ORIGINAL COVALENCY EFFECT ON DIELECTRIC AND FERROELECTRIC PROPERTIES OF KTN CERAMICS

The KTa1-xNbxO3 (KTN) system was largely studied in the case of very l ow concentration of substituted niobium (x < 0.04). The dielectric pro perties of KTN ceramics in the concentration range (0.3 less than or e qual to x less than or equal to 1) are presented in this work. The mos t original point is the investigation of the high frequency response ( 10(6) -10(9) Hz) as a function of temperature (250 - 450 K). Whatever the composition, a high frequency relaxation is observed in all the te mperature range. The corresponding relaxation frequency, f(Gamma)[HF], is strongly dependent on both temperature (minimum of f(Gamma) close to Tc) and composition. The interpretation of the results is based on the correlation chain model (cooperative hopping of off-centered ions) already used in numerous ferroelectric perovskites exhibiting a high frequency relaxation. In a previous study on complex lead based perovs kites (PSN, PST), strong correlations between [HF] relaxation frequenc y and chemical bonding were evidenced for the first time using extende d Huckel theory. The present study confirms the influence of covalency of Nb-O and Ta-O bonds on the f(Gamma)[HF] values in KTN. The obtaine d correlations which extend to the Te values valid the use of this ori ginal approach based on chemical bonding considerations for a better u nderstanding of dielectric and ferroelectric properties.