Comparison of (K_0.5Na_0.5)NbO₃ Single Crystals Grown by Seed-Free and Seeded Solid-State Single Crystal Growth

(K_0.5Na_0.5)NbO₃-based piezoelectric ceramics are of interest as a lead-free replacement for Pb(Zr,Ti)O₃. In recent years, single crystals of (K_0.5Na_0.5)NbO₃ with improved properties have been grown by the seed-free solid-state crystal growth method, in which the base composition is doped with a specific amount of donor dopant, inducing a few grains to grow abnormally large and form single crystals. Our laboratory experienced difficulty obtaining repeatable single crystal growth using this method. To try and overcome this problem, single crystals of 0.985(K_0.5Na_0.5)NbO₃-0.015Ba_1.05Nb_0.77O₃ and 0.985(K_0.5Na_0.5)NbO₃-0.015Ba(Cu_0.13Nb_0.66)O₃ were grown both by seed-free solid-state crystal growth and by seeded solid-state crystal growth using [001] and [110]-oriented KTaO₃ seed crystals. X-ray diffraction was carried out on the bulk samples to confirm that single-crystal growth had taken place. Scanning electron microscopy was used to study sample microstructure. Chemical analysis was carried out using electron-probe microanalysis. The single crystal growth behaviour is explained using the mixed control mechanism of grain growth. Single crystals of (K_0.5Na_0.5)NbO₃ could be grown by both seed-free and seeded solid-state crystal growth. Use of Ba(Cu_0.13Nb_0.66)O₃ allowed a significant reduction in porosity in the single crystals. For both compositions, single crystal growth on [001]-oriented KTaO₃ seed crystals was more extensive than previously reported in the literature. Large (~8 mm) and relatively dense (<8% porosity) single crystals of 0.985(K_0.5Na_0.5)NbO₃-0.015Ba(Cu_0.13Nb_0.66)O₃ can be grown using a [001]-oriented KTaO₃ seed crystal. However, the problem of repeatable single crystal growth remains.