The synthesis and the crystallization of the compound 1Cl have been performed by the usual electrochemical oxidation method which is further described in [1]. As referred in the experimental section, data collection with the Rigaku R-AXIS SPIDER Image Plate diffractometer using graphite monochromated radiation have revealed, in addition to the Bragg peaks, diffusion streaks characteristic for a series of these type of compounds and especially for those containing linear metal halides [12,13]. The compound 1Cl crystallizes in the non centrosymetric space group, I-42d and is isostructural with the 1Br analogue [1]. The unit cell contains eight donor molecules 1, 4 ordered anions and approximately other 3.5 anions in disordered positions, with an arrangement characteristic of the τ-phase [9]. More specifically the features of the diffuse streaks and the spots on them resemble that of τ-(EDO-S,S-DMEDT-TTF)₂ (AuBr₂)₁ (AuBr₂)_y [10]. The ordered part of the structure consists of a 2D layer [parallel to the (001) plane] formed by the donor molecules with their long axes parallel and their average plane normal to each other, forming in this way the characteristic tetragonal lattice of τ-phase (Figure 2). There are intermolecular contacts (Table 1, Figure S1) of S∙∙∙S type in the range from 3.478 Å to 3.523 Å and one S∙∙∙N type of 3.311 Å at RT, which are smaller than or close to the sum of the Van der Waals radii (3.70 Å and 3.35 Å respectively). The values of S∙∙∙S contacts at 260 K are smaller (3.463 Å–3.512 Å) and that of S∙∙∙N contact is slightly higher (3.33 Å). The ordered anions reside in the cavities of the tetragonal donor lattice (Figure 2). Calculations of difference Fourier maps during the structure solution process give electron density with appreciable values to be spread along the a or b crystallographic axis and in between the 2D donor layers, which is possible to be modeled with disordered AuCl₂ anions, “smeared” all along the b axis as in the case of the anions in other τ-phases [11]. As is mentioned in [10], these disordered chains of AuCl₂ anions are the origin of the intense diffusion streaks which are recorded on the diffraction images (Figure 3).

The spots observed on the diffusion streaks which are normal to a* or b* reciprocal cell axes respectively, (Figure 3a) have been interpreted [10] as two independent reciprocal lattices which are related by a 2 fold axis normal to the plane of the image of Figure 3a. As it is easily recognized by the arrangement of spots in white circles (Figure 3a), the extinction rule is that of a face-centered lattice. The two lattices of the anions have dimensions (8.1/7.33) a (=1/a’*) × 2b (=1/b’*) b or 2a × (8.1/7.33) b, and if there are no defects of anions in both lattices the y value of the disordered anions per formula unit is ≈0.9 (=7.33/8.1). Figure 3b shows the diffusion streaks as they have been recorded when the X-ray beams falls on the crystal parallel to the a* axis and the arrangement of the spots on them supports the interpretation of a face centered lattice.

The images show the same features down to 110 K |(Figure 4), where satellite peaks start to appear, which are clearly seen at the temperature of 100 K. These satellite peaks are clearly between the Bragg peaks which correspond to the ordered part of the structure (usually the “conducting” layer) and not on the diffusion lines which come from disordered AuCl₂ anions. This observation indicates that the probable origin of these satellites is a modulation of the ordered donor-anion part of the structure. The modulation wavelength is estimated to be approximately 5 × a* (or 5 × b*). Satellites between Bragg peaks have also been observed in an analogous system [10] and have been interpreted as a transition that couples the anionic layer with the conducting one. Our current measurements are not enough to support a more rigorous interpretation and farther experimental study is needed.

The results of the electrical measurements reported have been obtained from four crystals (No. 1–4) with the current parallel to a- or to c-axis. It was found that the room temperature values are σ_a = 10.89, σ_a = 13.87, σ_c = 0.023 and σ_c = 0.01 S cm⁻¹, for the crystals 1,2,3 and 4, respectively. This means that the anisotropy σ_a/σ_c is ca. 400–1400 at room temperature. Figure 5 shows the temperature dependence of resistivity of three samples. In all cases the crystals exhibit an increase of resistivity with the decreasing of temperature, i.e., a semiconducting behavior, with activation energy of ca 18 meV. In all cases the gradients of ρ change at ca 17 K. Also, it was found that under a magnetic field of 5 T the resistivity decreases as the temperature decreases.

Figure 6 shows the magnetic field dependence of ρ_a and ρ_c at 10 K. One can see that the magnetoresistance becomes negative by increasing the magnetic field. It is expected to become more negative at lower temperatures as in the cases of similar metallic crystals of τ-phase [3]. The magnetically hysteretic electronic properties at low temperature and a sudden appearance of negative magnetoresistance seem to suggest a close relation of these phenomena to electron correlation and magnetism (See [3,4,6,7]).