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

Abstract

To expand the process window for the rapid solid-state crystal growth (RSSCG) method, through which large unitary single crystals of a (K0.5Na0.5)NbO3 (KNN) system can be synthesized in a short time, we varied the dominant fabrication parameters, the B-site excess ratio and the Bi2O3 content, over a range wider than that utilized in previous studies. Unitary single crystals were obtained at the B-site excess ratios of 0.75%–1.75%; particularly, 1.00% and 1.25% were found to be suitable for the synthesis. Electron spin resonance (ESR) analysis showed that the concentration of oxygen vacancies increased with increasing B-site excess ratios and decreased with increasing amount of added Bi2O3 in the calcined powder. The optimization of the critical driving force for crystal growth by controlling oxygen vacancy concentration was found to be key to growing the large unitary single crystals appropriately by the rapid solid-state crystal growth method.

Comparison of (K0.5Na0.5)NbO3 Single Crystals Grown by Seed-Free and Seeded Solid-State Single Crystal Growth

“…Jiang et al also found that seed-free SSCG only took place within a certain range of LiBiO 3 addition [ 40 ]. Morimoto et al found that adjusting the alkali/niobium ratio and the addition of Bi 2 O 3 could control the number of single crystals that grew in K/Na-deficient samples, with too much Bi 2 O 3 addition preventing crystal growth [ 43 , 54 ]. Donor doping also affects grain-growth behaviour in BaTiO 3 and SrTiO 3 , with abnormal grain growth taking place up to a certain amount of donor dopant addition while further addition suppresses grain growth [ 79 , 80 ].…”

“…The processing conditions, size, output ratio (defined as the% area of the top face of the sample face which was converted into single crystals), and chemical composition of the KNN single crystals grown in the present work are compared with those grown by seed-free SSCG in the literature ( Table 7 ). The KNN single crystals grown in the present work are of comparable size to those grown by seed-free SSCG [ 33 , 39 , 40 , 41 , 42 , 43 , 46 , 54 ]. The processing temperatures and times are also similar to those in previous studies of seed-free SSCG.…”

“…To overcome the limitation of having to use KTaO 3 seed crystals, work has been carried out by several groups on seed-free solid-state crystal growth (seed-free SSCG). In this method, KNN-based single crystals are grown without using seed crystals by carefully controlling dopant addition and sintering schedule [ 39 , 40 , 41 , 42 , 43 ]. Grain growth is suppressed in the sample except for a small number of grains which grow rapidly to form abnormal grains.…”

“…These abnormal grains grow and consume all the matrix grains in the sample, forming single crystals. Single crystals of centimetre size have been grown with good piezoelectric properties in KNN and KNN-based systems [ 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 ]. KNN-based single crystals grown by this method were successfully used to prepare an intravascular photoacoustic probe [ 52 ] and show potential for use as actuators [ 46 , 53 ].…”

“…It is difficult to control the number and size of single crystals that grow in each sample, as the formation of abnormal grains takes place randomly. Morimoto et al were able to limit the single-crystal growth of KNN to one crystal per sample by careful control of the alkali/niobium ratio and the amount of CuO and Bi 2 O 3 dopants [ 43 , 54 ]. Sometimes, the grown single crystals are very porous [ 55 ].…”