High-temperature solution growth and vapour transport equilibration of (1−x)K_1−yNa_YNbO₃–xLiNbO₃ lead-free piezo-/ferroelectric single crystals

In order to develop lead-free piezo-/ferroelectric materials, single crystals of 0.98K_0.8Na_0.2NbO₃–0.02LiNbO₃ (KNN–LN) have been grown in the perovskite structure using the high-temperature solution growth method. Dielectric measurements reveal structural phase transitions at T_C (cubic to tetragonal)=411 °C and T_O–T (tetragonal to orthorhombic)=189 °C, respectively, and an additional phase transition at a lower temperature of −78 °C which corresponds to the transition from the orthorhombic to a rhombohedral phase. Single crystal structural refinements based on X-ray diffraction data indicate that there are no oxygen octahedral tilts present at room temperature and at −103 °C, which suggests that the crystal is K-rich. Composition analysis by energy dispersive spectroscopy and laser ablation – inductively coupled plasma – mass spectrometry confirms the K-rich composition. A vapour transport equilibration technique is successfully developed to optimise the composition of the as-grown crystals by enriching the Li-content and it is demonstrated to be a viable approach to increase the Li-concentration of KNN–LN and other piezo-/ferroelectric crystals.