Peter C. W. Leung

New cation-anion interaction motifs, electronic band structure, and electrical behavior in .beta.-(ET)2X salts [ET = bis(ethylenedithio)tetrathiafulvalene; X = dichloroiodide, chlorobromoiodide]

Etude structurale des composes du titre. Determination des temperatures de transition metal-isolant

Structural Characterization and Band Electronic Structure of α-(BEDT-TTF)2I3 below its 135 K Phase Transition

Abstract The nature of the 135 K metal-insulator (MI) transition in α-(BEDT-TTF)2l3, abbreviated α-(ET)2l3, was examined by determining the crystal structures above (298 K) and below (120 K) the phase transition and also by calculating the band electronic structures at both temperatures. This study demonstrates that both the crystal and band electronic structures of α-(ET)2l3 change only slightly upon passing through the MI transition. Within each sheetlike network of ET radical cations, the magnitudes of interactions between adjacent pairs of ET molecules (i-j) above and below the MI transition temperature (T MI) were evaluated by calculating their interaction energies, β ij = ⟨Ψ, H off|Ψ⟩, where Ψ1); and Ψ1 are the HOMOs of ET molecules i and j, respectively. The band electronic structures calculated by using single-zeta Slater type orbitals show that α-(ET)2l3 is a semiconductor with band gaps of 13 meV (298 K) and 35 meV (120 K). Thus, within the one-electron model, the apparently metallic properties...

The role of anions on the crystal structures and electrical properties of the organic metals and superconductors, (BEDT-TTF)2X (X = trihalide anions)

Abstract A systematic study of the crystal structures of (BEDT-TTF) 2 X salts [BEDT-TTF is bis-(ethylenedithio) tetrathiafulvalene, C 10 S 8 H 8 , and X is a linear trihalide anion] reveal that the different packing motifs for the BEDT-TTF molecules are directly related to the electrical properties of these materials. The packing of the organic molecules is influenced by the cation-anion interactions of the -CH 2 …X − type and is observed to change systematically with anion size.

Correlations of Anion Size and Symmetry with the Structure and Electronic Properties of β-(BEDTTTF)2X Conducting Salts with Trihalide Anions X = 13-, 12Br-, 1Br2-

Abstract In the isostructural series of β-(BEDT-TTF)2X organic metals, abbreviated β-(ET)2X, where X is a trihalide anion, I3-, I2Br- or IBr2-, and where ET is bis-(ethylenedithio) tetrathiafulvalene (C10S8H8), the salt containing the disordered asymmetric I2Br- anion has atypical structural and electronic properties compared to those containing the symmetrical and ordered I-3 and IBr2- anions.

A Systematic Study Of The Relationship Between Crystal Structurem, ESR Characteristics And Electerical Properties In Bedt-TTF Synthetic Metals

Abstract The crystal packing motifs exhibited by the BEDT-TTF (or “ET”) donor molecules in the various ET:X charge-transfer salts are summarized. In spite of the variable stoichiometries and crystall symmetries found for these synthetic metals, the number of different cyrstal packing motifs is limited — two interstack (L- and Z-modes) and three intrastack modes (a-, ab- and c- modes). Of these superconductivity has been observed only i the 2:1 (BEDT-TTF:X) salts which adopt a combination of the L- and a- modes of inter-and intrastack paking, respectibvely, ESR linewidth measurement provide a maans of separating different crystal phases within a given donoracceptor system and are especially useful in differentiating the non-superconducting a-(ET) 2× phases, x=13- and IBrz-, form the abient pressure superconduction g β-(ET)2x phases, x=I3 - IBrz- (TC;s = 1. 4 K and 2. 7 K. respectibely).

Hydrogen bond formation and anion ordering in superconducting (TMTSF)2ClO4 and (TMTSF)2AsF6

Abstract An examination of the low temperature (125K) X-ray diffraction determined structure of (TMTSF)2C104, the only ambient pressure organic superconductor known, reveals a previously unseen nearest-neighbor hydrogen bonding environment surrounding the anion resulting in short oxygen atom to methyl-hydrogen atom interactions that are likely responsible for the ordering of the C104- anion associated with a transition observed at 24K.1 Superconductivity is observed in (TMTSF)2ClO4 only when the anions adopt an ordered (relaxed1) configuration. In contrast to the previously reported single short Se…0 distance, the 0…H-C contacts are both short and numerous. The observed librational motion of the perchlorate anion provides further support for the importance of the 0…H interactions in the pinning of the anion. Additional discussion regarding the influence of F…H hydrogen bonds on the AsF6- anion disorder in (TMTSF)2AsF6, a superconductor under applied pressure only, is also presented.

Novel Low Temperature Modulated Structure of the Ambient Pressure Superconductor (BEDT-TTF)2I3 and a Design Strategy for New Superconducting Polyhalide Phases

Abstract The first ambient pressure S-based organic superconductor (BEDT-TTF)2I3, abbreviated (ET)2I3, exhibits a novel modulated structure below ∼ 200 K. At 125 K the ET molecules and I3 − anions exhibit large displacements 0.124 (3) A and 0.281 (1) A, respectively, from their positions in the “average structure.” New ambient pressure superconducting polyhalide derivatives such as (ET)2IBr2 are discussed with transition temperatures of 2.7 K, and in one sample, 4.2 K.

Neutron diffraction study of the β1-β2 transformation in Ta2D☆

Abstract The β1-β2 phase transformation in Ta2D was studied by neutron diffraction using the pulsed source at the Tohoku Linac and the Intense Pulsed Neutron Source, Argonne National Laboratory. It was found that the hh0 reflections almost disappear in the β2 phase, whereas the 001 reflections are the same as those in the β1 phase. The diffraction data show that the β2 phase has an orthorhombic unit cell which is identical with that of the β1 phase and contains four tantalum and two deuterium atoms in a volume which is approximately 2 1 2 × 1 × 2 1 2 that of the b.c.c. host lattice. The deuterium atoms fractionally occupy the tetrahedral sites in alternate (002)orth planes.

Exotic organic superconductors based on BEDT-TTF and the prospects of raising Tc'S

Invited paper Ambient-pressure superconductivity has now been discovered in five systems based on BEDT-TTF:X, X - = polyatomic, monovalent anion. The highest T c reported is 5 K, in (BEDT-TTF) 2 AuI 2 , and it appears that much higher T c s are possible if disorder (molecular and anionic), and structural modulation, can be avoided in new derivatives containing monovalent triatomic anions. A modulated structure has been observed in (BEDT-TTF) 2 1, using neutron and X-ray scattering, but does not appear to form in (BEDT-TFF) 2 X, X = I 2 Br - , AuI - 2 and IBr2 down to T = 10-20 K.

Synthesis, Structure and Electrical Conductivity of (BEDT-TTF)X(BrO4)Y Organic Metals

Abstract The electrochemical oxidation of BEDT-TTF (bis(ethylenedithio)-tetrathiafulvalene) in 1,1,2-trichloroethane solution in the presence of (n-Bu4N)Br04 as supporting electrolyte produces three distinct morphologies: needles, thick plates, and thin plates. These crystal habits have been identified with different crystallographic phases: needles as (BEDT-TTF) 2Br04, thick plates as (BEDT-TTF)2(Br04)(TCE)0.5, and thin plates as (BEDT-TTF)3(Br04)2. The structural characterization and conductivity for these materials is presented.