Mechanism Leading to Ferroelectricity Induced in Centrosymmetric Crystals by Antiferromagnetic Transitions

The appearance of ferroelectricity in conjunction with a paramagnetic-to-antiferromagnetic transition in centrosymmetric crystals is considered. Ferroelectricity is forbidden in centrosymmetric crystals. As a result of a magnetic transition such crystals may lose their center of symmetry. Hence, ferroelectricity, which is forbidden in the paramagnetic phase, may appear in the magnetically ordered one. To illustrate this effect from a symmetry point of view, an antiferromagnetic transition in a structure belonging to the space group Pmma is discussed in detail. An estimation of the ferroelectric moment in a model crystal with some "reasonably real" properties yields a moment of 10^-8 C/cm². It is also shown that a discontinuity in the dielectric constant may occur at the transition point, due to the ferroelectric moment. This discontinuous change is expected to be Δεε≈10^-6.