Ceramics with composition Ba1-xNaxTi1-xNbxO3 are of either classical ferroelectric (for 0 ⩽ x < 0.075) and ferro- or antiferroelectric (for 0.55 < x ⩽ 1) or relaxor ferroelectric type (for 0.075 ⩽ x ⩽ 0.55), the transition at Tc being only diffuse without any frequency dispersion for this last region. All the corresponding dielectric characteristics, i.e. diffusivity of the ferroelectric-paraelectric transition, frequency dispersion of ε'r, shift of Tm with frequency deviation from the Curie-Weiss law, are determined. The relaxor behaviour is more relaxor the more the composition deviates from BaTiO3 and NaNbO3. This study is in the field of preparation of relaxor ceramics free from lead in the interest of the environment, which present a transition temperature close to room temperature.

We report a large d 31 piezoelectric coefficient and corresponding electromechanical coupling factor, K p , of 0.5Ba(Zr 0.2 Ti 0.8 )O 3 -0.5(Ba 0.7 Ca 0.3 )TiO 3 (BCTZ50) and 0.68Ba(Zr 0.2 Ti 0.8 )O 3 -0.32 (Ba 0.7 Ca 0.3 )TiO 3 (BCTZ32) lead-free piezoceramics. The piezoelectric coefficient, d 31 , reaches a high value of 200 pC/N for BCTZ50 at room temperature which is comparable to the one of the soft PZT. This confirms the previously reported d 33 for the same material. A useful way to achieve such performances at the expense of a smaller thermal budget is suggested, enabling better control of the ceramics composition and microstructure. Based on pyroelectric and ferroelectric hysteresis loops measurements, we show that such outstanding properties are likely due to the high flexibility of polarization under thermal and electric stresses. V

Keywords: lead-free, ferroelectric relaxor, perovskites Relaxor ferroelectrics are used for applications in a wide variety of devices [1]. The great interest of these materials is related to their very high dielectric permittivity observed in the large range of temperature and the strong frequency dispersion at low temperature [2]. To understand the origin of this behaviour many works based on structural and physical models are performed. The relaxor behaviour occurs generally in complex perovskite of formula (A'A')'(B'B'')O 3 where two (or more) cations of different valences are located in the equivalent crystallographic positions. Relaxor materials actually used are lead-based ceramics which present a disadvantage due to the toxicity of PbO. The actual evolution of research is oriented to environment friendly application. In this way, the present work concerns new lead-free compositions of non-stoichiometric perovskite derived from the well known BaTiO 3. Dense ceramics were obtained by conventional mixed oxide method. Room temperature X-ray diffraction analysis allowed us to determine the limits of solid solution. Dielectric measurements were performed on ceramic disks. For all samples, the temperature and frequency variations of the real and imaginary part of permittivity are investigated. The results are discussed and compared to previous works concerning the Ba 1-x A 2x/ 3TiO 3 compositions where La and Bi are in the dodecahedral sites (A) [3,4]. The performed study has shown that the relaxor behaviour is not due only to the B-site order/disorder. The role of the cation in the A-site seems to be also important [5].

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A polycrystalline sample of BaTi0.85(Fe1∕2Nb1∕2)0.15O3 is synthesized by high temperature solid state reaction technique. This sample crystallizes in cubic pervoskite structure at room temperature. The dielectric properties have been investigated at temperature range from 80to450K at various frequencies (100–107Hz). Frequency dielectric dispersion phenomena in a BaTi0.85(Fe1∕2Nb1∕2)0.15O3 ceramic has been analyzed by impedance spectroscopy in the temperature range from 250to450K. The Cole-Cole relaxation equation modified by introducing the conductivity was used to describe the experimental dielectric spectra of a high permittivity. Excellent agreement has been obtained in wide frequency domain (100–107Hz) between the measured and calculated permittivities in the 250–450K temperature range. The dielectric relaxation frequency obeys the Arrhenius behavior with activation energy of 0.181eV. A separation of the grain and grain boundary properties has been achieved using an equivalent circuit model. The different parameters of this circuit have been determined using impedance and modulus spectroscopy studies. The inner grain conductivity also obeys the Arrhenius low behavior with activation energy of 0.183eV. The dielectric relaxation and the inner grain conductivity have thus similar activation energies which suggest a link between these two phenomena.

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