Relaxor behaviour of low lead and lead free ferroelectric ceramics of the Na0.5Bi0.5TiO3-PbTiO3 and Na0.5Bi0.5TiO3-K0.5Bi0.5TiO3 systems

Ceramics with compositions belonging to the Na0.5Bi0.5TiO3-PbTiO3 and Na0.5 Bi0.5TiO3-K0.5Bi0.5TiO3 systems were fabricated by natural sintering of pow ders prepared by thermal decomposition of adequate precursor solutions. The ferroelectric to paraelectric phase transitions were studied by variable t emperature X-ray diffractometry, differential scanning calorimetry and impe dance measurement in a wide range of temperature and frequency. In contrast with pure NBT, both the permittivity and dielectric loss of the NBT-rich s olid solutions show a strongly temperature and frequency dependent behaviou r. The permittivity decreases and its maximum is shifted towards high tempe ratures as the frequency increases. In the high temperature range, the ther mal variation of the permittivity is well described by 1/epsilon - 1/epsilo n (m) = C(T- T-m)(gamma) with gamma close to 1.5. Such a relaxor-like behav iour is interpreted in terms of cation disorder due to the statistical repa rtition of (Na,Bi) and Pb (or Na and K). This would be one very rare case o f relaxor phenomena correlated with the A-site occupancy in perovskite-like materials. (C) 2001 Published by Elsevier Science Ltd.