Clinical sparganosis in Hong Kong Ng, T H; Wong, W T; Fung, C F ...
Journal of the Royal Society of Health,
08/1989, Letnik:
109, Številka:
4
Journal Article
The first ambient pressure S-based organic superconductor (BEDT-TTF)
2
I
3
, abbreviated (ET)
2
I
3
, exhibits a novel modulated structure below ∼ 200 K. At 125 K the ET molecules and I
3
−
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)
2
IBr
2
are discussed with transition temperatures of 2.7 K, and in one sample, 4.2 K.
The nature of the 135 K metal-insulator (MI) transition in α-(BEDT-TTF)
2
l3, abbreviated α-(ET)
2
l
3
, 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)
2
l
3
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 Ψ
1
); and Ψ
1
are the HOMO's of ET molecules i and j, respectively. The band electronic structures calculated by using single-zeta Slater type orbitals show that α-(ET)
2
l
3
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 of α-(ET)
2
I
3
above T
MI
originate from this extremely small band gap. The band electronic structures calculated by using double-zeta Slater type orbitals also show that α-(ET)
2
l
3
is a semiconductor at both temperatures, but they do not provide a simple explanation for the MI transition. All valence and conduction bands calculated by using either single- or double-zeta basis set are very narrow (each less than 100 meV) and are not separated by large energy gaps. Consequently, the one-electron viewpoint may not he adequate in describing the MI transition of α-(ET)
2
I
3
, and the possibility of electron localization in the ET stacks as a result of either electron-electron (Coulomb) repulsion or mixed valence may then be considered.
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
intras
tack 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).
In the isostructural series of β-(BEDT-TTF)
2
X organic metals, abbreviated β-(ET)
2
X, where X is a trihalide anion, I
3
-, I
2
Br- or IBr
2
-, and where ET is bis-(ethylenedithio) ...tetrathiafulvalene (C
10
S
8
H
8
), the salt containing the disordered asymmetric I
2
Br- anion has atypical structural and electronic properties compared to those containing the symmetrical and ordered I-
3
and IBr
2
- anions.
