On multiferroicity of TTF-CA molecular crystal.

Magnetic properties of the TTF-CA molecular crystal below the neutral to ionic transition temperature are studied using the embedded cluster approach in combination with density functional theory. The calculated values of the Heisenberg exchange integral between the neighboring TTF and CA molecules stacked along the crystallographic axis a suggest that the ionic phase of the TTF-CA can be described as an alternating antiferromagnetic spin-1/2 Heisenberg chain with the exchange integral J = 1124 cm(-1) and the alternation parameter δ = 0.46. Although the combination of ferroelectricity of the ionic phase with the antiferromagnetic ordering renders TTF-CA multiferroic (as predicted theoretically in G. Giovannetti et al., Phys. Rev. Lett., 2009, 103, 266401), the large value of the alternation parameter should result in a nonmagnetic ground state of this phase. The dependence of the magnetic coupling parameters on the crystal structure is studied and the implications for experimental observation of magnetic properties of TTF-CA are discussed.