Influence of Ta⁵⁺ content on the crystallographic structure and electrical properties of [001]_PC-oriented (Li,Na,K)(Nb,Ta)O₃ single crystals

A series of centimeter-sized lead-free piezoelectric Li⁺- and Ta⁵⁺-modified (Na,K)NbO₃ single crystals with an ABO₃ perovskite structure was successfully grown by the top-seeded solution growth method. Effective segregation of the elements was considered in order to further develop the growth of Li⁺- and Ta⁵⁺-modified (Na,K)NbO₃ single crystals. The crystallographic structure and electrical behaviour along the [001]_PC orientation were studied. X-ray diffraction, Raman spectroscopy and dielectric studies reveal that the increase in Ta⁵⁺ content reduces the orthorhombic-tetragonal phase transition. Bipolar and unipolar strain curves were investigated at 2 kV mm^-1. The highest bipolar and unipolar strains associated with a large-signal piezoelectric constant of 368 pm V^-1 were achieved for Li_0.02Na_0.626K_0.354Nb_0.808Ta_0.192O₃. The asymmetric bipolar strain curves were observed and related to the existence of internal bias fields induced by (V′_A-V_O) defect dipoles, which were created during the crystal growth process.