Danilo Suvorov, Jakob König, Matjaz Spreitzer

Advanced Materials Department, Jožef Stefan Institute, Ljubljana, Slovenia

Investigating the literature on the lead-free (K_1-xNa_x)_0.5Bi_0.5TiO₃ based piezoelectric materials, a strong inconsistency of the reported data and properties can be observed.
To clarify the inconsistencies, we investigated the formation mechanism of Na_0.5Bi_0.5TiO₃ (NBT) and K_0.5Bi_0.5TiO₃ (KBT). Synthesis of NBT commences with the formation of a sodium deficient end member, which then reacts further towards a nominal composition. During the reaction, the symmetry changes from cubic to rhombohedral. The change in the symmetry and microstructure result in the differences in the electrical properties. The most striking difference is the change of piezoelectric constant d33, which increases from 82 pC/N to 97 pC/N with increasing sodium deficiency. The A-site deficiency was shown to be responsible for high leakage current, which can be used for high-temperature ionic conduction.  A-site deficiency exists also in KBT compound, where the formation of a stoichiometric compound is not possible due to thermal instability of the matrix phase. During the solid-state synthesis of KBT, several secondary phases are formed. Among them, the hygroscopic K₂O-rich liquid phase is the most important as it deteriorates the electrical properties of the ceramics. This phase is formed by transformation of potassium tetra-titanate to potassium hexa-titanate. Inevitably, polytitanates form at low temperatures and they are stable. The K₂O-rich phase was identified by careful investigation of the phase relations around KBT and microstructural changes of thermally and chemically etched samples. By eliminating the hygroscopic liquid phase, the electrical properties are greatly improved: a two-fold increase in the dielectric constant, an increase in the remanent polarization to 29 mC/cm² and a 3-fold increase in the piezoelectric d33 coefficient were observed.

In this work, we will show the evidences leading to the conclusion that the main reasons for the inconsistencies are an inappropriate selection of the synthesis conditions and insufficient characterization of the synthesized ceramics before measuring their electrical properties. Selected synthesis peculiarities and their impact on the dielectric and ferroelectric properties will be presented. In addition, the methods for identification and analyses of the minor factors influencing the properties of the ceramics will be discussed.

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1. Li et all, Nature Materials 2013