (x ∼ 0.8), is synthesized and its crystal and electronic structure as well as transport and magnetotransport properties are studied. The single crystals have monoclinic C2/c symmetry and β′′-packing of the conducting BEDT-TTF layer. The samples were found to be superconductors with Tc = 2.8–6.3 K (depending on the sample). Well pronounced Shubnikov–de Haas oscillations are observed in the field range 7–17 T. The Fourier spectrum of the oscillations mainly consists of two frequencies, which correspond to the Fermi surface cross sections 6.3% and 0.85% of the Brillouin zone. The first one with good accuracy corresponds to the calculated cross section 7.5%, the origin of the second one is discussed. Semiconducting orthorhombic phase ‘pseudo-κ’-(BEDT-TTF)4Kx(H3O)1−x[RuIII(ox)3]C6H5CN is also synthesized and studied.

Spin Dynamics in (( BEDO - TTF ) 6 ( M (CN) 6 )(H 3 O,CH 3 CN) 2 ( M = Fe, Cr) Crystals

Layered single crystals of the (BEDO-TTF)6[M(CN)6](H3O,CH3CN)2 (M = Fe, Cr) compounds with alternating conducting layers of BEDO-TTF and [M(CN)6](H3O,CH3CN)2 are studied. The contributions to the magnetic susceptibility from charge carriers in BEDO-TTF layers and from the subsystem of localized magnetic moments of iron (or chromium) transition metal complexes are separated for both compounds under investigation. It is revealed that the crystals with [Fe(CN))6]3− anions at a temperature of ∼80 K and the crystals with [Cr(CN))6]3− anions at ∼30 K undergo magnetic transitions which are accompanied by drastic changes in the parameters of the EPR lines associated with the BEDO-TTF layers and the subsystem of localized spins of transition metal complexes. It is established that the presence of the BEDO-TTF layers in the structure affects the magnetic properties of iron and chromium hexacyanide complexes.