Growth of large unitary (K,Na)NbO₃ single crystals using a seed-free solid-state crystal growth method by adjusting the B-site excess ratios and additional Bi₂O₃

Abstract

We adjusted the B site excess ratio and the quantity of additional Bi2O3 in the KNN calcined powder in order to establish a process to fabricate, (K,Na)NbO3 (KNN)-based single crystals, with large and uniform configurations and relatively stable low porosity, using the seed-free solid-state crystal growth method. When the activity of abnormal grain growth during sample heating was controlled appropriately by adding 0.5 wt% Bi2O3 into the calcined powder with a B site excess ratio of 1%, extraordinarily large unitary rectangular single crystals were grown successfully. The individual crystals started to grow from the nucleus around the center of the green compacts formed by the calcined powder. Then they expanded until they became larger than 15 × 15 mm2 by heating at 1060 °C for 15 h. The relative density of the fabricated single crystals was higher than 99%.

Conditions of large unitary (K, Na)NbO3 system single crystals for rapid solid-state crystal growth method

“…When a = 0.50-0.60 wt%, large tortoiseshell-colored single crystals with diameters exceeding 2-3 mm were grown in a specific temperature range. As reported in our previous paper, 22) since the Bi 2 O 3 concentration in the single crystal are equivalent to that in the polycrystal regions of the sintered sample, Bi ions are known to diffuse homogeneously into the grains and the grain boundaries during heating. Although black polycrystalline sintered bodies were formed throughout the compact at a lower firing temperature when a = 0.55 wt% and 0.60 wt%, large single crystals appeared surrounded by the black polycrystalline regions at a higher temperature.…”

“…Consequently, the combinations for which the growth of large unitary single crystals were observed to be limited to x = 1.0% and a = 0.5 wt%. 22) In the present study, the upper limit of the B-site excess composition of the calcined powder was extended to 1.75% to expand the process window of the RSSCG method. In a previous report, the authors investigated the effect of the amount of added CuO, b, 0.5%-6.0% during the RSSCG method.…”

“…To control AGG, instead of using conventional special sintering aids, the authors have utilized the B-site excess composition of Cu-doped KNN calcined powder and conducted the post-addition of Bi 2 O 3 to realize high-speed solidstate KNN single crystal growth. 22) The B-site excess composition, that is, the composition with a positive B-site (Nb) excess ratio x: (B/A -1) relative to the A-site (Na, K) ions, promotes AGG, whereas the post-addition of Bi 2 O 3 suppresses AGG. Using this approach, known as the rapid solid-state crystal growth (RSSCG) method, large rectangular crystals of 15 × 15 mm 2 or larger were grown at heating times of <12 h. 22) Single crystal X-ray diffraction and Laue diffraction measurements revealed that they are unitary single crystals having the KNN perovskite structure with space group Amm2.…”

“…22) The B-site excess composition, that is, the composition with a positive B-site (Nb) excess ratio x: (B/A -1) relative to the A-site (Na, K) ions, promotes AGG, whereas the post-addition of Bi 2 O 3 suppresses AGG. Using this approach, known as the rapid solid-state crystal growth (RSSCG) method, large rectangular crystals of 15 × 15 mm 2 or larger were grown at heating times of <12 h. 22) Single crystal X-ray diffraction and Laue diffraction measurements revealed that they are unitary single crystals having the KNN perovskite structure with space group Amm2. 22) In the author's previous study, the ranges of the B-site excess ratio, x, and the amount of added Bi 2 O 3 , a, were investigated when the single crystal growth conditions were limited to 0.6% ⩽ x ⩽ 1% and 0.3 wt% ⩽ a ⩽ 0.6 wt%, respectively.…”

“…Using this approach, known as the rapid solid-state crystal growth (RSSCG) method, large rectangular crystals of 15 × 15 mm 2 or larger were grown at heating times of <12 h. 22) Single crystal X-ray diffraction and Laue diffraction measurements revealed that they are unitary single crystals having the KNN perovskite structure with space group Amm2. 22) In the author's previous study, the ranges of the B-site excess ratio, x, and the amount of added Bi 2 O 3 , a, were investigated when the single crystal growth conditions were limited to 0.6% ⩽ x ⩽ 1% and 0.3 wt% ⩽ a ⩽ 0.6 wt%, respectively. Consequently, the combinations for which the growth of large unitary single crystals were observed to be limited to x = 1.0% and a = 0.5 wt%.…”