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Donor doping is commonly applied for softening of the piezoelectric and dielectric properties and facilitation of polarization switching in the ubiquitous Pb(Zr,Ti)O3 [ PZT] ceramics. The origin of the donor-dopant effects is not entirely clear. (Pb,Ba)ZrO3 [ PBZ] is a related ferroelectric material, its perovskite A-site being partially occupied by the larger Ba+2 cation, less prone to evaporation than Pb+2, and the B-site is occupied entirely by the valency-stable Zr+4. Here we report on our studies of Nb+5 doping effects in (Pb,Ba)ZrO3. Similarly, to past observations on La+3 and Nb+5 doped PZT, we find a strong reduction in relative density of PBZ when the doping is <0.5 atomic %. This is accompanied by lattice parameter reduction, enhanced PbO loss, smaller grain size and deterioration of dielectric, piezoelectric and polarization switching properties, the latter being opposite of expected softening effect. All those observations can be interpreted in terms of the Nb entering A-site at small concentrations. This is supported by ab-inito calculations and analysis of the possible defect reaction equations. The structure and microstructure of PBZ with Nb>0.2% are consistent with Nb+5 entering the B-site and softening effects are observed. The study supports the scenario of hardening due to domain walls pinning by VPb- VO divacancies and softening upon decrease in their concentration. [ABSTRACT FROM AUTHOR]

Ferroelectric orthorhombic lead metaniobate ( PbNb2 O6) is known to be metastable with respect to the thermodynamically stable nonferroelectric rhombohedral polymorph. The high-temperature tetragonal to low temperature rhombohedral phase transition is reconstructive and thereby sluggish; ferroelectric PbNb2 O6 is obtained by quenching from the stable phase field of the tetragonal polymorph. We report on the stabilization of the ferroelectric tungsten bronze polymorphs of PbNb2 O6 by minor chemical substitution in the series [(1 − x) PbNb2 O6- x BiTiNbO6], [(1 − x) PbNb2O6- x Na0.5 Bi0.5 Nb2 O6], [(1 − x) PbNb2 O6- x K0.5 Bi0.5 Nb2 O6], and [(1 − x) PbNb2 O6- x CaTiO3]. The high-temperature tungsten bronze polymorph is entropy stabilized with respect to the stable rhombohedral polymorph, and we propose that the tungsten bronze is further entropy stabilized by chemical substitution, reducing the transition temperature of the rhombohedral polymorph and further disfavoring the kinetics of the undesired phase transition. Optimized solid-state synthesis and processing to obtain dense ceramics were developed for the solid solutions, and the dielectric, ferroelectric, and piezoelectric properties of the PbNb2 O6 solid solutions are reported. Curie temperature is suppressed with chemical substitution in all the systems. Lattice cell parameters display systematic variation with composition, reducing the molar volume, and the lattice parameter ratio 2 b/ a with increasing degree of substitution, reflecting a suppression of the polarization along the (010) direction due to chemical substitution. The piezoelectric properties improved with increasing substitution level probably due to the ease of poling of the materials with lower Tc. However, some improvements seen with 2% CaTiO3 were not accompanied by Tc decrease. [ABSTRACT FROM AUTHOR]

This paper addresses the high-temperature instability of Li- and Ta-modified (K,Na)NbO3 piezoceramics. The grains with abnormal size evolve out of the fine matrix grains during high-temperature annealing. They are found to be precipitates with a tetragonal tungsten bronze structure, which result from the volatilization and segregation of the alkali metal elements. With the growth of the abnormal grains the composition of the perovskite matrix phase also changes remarkably, as has been suggested by EDX analysis (for Na) and electric measurements (for Li). These variations lead to a large increase in the tetragonal/orthorhombic phase transition temperature and appreciable variations in the dielectric, ferroelectric, and piezoelectric properties of the ceramic samples. Control of the volatilization of the alkali metal elements can efficiently depress the abnormal grain growth and the compositional segregation. [ABSTRACT FROM AUTHOR]

Compositional inhomogeneity is examined in Li- and Ta-modified potassium sodium niobate (K,Na)NbO3 perovskite ceramics. The inhomogeneous distribution of the A-site (K and Na) and B-site (Nb and Ta) cations is found to be correlative. The inhomogeneity could not be eliminated by prolonged high-temperature annealing in samples prepared by direct mixing of alkaline carbonates and Ta and Nb oxides. In contrast, the precursor method, where a (Nb xTa1− x)2O5 solid solution was first formed, led to a considerably improved compositional homogeneity and appreciably enhanced dielectric, ferroelectric, and piezoelectric properties. Our results suggest that more attention needs to be paid toward controlling the compositional fluctuation of this complex solid solution system. [ABSTRACT FROM AUTHOR]

The processing and characterization of electrical properties of potassium niobate ceramic, KNbO3 (KN) have been studied. The difficulty of obtaining dense samples by conventional methods limits the knowledge on electrical properties of this material. In this paper, a complete route for processing of KN with density over 94% is described. Piezoelectric and dielectric data are presented. It is observed that the major problem concerning the density is related to incorrect stoichiometry, which is believed to be a more critical issue for this system compared with other electro-ceramics. [ABSTRACT FROM AUTHOR]