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Marc Fourmigué
“Sciences Chimiques de Rennes”, UMR 6226 CNRS-UR1 Equipe “Matière Condensée et Systèmes Electroactifs”
Campus de Beaulieu, 35042 RENNES , France
A Variety of Ground States in Salts of Unsymmetrical TTFs
E mail: [email protected]
http://scienceschimiques.univ-rennes1fr/macse
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1D Organic Conductors
Charge transfer salts : TTF-TCNQ, the first organic metal
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Pseudo 1D Organic Conductors
Electrocrystallization of cation radical salts
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2D, 3D Organic Conductors
Strongly correlated electrons: ●A variety of competing ground states: Metallic, Mott insulator, Diamagnetic insulator, Antiferromagnet, Superconductor, …
●A variety of phases transitions (T, p): Localisation (Tρ), Peierls, Spin Peierls, Charge Order (CO), …
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Organic Conductors: Preparation
Electrocrystallization of cation radical salts:
●high purity of donor and electrolyte
●solvent nature and polarity
●low current density
●controlled T
● See Poster P13 by C. Mézière et O. Jeannin ● P. Batail, K. Boubekeur, M. Fourmigué, J.-C. P. Gabriel, Chem. Mater. 1998, 10, 3005
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Unsymmetrical TTFs
ClO4– and ReO4– salts exhibit non-dimerized chains, 4kF vs. 2kF instabilities
Br– salt described Br– salt described structure unknown structure unknown metallic state AF ground state Metal-Insul. trans TNeel = 33 K
● C. Coulon, J. Amiell, D. Chasseau, E. Manhal, J.-M. Fabre, J. Physique 1986, 47, 157-161 ● S. Ravy, P. Foury-Leylekian, D. Le Bolloc'h, J.-P. Pouget, J.-M. Fabre, R. J. Prado, P. Lagarde, J. Physique IV 2004, 114, 81-85
Low quality and small crystals
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Unsymmetrical TTFs
M. Fourmigué, F. C. Krebs and J. Larsen, Synthesis 1993, 509 F. Gerson, A. Lamprecht and M. Fourmigué, J. Chem. Soc. Perkin Trans. II, 1996, 1409
Selective coupling method
Purification problems from statistical cross coupling reaction
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Unsymmetrical TTFs
Cl–, Br–, I– salts prepared, structurally characterized
Transport and magnetic properties investigated
(p, T) phase diagram determined
Cl–, Br–, I– salts prepared, structurally characterized
Transport and magnetic properties investigated
Calculations on the AF ground state
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(tTTF)2Br
Electrocrystallization of tTTF with Br- affords a 2:1 salt, reported by Coulon et al. to exhibit, from EPR measurements, an AF ground state below TNeel = 33 K
● Highest Néel Temperature known among organics ?
● Structure unknown
● Cl- or I- salts unknown
P. Vaca, C. Coulon, S. Ravy, J. P. Pouget, J. M. Fabre J. Physique I 1991, 1, 125-140
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(tTTF)2Br
Electrocrystallization of tTTF with Br- affords indeed a 2:1 salt
Triclinic system, space group P-1
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(tTTF)2Br
I
II
II
II
III
III
III
III
IV
I
● Strong intrastack Dimerisation
βI = –0.6078
βII = 0.0734 (along b)
● Large lateral interactions
βIII = –0.185 (2 × along a)
βIV = –0.148 (along b-a)
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(tTTF)2Br: transport
Semi-conducting behaviour
ρRT = 15 Ω cm
σRT = 0.067 S cm-1
Pressure dependence
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(tTTF)2Br: magnetism
SQUID measurements performed on 39 mg show a strong field dependence below 35-36 K AF ground state confirmed !
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Band structure calculations II: DFT (Canadell)
● Evaluation of the magnetic couplings
a a
b a-b Ja =
Jb =
Ja-b =
-164 K
-0.5 K
-31.8K
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Spin-polarized calculations of the AF state
Three cases to be considered
a a
b a-b
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● Stability of different states for (tTTF)2Br (in K per formula unit)
Spin-polarized calculations: the AF states
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Spin-polarized calculations: perspectives
Weak and comparable spin densities in the metallic states
FM: +0.122 AFb: +0.167 AF: +0.198
Larger (albeit < 0.5) spin density in the AF state
AFb: +0.322
Questions and opportunities:
- Can we observe experimentally the a cell doubling ? - Neutron-polarized diffraction experiments ?
- Comparison with the isostructural I– salt - Comparison with the related AsF6– salt
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Unsymmetrical TTFs
Cl–, Br–, I– salts prepared, structurally characterized
Transport and magnetic properties investigated
(p, T) phase diagram determined
Cl–, Br–, I– salts prepared, structurally characterized
Transport and magnetic properties investigated
Calculations on the AF ground state
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Tetragonal halide salts
Electrocrystallization with Cl−, Br−, I−, affords 2:1 salts, tetragonal system space group I-42d
Anion I– Br– Cl– a (Å) 17.4031(2) 17.0920(3), –1.8% 16.9376(6), –2.7%
c (Å) 7.0978(1) 7.0582(2), –0.6% 7.0400(3), –0.8%
V (Å3) 2149.70(5) 2061.96(8), –4.1% 2019.6(1), –6.0%
1) o-Me2TTF molecules on the 2-fold axis of the I-42d space group Uniform stacks by symmetry Strict ¼ filling on the bands
2) Stacks perpendicular to each other Strong 1D character
Preliminary results: A. Abderraba, R. Laversanne, E. Dupart, C. Coulon, P. Delhaes, C. Hauw, J. Physique Coll. C3, 1983, 44, 1243
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Band structure
● 1D calculations give a ¾ filled system without gap and large dispersion (1 eV)
● 3D calculations give avoided crossings at the Z point
● Strong anisotropy (compared with TMTTF salts) with βintra/βinter1: 40-30
Anion βintra (eV) βinter1 (eV) βinter2 (eV) βintra/βinter1 I– 0.427 0.011 0.007 39
Br– 0.466 0.015 0.002 31
Cl– 0.461 0.017 0.003 27
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Conductivity properties of the Br- salt
Longitudinal conductivity:
● σRT = 150 S cm–1 ● metallic at RT, Tρ ≈ 200K ● two-step transition ● transition to insulating state @ 50K ● Application of pressure: σRT: only +15% per kbar (half of TMTTF salts) Limited compressibility, role of C—H•••Hal− ) Transition shifted to 39 K at 7.9 kbar
Transverse conductivity:
● σ⊥ = 0.25 S cm–1 at room temperature
● MI transition at the same temperature than in longitudinal configuration.
● Anisotropy ratio σ// / σ⊥ ≈ 400 In TMTTF salts: σ// / σ⊥1 = 100 and σ// / σ⊥2 = 104
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Combined phase diagram
(Pc, Tc)
- Tricritical point at pc, Tc
- Counter-intuitive pressure effect of the anions
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Unit Cell Evolutions
Anion I– Br– Cl– a (Å) 17.4031(2) 17.0920(3), –1.8% 16.9376(6), –2.7%
c (Å) 7.0978(1) 7.0582(2), –0.6% 7.0400(3), –0.8%
V (Å3) 2149.70(5) 2061.96(8), –4.1% 2019.6(1), –6.0%
Very limited dependence of the stacking c axis
Role of C–H●●●X– hydrogen bonds
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Solid Solutions
Evolutions with the anion size within (o-DMTTF)2(Cl)x(Br)1-x (o-DMTTF)2(Br)x(I)1-x
Preparation:
● Electrocrystallization with fractions of Br–/I– or Cl–/Br– electrolytes
Analysis:
Determination of Br/I and Cl/Br ratio from: ● Energy Dispersion Spectrometry (EDS) on SEM ● Refinement of the ratio from single crystals X-ray data ● Evolution of unit cell volume
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Cl-/Br- Br-/I-
● Preferential Br- insertion in Cl-/Br- and Br-/I- solid solutions ● Continuous evolution of physical properties
Solid Solutions
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Conclusions on (o-Me2TTF)2X salts
● An original series of isomorphous salts with - tetragonal symmetry - strictly uniform chains - strong anisotropy of the conductivity (σ// / σ⊥ ≈ 400) - weak compressibility - SP-type phase transition around 50 K - Alloys Cl/Br and Br/I with preferential Br– insertion - Counter-intuitive pressure effect of the anions - Role of intermolecular C–H●●●X– hydrogen bonds
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Conclusions
● Electrocrystallization is a useful tool for the elaboration of conducting salts with a variety of physical ground states
- Antiferromagnetic ground state in (tTTF)2X - Spin-Peierls transition in (oMe2TTF)2X
● Evolutions with anion size: physical pressure vs. chemical pressure ● Role of weak intermolecular C–H●●●X– ● Possibility for elaboration of solid solutions (preferential insertion vs. statistical insertion)
a a
b a-b
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Acknowledgments
Eric W. Reinheimer Olivier Jeannin
Pascale Auban-Senzier Pr. Claude Pasquier Pascale Foury Jean-Paul Pouget
Pr. Claude Coulon (Bordeaux)
Roman Swietlik (Poznan)
Enric Canadell (Barcelona)