The reasons for the lower piezoelectric properties in the most studied lead-free piezoelectrics, modified (K,Na)NbO 3 and (Bi 0.5 Na 0.5 )TiO 3 , are discussed. Contributions from domain wall motion and properties at the morphotropic phase boundary are considered and are compared to those in PZT. Lead-free, non-piezoelectric solutions to electromechanical coupling are discussed.

Compositional inhomogeneity is examined in Li-and Ta-modified potassium sodium niobate (K,Na)NbO 3 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 hightemperature annealing in samples prepared by direct mixing of alkaline carbonates and Ta and Nb oxides. In contrast, the precursor method, where a (Nb x Ta 1Àx ) 2 O 5 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.

A composition-temperature phase diagram of the system ͑1−x͒͑K 0.5 Na 0.5 ͒NbO 3-xLiNbO 3 is presented for 0 ഛ x ഛ 0.1. Using dielectric and piezoelectric resonance measurements, and Raman spectroscopy, ceramic samples containing 2%-10% LiNbO 3 were studied over a temperature range of 7-770 K showing a complex sequence of phase transitions. Analysis of the different Raman, piezoelectric, and dielectric data shows distinct transitions from cubic to tetragonal to orthorhombic to rhombohedral phase for x = 0.02-0.05. The symmetries of the phases were assigned using analogy to phase diagram of ͑K 0.5 Na 0.5 ͒NbO 3 single crystals and ceramics ͑x =0͒. At x Ͼ 0.07 only one transition between ferroelectric phases occurs where tetragonal phase transforms to another phase, possibly of rhombohedral, orthorhombic, or monoclinic symmetry. In the region between x = 0.05 and x = 0.08, the phase transition sequence is more complex. Below 100 K this phase with unidentified symmetry creates a vertical boundary with the rhombohedral phase present near 5% Li. A triple point between the tetragonal, orthorhombic, and the new phase is identified.

The structural phase diagram of the Pb-free ferroelectric (Na ½ Bi ½ ) 1-x Ba x TiO 3 (NBT-BT), x<0.1, has been explored by Raman spectroscopy at temperatures from 10 to 470 K. The data provide clear evidence for a proposed temperature-independent morphotropic phase boundary at x ≈ 0.055. However, there is no evidence for a structural phase transition across T ≈ 370 K for x > 0.055, where bulk-property anomalies appear to signal a transition to a nonpolar or antiferroelectric phase. The results identify that the phase above 370 K shows short-range ionic displacements that are identical to those in the long-range-ordered phase below 370 K. These conclusions provide a natural interpretation of the weak piezoelectric response in this system and have important implications for the search for Pb-free piezoelectrics.

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.