A Variety of Ground States in Salts of Unsymmetrical TTFs. Marc Fourmigué

A Variety of Ground States in Salts of Unsymmetrical TTFs Marc Fourmigué Sciences Chimiques de Rennes, UMR 6226 CNRS-UR1 Equipe Matière Condensée et Systèmes Electroactifs Campus de Beaulieu, 35042 RENNES, France E mail: marc.fourmigue@univ-rennes1.fr http://scienceschimiques.univ-rennes1fr/macse

1D Organic Conductors Charge transfer salts : TTF-TCNQ, the first organic metal

Electrocrystallization of cation radical salts Pseudo 1D Organic Conductors

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 ClO 4 and ReO 4 salts exhibit non-dimerized chains, 4k F vs. 2k F instabilities Br salt described Br salt described structure unknown structure unknown metallic state AF ground state Metal-Insul. trans T Neel = 33 K Low quality and small crystals 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

Unsymmetrical TTFs Purification problems from statistical cross coupling reaction Selective coupling method 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

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) 2 Br 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 T Neel = 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) 2 Br Electrocrystallization of tttf with Br - affords indeed a 2:1 salt Triclinic system, space group P-1

(tttf) 2 Br Strong intrastack Dimerisation β I = 0.6078 II β II = 0.0734 (along b) I Large lateral interactions III IV II III β III = 0.185 (2 along a) I β IV = 0.148 (along b-a) III II III

(tttf) 2 Br: transport Semi-conducting behaviour ρ RT = 15 Ω cm σ RT = 0.067 S cm -1 Pressure dependence

SQUID measurements performed on 39 mg show a strong field dependence below 35-36 K AF ground state confirmed! (tttf) 2 Br: magnetism

Band structure calculations II: DFT (Canadell) Evaluation of the magnetic couplings J a = -164 K a-b b J b = -0.5 K a a J a-b = -31.8K

Spin-polarized calculations of the AF state a-b b a a Three cases to be considered

Spin-polarized calculations: the AF states Stability of different states for (tttf) 2 Br (in K per formula unit)

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 AsF 6 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 β intra /β inter1 : 40-30 Anion β intra (ev) β inter1 (ev) β inter2 (ev) β intra /β inter1 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: σ RT = 150 S cm 1 metallic at RT, Tρ 200K two-step transition transition to insulating state @ 50K Application of pressure: σ RT : 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 = 10 4

Combined phase diagram (P c, T c ) - Tricritical point at p c, T c - 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 (Å 3 ) 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

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

Conclusions on (o-me 2 TTF) 2 X 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-b a b a

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)