Below a critical temperature, some compounds exhibit a finite electric spontaneous polarization. This quantity is similar to the force, one encounter when a magnet is approached from another one or a metallic plane: some magnets are ‘soft’, other are ‘hard’. For a magnet, this phenomenon is the consequence of ferromagnetism, which corresponds, at the microscopic level, to the alignment of the spins of the electrons (=magnetic moment) along a unique direction. In dielectrics, ferroelectricity is due to the alignment of dipole moments between atoms/ions of different charge in a unique direction. Since the discovery of ferroelectricity in the 1920’s in the Rochelle salt, researchers are always looking for new materials with novel properties and in particular with a large electric spontaneous polarization. The most well-known ferroelectrics are the perovskites BaTiO₃, KTaO₃ or more recently, BiFeO₃ with the highest observed spontaneous polarization and, in addition, magnetic properties. On the search for applications always wider in the current life, in particular towards the synthesis of cheap computer memories, it is important to synthesize materials that exhibit ferroelectricity at room temperature at low cost. In this respect, hybrid organic-inorganic compounds are excellent candidates as their synthesis occurs at moderate temperatures (below 200°C).

 

Polarization cycle of the compound (MV)[BiI₃Cl₂] at different temperatures near room temperature. In insert, structure of this compound showing the stacking of the inorganic chains made of octahedra of BiI₃Cl₂ (top view on top, side view at bottom) separating the organic molecules of methylviologen.

 

A group of the laboratory, in collaboration with the group of Pr. Nicolas Mercier from MOLTECH-Anjou at Angers University (France), has measured a very large electric spontaneous polarization in the hybrid compound (MV)[BiI₃Cl₂] where MV stands for methylviologen (see figure), recently synthesized by the Angers group. The measured polarization is of the same order of magnitude as in the best inorganic oxides. This observation opens the door to a wide development of these hybrid materials in order to synthesize new ferroelectrics, but also, in a near future, to couple ferroelectricity with magnetic properties (multiferroicity) in hybrid materials. The objective is to easily control magnetic properties with a voltage source.

 

Reference:

N.Leblanc, N.Mercier, L.Zorina, S.Simonov, P.Auban-Senzier and C.Pasquier, J. Am. Chem. Soc. 133, 14924 (2011)

 

Contact :

Claude Pasquier : (pasquier@lps.u-psud.fr)