Charge-ordering induces magnetic axes rotation in organic materials (TMTTF) 2 X (with X = SbF 6 , AsF 6 , and PF 6 )

We have performed a comprehensive EPR investigation of the quasi-one-dimensional organic systems (TMTTF) 2 X with centrosymmetrical anions X (X = SbF 6 , AsF 6 , and PF 6). We observe a strong rotation of g-factor principal axes when the temperature decreases below charge-ordering temperature T CO. The possible origin of this rotation is analyzed on the basis of quantum chemical calculations performed using density functional theory method. A good agreement between theory and experiment is found. PACS: 71.27.+a Strongly correlated electron systems; heavy fermions; 75.85.+t Magnetoelectric effects, multiferroics; 76.30.–v Electron paramagnetic resonance and relaxation. Keywords: organic salt, electron paramagnetic resonance, charge-ordering, loss of the symmetry centre. Quasi-one-dimensional systems such as organic charge-transfert salts have been intensively studied during the last decades. Among these organic salts, (TMTTF) 2 X molecules , where X represent a inorganic anion, exhibit a very rich phase diagram [1] and display phase transitions such as antiferromagnetic, charge-ordering, spin-peierls, etc. They cristallize in the triclinic 1 P space group with two donor molecules and one anion in the unit cell. TMTTF molecules are arranged in a " zigzag " configuration, forming stacks along the a direction and layers in ab plane which alternate with the anion X along the c axis. Charge-ordering (CO) is observed in these compounds at T CO = 65 K, 105 K [2], and 154 K [3] for X = SbF 6 , AsF 6 , and PF 6 , respectively. Here, we report a continuous wave (cw) electron para-magnetic resonance (EPR) study of (TMTTF) 2 X (X = SbF 6 , AsF 6 , and PF 6). The rotation of g-factor principal axes was observed in these three salts when the temperature decreases from 300 to 4 K. This rotation is considerably enhanced below the charge-ordering transition. We discuss the possible origin of this rotation by comparing experimental results with those obtained from our quantum chemical calculations performed with density functional theory (DFT). When considering only a thermal contraction of the TMTTF structure, as temperature decreases, the DFT calculations show a rotation of about 4–5° between T = 300 and 4 K. But when a uniform anion displacements due to the charge ordering transition were considered [4] we obtain a much stronger rotation ranging up to 26 in the case of (TMTTF) 2 SbF 6 salt, in fair agreement with the experimental data. Single crystals of (TMTTF) 2 X were