Multiferroicity and magnetic-ferroelectric crossover. The PJTE theory of ferroelectricity in ABO3 crystals was expanded to show that, depending on the number of electrons in the d shell of the transition metal ion B and their low spin or high spin arrangement (which controls the symmetry and spin multiplicity of the ground and PJTE active excited states of the [BO 6 ] center), the ferroelectricity may coexist with a magnetic moment (multiferroicity). Moreover, in combination with the temperature dependent spin crossover phenomenon (which changes the spin multiplicity), this kind of multiferroicity may lead to a novel effect known as a magnetic-ferroelectric crossover.