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: marc.fourmigue@univ-rennes1.fr

http://scienceschimiques.univ-rennes1fr/macse

1D Organic Conductors

Charge transfer salts : TTF-TCNQ, the first organic metal

Pseudo 1D Organic Conductors

Electrocrystallization of cation radical salts

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), …

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

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

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

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

(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

(tTTF)2Br

Electrocrystallization of tTTF with Br- affords indeed a 2:1 salt

Triclinic system, space group P-1

(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)

(tTTF)2Br: transport

Semi-conducting behaviour

ρRT = 15 Ω cm

σRT = 0.067 S cm-1

Pressure dependence

(tTTF)2Br: magnetism

SQUID measurements performed on 39 mg show a strong field dependence below 35-36 K AF ground state confirmed !

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

Spin-polarized calculations of the AF state

Three cases to be considered

a a

b a-b

● Stability of different states for (tTTF)2Br (in K per formula unit)

Spin-polarized calculations: the AF states

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

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

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

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

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

Combined phase diagram

(Pc, Tc)

-  Tricritical point at pc, Tc

-  Counter-intuitive pressure effect of the anions

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

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

Cl-/Br- Br-/I-

● Preferential Br- insertion in Cl-/Br- and Br-/I- solid solutions ● Continuous evolution of physical properties

Solid Solutions

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

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

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)