Prediction of ferroelectric properties in niobates and tantalates based oncovalency considerations

The modulation of ferroelectric material properties using ionic substitutio ns is well known. The Curie temperature, in particular, was previously rela ted to chemical bonding considerations. The present work deals with niobate s and tantalates, ferroelectric families particularly interesting for appli cations. The metal-oxygen covalency is investigated using quantum chemistry methods. The role of covalency on the ferroelectric distortion is discusse d and relations between covalency on one side, Curie temperature and microw ave relaxation frequency on the other side, are evidenced. Covalency not on ly softens the short-range repulsions, but also stiffens and stabilizes the metal-oxygen network. This last effect appears to dominate in niobates and tantalates. As an unusual result, covalency tends to inhibit the ferroelec tric distortion in these compounds. Covalency effects can be used in a pred ictive way to modulate chemically ferroelectric and dielectric properties. This approach is validated through various examples of niobates and tantala tes.