Crystal chemistry aspects of the magnetically induced ferroelectricity in TbMn₂O₅ and BiMn₂O₅

The origin of magnetic frustration was stated and the ions, whose shift is accompanied by emerging magnetic ordering and ferroelectricity in TbMn₂O₅ and BiMn₂O₅, were determined on the basis of calculating the magnetic coupling parameters by using the structural data. The displacements accompanying the magnetic ordering are not polar, they just induce changes of bond valence (charge disordering) of Mn1 and Mn2, thus creating instability in the crystal structure. The approximation of the bond valence to the initial value (charge ordering) under magnetic ordering conditions is only possible again due to polar displacement of Mn2 (or O1) and O4 ions along the b axis which is the cause of the ferroelectric transition.