Prediction of ferroelectric properties in niobates and tantalates based on covalency considerations

The modulation of ferroelectric material properties using ionic substitutions is well known. The Curie temperature, in particular, was previously related to chemical bonding considerations. The present work deals with niobates and tantalates, ferroelectric families particularly interesting for applications. The metal-oxygen covalency is investigated using quantum chemistry methods. The role of covalency on the ferroelectric distortion is discussed and relations between covalency on one side, Curie temperature and microwave relaxation frequency on the other side, are evidenced. Covalency not only 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 ferroelectric distortion in these compounds. Covalency effects can be used in a predictive way to modulate chemically ferroelectric and dielectric properties. This approach is validated through various examples of niobates and tantalates.