Anion Chain Structure Controlled Behavior of Phase Transition in

ABSTRACT: The correlation between the structure of anionic chains in crystals of quasi-two-dimensional organic metal (EDT-. TTF)4[Hg3I8]1-x and the ty...
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CRYSTAL GROWTH & DESIGN

Anion Chain Structure Controlled Behavior of Phase Transition in Quasi-Two-Dimensional Organic Metal (EDT-TTF)4[Hg3I8]1-x

2007 VOL. 7, NO. 12 2768–2773

Elena I. Zhilyaeva,*,† Andrey Y. Kovalevsky,‡ Rustem B. Lyubovskii,† Svetlana A. Torunova,† George A. Mousdis,§ George C. Papavassiliou,§ and Rimma N. Lyubovskaya† Institute of Problems of Chemical Physics RAS, ChernogoloVka, Moscow region 142432 Russia, Department of Chemistry, State UniVersity of New York at Buffalo, Buffalo, New York 14214, and Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, Athens, 11635, Greece ReceiVed April 6, 2007

ABSTRACT: The correlation between the structure of anionic chains in crystals of quasi-two-dimensional organic metal (EDTTTF)4[Hg3I8]1-x and the type of phase transition (metal-superconductor or metal–insulator) was found. Crystal structures of four single crystals of ethylenedithiotetrathiafulvalene (EDT-TTF) octaiodomercurate, which differed in the resistivity behavior, were determined at 90 K. The crystals 1–3 underwent a superconducting transition: Crystal 1 became a superconductor at ambient pressure, and crystals 2 and 3 became superconductors at 0.3 kbar. Crystal 4 underwent a metal–insulator transition. It was shown that 1–3 have a nonstoichiometric composition, (EDT-TTF)4[Hg3I8]1-x, while 4 has a stoichiometric composition (EDT-TTF)4Hg3I8. In all of the crystals, the structures are formed by conducting β-like layers of EDT-TTF and insulating layers built of {[Hg3I8]2-}∞ chains, in which the [Hg2I6]2- anions alternate with the HgI2 molecules. In 4, the chains are fully ordered. In 1–3, the [Hg3I8]2- units are disordered in two positions related by the approximate inversion center, the total occupancy of both positions being less than 1.0. It implies that in superconducting crystals iodomercurate chains involve defect sections with the inverse alternation of the HgI2 molecules and the [Hg2I6]2- anions. In the anion chains of 1, the defects occur more often than in 2 and 3. Introduction The stoichiometry of organic superconductors based on radical cation salts is known to define the charge of the donor molecules of a conducting organic layer and the filling of the conduction band and, consequently, electron and transport properties of compounds. Most organic superconductors based on radical cation salts have a stoichiometric composition: donor2A, donor4A, donor3A, and donor3A2,1,2 where A is an anion. However, there are superconductors of a nonintegral composition, which are called nonstoichiometric superconductors.3 The first examples of nonstoichiometric superconductors were found among radical cation salts of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF): (BEDT-TTF)4Hg2.78Cl8 and (BEDT-TTF)4Hg2.89Br84,5 with the donor molecule charge being equal to +0.61 and +0.555, respectively. The salts have rather unusual physical properties; in particular, the Tc increasing with pressure, positive curvature of upper critical fields, and the exceeding of Clogston paramagnetic limit.6,7 Recently,8,9 several more nonstoichiometric superconductors have been synthesized from two unsymmetrical MDT-TSF and MDT-ST donor molecules, which have the same carbon skeleton but differ in heteroatoms. The salts involve linear anions, and their compositions are (MDT-TSF)[AuI2]0.436, (MDT-TSF)Ax, A ) I3, I3-δBrδ, I3-δClδ,3,8 and (MDT-ST)[I3]0.4179 with the donor charge being smaller than +0.5 per molecule. The structural feature of the nonstoichiometric superconductors is the presence of two incommensurate sublattices with different periodicities along one of crystallographic axes. For (BEDT-TTF)4Hg2.78Cl8 and (BEDT-TTF)4Hg2.89Br8, the BEDT-TTF-donor molecules and the halogen atoms are associated with one lattice, whereas * To whom correspondence should be addressed. Fax: (+7)4965155420. E-mail: [email protected]. † Institute of Problems of Chemical Physics RAS. ‡ State University of New York at Buffalo. § National Hellenic Research Foundation.

Chart 1

the mercury atoms possess the periodicity of the other sublattice.6,10,11 For MDT-TSF and MDT-ST salts with linear anions, the anions have the incommensurate periodicity with the donor lattice.3,8,9 Recently, we have prepared a new organic superconductor with an unsymmetrical ethylenedithiotetrathiafulvalene donor (EDT-TTF, Chart 1) and a mercury-containing anion [Hg3I8]2-, which undergoes a transition to a superconducting state at Tc ) 8.1 K at ambient pressure.12,13 Electron probe microanalysis (EPMA) showed that this compound has a nonstoichiometric composition. The resistivity behavior of the EDT-TTF octaiodomercurate crystals of the same habitus was strongly different at low temperatures. Three types of crystals were found as follows: the crystals with superconducting temperature Tc ) 8.1 K at ambient pressure, those transiting to a superconducting state under applied pressure around 0.3 kbar at 5 < Tc < 8 K, and the crystals undergoing a metal–insulator (M-I) transition below 35 K. The application of 0.3 kbar pressure to the crystal with a M–I transition only slightly decreased the M-I transition temperature.13 Undoubtedly, the study of the structure of each phase was of great interest. It should be noted that the selenium analogue, EDTDTDSF, formed a stoichiometric organic metal (EDTDTDSF)4Hg3I8. No indications of superconductivity were observed in it in resistivity measurements down to 1.5 K.14 In this work, we studied the crystal and molecular structures of (EDT-TTF)4[Hg3I8]1-x single crystals, which undergo a transition to a superconducting state at ambient pressure and at 0.3 kbar, and that of the (EDT-TTF)4Hg3I8 crystal with a transition to insulator at 35 K. Exact compositions of each phase and structural features were established. The influence of the

10.1021/cg070339y CCC: $37.00  2007 American Chemical Society Published on Web 11/20/2007

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Table 1. Crystallographic Data for (EDT-TTF)4[Hg3I8]1-x (x ) 0/0.03) crystal

1

2

3

4

type of phase transition formula Mr (g mol-1) crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z R [I > 2σ(I)]

metal- superconductor, Tc ) 8.1 K C32H24S24 (I8Hg3)0.973 2751.59 triclinic P1j 12.7505(11) 14.0737(12) 18.1228(15) 105.832(2) 92.573(2) 97.319(3) 3092.4(5) 2 0.059

metal- superconductor, Tc ) 7 K/0.3 kbar C32H24S24 (I8Hg3)0.981 2764.85 triclinic P1j 12.7610(10) 14.0790(12) 18.1327(15) 105.866(2) 92.578(2) 97.347(2) 3097.0(4) 2 0.053

metal- superconductor, Tc ) 7.5 K/0.3 kbar C32H24S24 (I8Hg3)0.981 2764.85 triclinic P1j 12.7608(8) 14.0795(8) 18.1161(11) 105.826(2) 92.567(2) 97.383(2) 3094.6(3) 2 0.046

metal-insulator, TM-I ) 30 K C32H24S24 I8Hg3 2794.92 triclinic P1j 12.7655(5) 14.0808(5) 18.1156(6) 105.791(1) 92.559(1) 97.389(1) 3096.5(2) 2 0.036

structure of anionic chains in crystals of quasi-two-dimensional organic metal (EDT-TTF)4[Hg3I8]1-x on type of transition (metal-superconductor or M–I) was found. Experimental Section Synthesis. The donor EDT-TTF was synthesized according to the procedure described in ref 15. The electrolyte [n-Bu4N]HgI3 was prepared as described in ref 16. Single crystals of EDT-TTF iodomercurates were grown by electrocrystallization on a 0.5 mm platinum wire electrode in a U-shaped cell at 0.5 µA constant current and 16 °C. In a typical experiment, 82.3 mg (0.1 mmol) of [n-Bu4N]HgI3 and 22.8 mg (0.05 mmol) of HgI2 were dissolved in 6 mL of chlorobenzene (PhCl) and 0.6 mL of ethanol and then shared equally between two arms of a two-compartment cell. EDT-TTF (8.8 mg, 3 mmol) dissolved in 2 mL of hot PhCl was placed in the anodic chamber, and 1.1 mL of PhCl was added in the cathodic one. The crystals appeared as hexagon shiny plates and were collected after 2 weeks. The crystals were certified by resistivity measurements. Usually, the crystals with different types of transitions could be found in the same batch, but the four crystals described below were picked from different batches. Electrical Resistivity Measurements. Direct current (dc) resistivity of single crystals was measured down to 4 K by a standard four-probe technique with current flow parallel to the EDT-TTF layers. Contacts were glued to the crystals with a graphite paste using 0.01–0.02 mm diameter platinum wire. To achieve hydrostatic pressure of ∼0.3 kbar upon cooling, a crystal was immersed into a drop of Apieson grease. X-Ray Crystallography. Crystal structures were determined for four single crystals of EDT-TTF octaiodomercurate (numbers 1–4). X-ray diffraction data were collected at 90(1) K using a Bruker SMART1000 CCD diffractometer installed at a rotating anode source (Mo KR radiation, λ ) 0.71073 Å) and equipped with an Oxford Cryosystems nitrogen gas-flow apparatus. The data were collected by the rotation method with 0.3° frame-width (ω scan) and 10 s exposure time per frame. Four sets of data (600 frames in each set) were collected, nominally covering half of the reciprocal space. The data were integrated, scaled, sorted, and averaged using the SMART software package.17 Absorption correction was done by employing program SADABS.17 The structures were solved by the direct methods using SHELXTL NT, Version 5.10.18 The structure was refined by full-matrix least-squares against F2. Nonhydrogen atoms were refined in the anisotropic approximation. Positions of hydrogen atoms were found by the difference electron density Fourier synthesis. Subsequently, the positions of H-atoms were refined by the “riding” model with Uiso ) 1.2Ueq of the connected nonhydrogen atom. The main crystallographic data for four crystals of (EDT-TTF)4(Hg3I8)(1-x) are given in Table 1. CCDC 628846–628849 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif or by e-mailing [email protected].

octaiodomercurate crystals, we performed X-ray structural analysis of four single crystals (numbers 1–4, taken from different batches), which demonstrated the following peculiarities of electric resistivity with the temperature decrease (Figure 1). Single crystal 1 underwent a superconducting transition at 8.1 K under ambient pressure (Figure 1a). In addition to zero electrical resistivity, the Meissner effect was found,13 which unambiguously indicated superconductivity. Crystal 2 behaved as a metal with a transition to insulator below 12 K at ambient pressure, but it became a superconductor at ∼0.3 kbar and 7.1 K (Figure 1b). For crystal 3, the resistance sharply grew upon cooling below 22 K at ambient pressure, but at a hydrostatic pressure of ∼0.3 kbar, a slight increase in resistance was observed below 16 K followed by a sharp superconducting transition near 7.5 K (Figure 1c). Finally, crystal 4 behaved as a metal with a transition to insulator below 35 K at ambient pressure. The application of 0.3 kbar pressure slightly decreased TM-I (Figure 1d) for 4. Crystal Structures of 1–4. Crystal structures were investigated on the very same crystals used for transport measurements. Crystal structures were determined at 90(1) K. Table 1 summarizes the compositions found, the metal-superconductor or M–I transition temperatures, Tc or TM-I, the space groups, and the lattice parameters for 1–4. It is evident that all of the crystals studied have very close lattice parameters. Crystals 1–4 are composed of negatively charged [Hg2I6]2- ions, neutral HgI2 molecules, and positively charged EDT-TTF radical cations. All

Results Temperature Variable Resistivity. To understand the differences in the behavior of resistance of different EDT-TTF

Figure 1. Resistivity vs temperature for EDT-TTF octaiodomercurates: a, single crystal 1; b, 2; c, 3; and d, 4.

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Figure 2. View of crystal packing in 4 along the a-axis.

Figure 3. Structure of stacks in crystals 1–4. Short S · · · S contacts are denoted by dashed lines. Left: sequence and labeling of radical cations in stacks. The box shows the smallest repeating motif. Right: interstack interactions in 1–4.

of the crystals have a layered structure, organic and inorganic layers alternating along the c-axis (Figure 2). The organic layers of 1–4 consist of stacks of EDT-TTF radical cations. Molecular Structure of the Radical Cations. The unsymmetrical donor molecules comprising two five-membered and one six-membered heterocyclic ring systems are noticeably distorted from planar geometry. The six-membered rings of the EDT-TTF radical cations adopt half-chair conformations, and the two sp3 carbon atoms are displaced from the plane of the other four atoms of the cycle by about 0.3-0.5 Å to the opposite directions in each molecule for all four structures. In addition, the π-systems of the cations are not planar but bent through the sulfur atoms of the five-membered rings fused with sixmembered rings. The angles between the distant five-membered ring and S-CdC-S moiety of the tetrathiabicycle’s fivemembered ring are very similar for the same molecules in all of the structures. The values are 7–8, ∼8.5, ∼9.0, and ∼10.0° for the molecules containing C1-C8 atoms (molecule A), C9-C16 (molecule B), C17-C24 (molecule C), and C25-C32 (molecule D), respectively. The lengths of the central CdC bonds in molecules A-D are in the 1.36(1)-1.39(1) Å range, the values being within three standard deviations, and they can therefore be considered to be equal for all of the compounds. Structure of Organic Layers. In the organic layers of 1–4, the EDT-TTF radical cations form infinite stacks of the same type along the 2b + a direction (Figure 3). The stacks are parallel to each other and are united into sheets parallel to the crystallographic ab plane. In each stack, the molecules are arranged head-to-tail in the following polymeric sequence, (ABBADCCD)n. In all crystals, the pairs of the EDT-TTF radical cations AB, BA, DC, and CD with short S · · · S contacts inside (3.63/3.69 Å) and overlapping arrangement of a bond-

Zhilyaeva et al.

Figure 4. Structure of the anion layer in crystal 4.

overpartially shifted bond can be singled out in the stack (Figure 3, left). The overlapping modes of the EDT-TTF radical cations between the pairs are ring-overbond. No S · · · S contacts shorter than the sulfur double van-der-Waals radius of 3.70 Å19 were found between the pairs. A stack in each compound contains an equal number of shortened S · · · S contacts. The interactions are between the sulfur atoms of the TTF moieties only; the sulfur atoms of the six-membered rings are not involved in these contacts. In addition to the shortened intrastack contacts, numerous shortened interstack interactions were found (Figure 3, right). Because of very short side-by-side S · · · S contacts (3.37 and 3.43 Å, which are shorter than those found in the stacks), the EDT-TTF radical cations from the neighboring stacks form infinite chains (“step chains”) parallel to the a-axis. The radical cations are located “head-to-head” in these chains. These step chains are seen well in Figures 2 and 3. It should be emphasized that shortened side-by-side S · · · S contacts are formed by both S atoms from the five-membered rings and sulfur atoms from the six-membered rings. The number and values of S · · · S short interstack contacts found for 1–4 are the same. The overall donor cation packing shows a β-like type donor arrangement, which is often observed in conducting BEDT-TTF salts. Structure of Inorganic Layers. The structure of the anion layer in 4 is shown in Figure 4. The [Hg2I6]2- anions alternate with neutral HgI2 molecules along the a-axis of the crystal. The Hg1 and Hg2 atoms in the [Hg2I6]2- dimer have a distorted tetrahedral configuration of the Hg-I bonds, with the (bridging I)-Hg bond lengths being ∼0.13/0.20 Å longer than those of the terminal I atoms. The HgI2 molecule is almost linear, the bond angle I7-Hg3-I8 being 175.34(2)°. The Hg atom of the HgI2 molecule makes 3.533, 3.742, 3.806, and 3.867 Å contacts with the iodine atoms of the two neighboring [Hg2I6]2- anions, imitating octahedral arrangement of ligands around the central mercury atom. In fact, the HgI2 molecules and the [Hg2I6]2anions form infinite chains {[Hg3I8]2-}∞ due to the presence of these contacts. Besides, the iodine atoms of each chain form a number of interchain shortened van der Waals contacts with the other iodine atoms of 3.964–4.167 Å. The structures of anion chains and the anion layer of 4 are similar to those of Se analogue 5.14 In contrast to 4, in crystals 1–3, the [Hg3I8]2- units are disordered over two positions (as depicted in Figure 5) with distinctly different occupancies. The two alternate positions are related by the approximate inversion center located between and equidistant from Hg2 and Hg1A atoms. For structures of 1-3, the occupancies of the two positions were refined independently. The resulting values are presented in Table 2, the total occupancy of both positions being less than 1.0 for each crystal, while the population of [Hg3I8]2- units in 4 was refined to be

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Crystal Growth & Design, Vol. 7, No. 12, 2007 2771

Figure 5. [Hg3I8]1–0.03 units in 1. Solid and empty lines depict the [Hg3I8]2- units with high and low occupancy, respectively. Table 2. Occupancies of the [Hg3I8]2- Positions in 1–5 crystal

1

2

3

4

5a

major position minor position total occupancy vacant positions, x

0.910(2) 0.063(2) 0.973(3) 0.027(3)

0.922(2) 0.059(2) 0.981(3) 0.019(3)

0.945(2) 0.036(2) 0.981(3) 0.019(3)

1.00 0 1.00 0

1.00 0 1.00 0

a

Ref 14.

Table 3. Angle of the HgI2 Molecule (Deg) in the “Major” and “Minor” Parts of [Hg3I8]2- Units in 1–5 crystal

1

2

3

4

Figure 6. Shortened I · · · S contacts between inorganic and organic layers in crystal 3. Similar interactions were found in 1 and 2.

5a

I7-Hg3-I8 in 175.21(4) 175.16(3) 175.13(2) 175.34(2) 175.23 “major” part I7A-Hg3A-I8A in 156.9(6) 155.1(7) 148.0(1) “minor” part a

Ref 14.

1.0. These data imply that there are empty spaces populated neither by HgI2 molecules nor by [Hg2I6]2- anions in the inorganic layers of the 1–3 crystals. The shares of empty spaces in the {[Hg3I8]2-}∞ chains (x) are listed in Table 2 as well. Therefore, the composition of the ambient pressure superconducting crystal 1 could be presented by the formula (EDTTTF)4[Hg3I8]0.973(3), those of the 0.3 kbar pressure superconductors 2 and 3 could be presented as (EDT-TTF)4[Hg3I8]0.981(3), and the composition of 4 fully conforms to (EDT-TTF)4Hg3I8. It should be noted that in the Se analogue (5) the anion is ordered as in 4, and the occupancy of the [Hg3I8]2- positions is 1.0, similarly to that in 4.14 The structure of the [Hg3I8]2- units with high occupancy of positions in crystals 1–3 is similar to those in 4 and 5 with almost linear HgI2 molecules (the I7-Hg3-I8 angles are close to ∼175°; see Table 3), which form an octahedron through contacts with the iodine atoms of the two neighboring [Hg2I6]2anions. However, in “minor” part of the [Hg3I8]2- units of 1–3, the HgI2 molecules are not linear and the I7A-Hg3A-I8A bond angles are substantially smaller than 175° (Table 3). In crystal 3, the HgI2 molecule is most bent. Donor · · · Anion Interactions. The inorganic and organic layers have close interactions with each other. I · · · S van der Waals contacts are formed between the iodine atoms and the sulfur atoms of the six-membered rings. The distances range from 3.43 to 4.02 Å for the “major” [Hg3I8]2- part of 1–3 and 4 and from 3.07 to 3.9 Å for the “minor” iodomercurate part in 1–3. Figure 6 illustrates the interactions for the “minor” iodomercurate part of 3. The remarkable difference between the interactions of the “major” and “minor” [Hg3I8]2- units with sulfur atoms is that the latter forms two very short contacts of 3.07 (I1A · · · S) and 3.30 Å (I2A · · · S) not observed for the “major” part iodine. Discussion Peculiarities of Structure of Anion Layers in Superconducting 1–3 Crystals. Structure of Single Anion Chain. Single crystal X-ray diffraction analysis revealed that

Figure 7. Structure of a single iodomercurate chain in 1. The positions of the [Hg3I8]2- units of both high (solid lines) and low occupancy (empty lines) are shown.

in all EDT-TTF octaiodomercurate crystals with a superconducting transition, the “major” and the “minor” positions of the [Hg3I8]2- unit are related by an approximate inversion center located between Hg2 and Hg1A. This implies that in contrast to continuous {[Hg3I8]2-}∞ chains in 4, some discontinuity should occur in the iodomercurate chain in 1–3 when the “major” [Hg3I8]2- unit changes for the “minor” [Hg3I8]2- unit. Empty space would appear that results in incomplete occupancy of the [Hg3I8]2- positions. Therefore, a single chain would consist of separate periodic sections (hereinafter subchains) composed of either the “major” [Hg3I8]2- units or the “minor” [Hg3I8]2- units. The terminal HgI2 molecule of every subchain would form contacts with two iodine atoms from only one neighboring [Hg2I6]2- anion to imitate a tetrahedral arrangement of ligands. As a result, deviation from linearity of HgI2 occurs. In fact, these speculations are justified by less than 1.0 occupancy of the [Hg3I8]2- positions and the essential deviation of the HgI2 molecule from linearity found in the X-ray analysis of 1–3. To estimate a number of the “minor” subchaines in single {[Hg3I8]2-}∞ chain, we made an attempt to establish its dependency on a number of vacant positions of [Hg3I8]2- (x) in a single {[Hg3I8]2-}∞ chain. We suggested that each “minor” subchain involves an integer of [Hg3I8]2- units. The examination of empty spaces in disruptions of a single chain (see ref 20 and Figure 7) allowed us to conclude that one vacant volume of the [Hg3I8]2- unit exists per each “minor” subchain. Therefore, the number of “minor” subchains is equal to x, the number of vacant volumes of [Hg3I8]2-: 2.7(3)% for 1 and 1.9(3)% for 2 and 3 (Table 2). The estimation of an average number of [Hg3I8]2- units (z) in the “minor” subchains affords 6.3/2.7 ) 2.3(2) for 1, 5.9/1.9 ) 3.1(3) for 2, and 3.6/1.9 ) 1.9(2) for 3. The number of “major” subchains in each single chain should be equal to that of “minor” ones. The estimated averaged number of the [Hg3I8]2- units in “major”

2772 Crystal Growth & Design, Vol. 7, No. 12, 2007 Chart 2. Schematic sketch of the “minor” subchaines in 3, 2, and 1 crystals (formulas I, II, and III, respectively)

subchains is 91/2.7 ) 34(4) for 1, 92.2/1.9 ) 49(5) for 2, and 94.5/1.9 ) 50(5) for 3. Thus, “minor” subchains are short chain sections, and the “major” subchains are long chain sections. Though the anion chains of crystals 2 and 3 contain an equal number of subchains, there is a significant difference in the averaged number of the [Hg3I8]2- units in their “minor” subchains. It should be noted that for crystal 1 the averaged length of the “minor” subchaines is not an integer. Consequently, for 1, the dependency of a number of the “minor” subchaines in single anion chain on x should be different from those for 2 and 3. Peculiarities of the structure of the “minor” subchains in 1 are considered below. Peculiarities of the Structure of the “Minor” Subchains in 1 as Compared with 2 and 3. The value of the bend angle of the HgI2 molecule is important for understanding the peculiarities of the structure of the “minor” subchains in 1–3. It is obvious that the X-ray experiment provides the value of the bond angle in the HgI2 molecule, which is an averaged value of the bond angles of all HgI2 molecules of the “minor” subchain, namely, a strongly bent terminal molecule and all linear molecules from the subchain. At the length of the “minor” subchain equal to ∼two [Hg3I8]2- units (see Chart 2, structure I), the “minor” subchain contains one linear HgI2 and one terminal HgI2 with configuration of Hg atom bonds close to a tetrahedral one (an ideal tetrahedral angle is 109.5°). A rough estimation of the averaged angle as an arithmetic average of two angles gives the value equal to ∼145° [(109.5° + 180°)/2] that is close to the angle of 148° found for crystal 3 (Table 3). At the length of the “minor” subchain equal to ∼three [Hg3I8]2- units, the “minor” subchain would contain two linear and one terminal HgI2 molecule (Chart 2, structure II). An arithmetic average of three angles gives the value equal to 156.5° that is close to the angle of 155.1° found for crystal 2. Of importance is the fact that the bond angle found in 1 is close to 156° rather than to 145°. It implies that two linear HgI2 molecules rather than one fit per one bent HgI2 molecule in the crystal of the ambient pressure superconductor. This is possible if “minor” subchains of two types are present in the anion layer: (i) subchains comprising two [Hg3I8]2- units and (ii) subchains comprising {2[Hg3I8]2- units + HgI2 molecule}, two terminal HgI2 molecules being bent in opposite directions (see Chart 2, structure III). Seemingly, the ratio between i and ii is approximately 3:1. The estimation of the averaged length of the “minor” and “major” subchains made by us for this model of the anion chain (see ref 21) gives 2.0(2) [Hg3I8]2- units for the “minor” subchains and 30(4) ones for the “major” subchains. Role of Defects and Noninteger Donor Charge. In summary, the main difference in the structures of superconducting and nonsuperconducting crystals is the difference in structures of iodomercurate chains of insulating layers. In the crystal with the M–I transition, the anion chains are regular and continuous. The anion chains of superconducting crystals involve defect sections as short subchains with inverse alternation of the HgI2

Zhilyaeva et al.

molecule and the [Hg2I6]2- anion. Defect sections alternate with the “major” subchains and are separated from them by empty spaces. The averaged lengths of defect sections are within 2–3 [Hg3I8]2- units. The “major” subchains are 15/25 times longer. Apparently, the formation of defect sections in iodomercurate chain can be considered as a result of a violation in the alternation of HgI2 and [Hg2I6]2- in a chain in the process of crystal growth. Crystal 1 essentially differs from crystals 2–3 in the lengths of the “major” subchains. In addition, defect sections of the two types {(Hg3I8)2} and {(Hg3I8)2HgI2} are present in anion chains of 1, while defect sections of only one type, {(Hg3I8)z}, are observed in anion chains of 2 and 3. The smaller length of the “major” subchains implies that defect sections are observed more often in the anion chain (one defect per approximately 30 [Hg3I8]2- units in crystal 1 as compared with 50 [Hg3I8]2units in crystals 2 and 3). Therefore, the defect concentration in 1 is higher than in 2 and 3. The presence of defect sections of two kinds in the anion chain enhances disorder in the anion layer of 1. Thus, the degree of disorder in the ambient pressure superconducting crystal 1 is higher than in 2 and 3. Apparently, in superconducting crystals 1–3, the disorder in the anion chains running along the a-axis generates a random potential, which due to very short I · · · S contacts between the defect sections of iodomercurate chains and the radical cations is transferred to continuous EDT-TTF step chains. Interestingly, the crystals 2 and 3, which undergo a superconducting transition at almost the same Tc at 0.3 kbar pressure, have the same averaged lengths of the “major” subchains (i.e., the defect concentrations) but differ substantially in averaged lengths of the defect sections. Moreover, the presence of vacancies in the anion chains results in that the donor charge is not +0.5 per molecule, but slightly smaller, +0.486(3) for 1 and +0.490(3) for 2 and 3. Previously, the donor charge values equal to +0.61, +0.555, +0.436, and +0.417 were found for nonstoichiometric superconductors with incommensurate sublattices.3,4,6,9 Because the donor charge values for 1–3 are considered to be equal for all three superconducting crystals within the experimental error, the difference in conductivity behavior between 1 and 2 and 3 is most likely due to the difference in the defect concentration in the anion chains. The above-considered structures of the (EDT-TTF)4[Hg3I8]1-x superconducting crystals are essentially different from those of nonstoichiometric superconductors with two incommensurate sublattices in a crystal. To our knowledge, (EDTTTF)4[Hg3I8]1-x is the first example among organic superconductors based on radical cation salts where the occurrence of a superconducting transition is due to the presence of defects in the chains of insulating layers. Conclusion Four single crystals of octaiodomercurate salts of ethylenedithiotetrathiafulvalene with different type transitions in electrical resistivity have been structurally characterized. For all of these crystals, the structure of insulating anion chains plays an important role in resistivity behavior. Interestingly, ordered or disordered modes of the [Hg3I8]2- sequences result in different types of transitions, namely, to insulating or superconducting states. In all superconducting crystals, the anion chains contain defect sections as short subchains with inverse alternations of [Hg2I6]2- and HgI2. Averaged lengths of short and “major” subchains in a single anion chain have been evaluated to be 2–3 [Hg3I8]2- units and 30/50 units, respectively. In the anion chains of ambient pressure superconductor, the defect concen-

Anion Chain Structure-Controlled Transition

tration is higher than in 0.3 kbar pressure superconductors. It is assumed that short defect sections transfer the distortion generated in the anion chain to the conducting layers due to very short I · · · S contacts with the EDT-TTF radical cations. Acknowledgment. We thank the Russian Foundation for Basic Research (Grant 04-03-32296à) for financial support. Supporting Information Available: CIF file for tetra(ethylenedithiotetrathiafulvalenium) iodomercurate. This material is available free of charge via the Internet at http://pubs.acs.org.

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Crystal Growth & Design, Vol. 7, No. 12, 2007 2773 (14) Zhilyaeva, E. I.; Kovalevskyi, A. Yu.; Torunova, S. A.; Mousdis, G. A.; Lyubovskii, R. B.; Papavassiliou, G. C.; Coppens, P.; Lyubovskaya, R. N. Synth. Met. 2005, 150, 245. (15) Mousdis, G. A.; Kakoussis, V. C.; Papavassiliou, G. C. LowerDimensional Systems and Molecular Electronics; Plenum Press: New York, 1990; Vol. B248, p 181. (16) Goggin, P. L.; King, P.; McEwan, D. M.; Taylor, G. E.; Woodward, P.; Sandstrom, M. J. Chem. Soc. Dalton Trans 1982, 875. (17) SMART and SAINTPLUS, Area Detector Control and Integration Software, Version 6.01; Bruker Analytical X-Ray Systems: Madison, Wisconsin, 1999. (18) SHELXTL, An Integrated System for SolVing, Refining and Displaying Crystal Structures from Diffraction Data, Version 5.10; Bruker Analytical X-Ray Systems: Madison, Wisconsin, 1997. (19) Bondi, A. J. Phys. Chem. 1964, 68, 441. (20) Obviously, two disruptions occur per one “minor” {[Hg3I8]2-}z subchain. If a “major” subchain ends by HgI2, the first unit of a “minor” subchain must be HgI2 since “major” and “minor” positions are related by the approximate inversion center. In this case, the empty space between these subchains corresponds to ∼one-third of the volume of the [Hg2I6]2- anion (see Figure 7). The “minor” subchain, which starts from HgI2, should apparently be finished by [Hg2I6]2- anion; otherwise, the two terminal HgI2 molecules located symmetrically in the “minor” subchain must be bent in opposite directions and, being averaged, give a picture of a linear rather than bent HgI2 molecule that is contradictory to the X-ray data. The “major” subchain next to this “minor” subchain should have the [Hg2I6]2- anion as a first unit since “minor” positions are transformed to “major” ones through the approximate inversion center. The empty space between them should correspond to the volume of the HgI2 molecule plus two-thirds of the volume of the [Hg2I6]2- anion. The total volume of the two empty spaces per one “minor” subchain is to be equal to one volume of [Hg3I8]2- unit (1/3[Hg2I6]2- + HgI2 + 2/3[Hg2I6]2-). (21) If both “minor” and “major” subchains start from and finish by HgI2 molecules, an empty space of a total volume equal to one-half of that of [Hg3I8]2- unit fits per each “minor” subchain (see Figure 7). If a number of “minor” subchains with one terminal HgI2 molecule is n, a number of “minor” subchains with two terminal HgI2 is 1/3n and a total number of “minor” subchains should be 4/3n. There are n vacant volumes of [Hg3I8]2- per n subchains with one terminal HgI2 molecule and 1/6n vacant volumes of [Hg3I8]2- per 1/3n subchains with two terminal HgI2 molecules in crystal 1. A total number of vacant [Hg3I8]2- volumes in crystal 1 derived from the X-ray analysis is 0.027 (see Table 2); 0.027 ) n + 1/6n; n ) 0.023. The total number of the “minor” subchains in 1, considering the subchains with both one terminal and two terminal HgI2 molecules, is 4/3(0.023) ) 0.031. The averaged length of “major” and “minor” subchains in 1 is 6.3/3.1 ) 2.0(2) for “minor” and 91/3.1 ) 30(4) for “major” ones, that is, 7/8 from the value calculated in the former model.

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