Effects of Cu addition on rapid solid-state crystal growth of (K,Na)NbO₃ single crystals

Abstract

We investigated the effects of 0.5 mol% to 6.0 mol% CuO addition on crystal growth by the rapid solid-state crystal growth method to synthesize large unitary rectangular (K,Na)NbO3 (KNN) single crystals several square centimeters within 24 h. X-ray photoelectron spectroscopy and X-ray powder diffraction measurements revealed that CuO and Cu2O segregated in the region near the coarsened single crystal/polycrystal boundary. A positive correlation between crystal growth and the addition of CuO up to 1.5 mol% was observed; however, for CuO amounts larger than 1.5 mol%, this correlation became negative. To investigate these phenomena, a high-precision synchrotron powder diffraction measurement was performed. Significant precipitation of the CuO phase was detected in the calcined powders with a CuO addition larger than 1.5 mol%. The effects of this precipitation on the growth mechanism for coarsening the single crystals were discussed.

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

“…In these black regions, the liquid phase of CuO segregated from the enlarged crystal nucleus should have formed during firing, promoting the continued growth of single crystals. 23) The firing temperature ranges for the growth of the large unitary single crystals were 1020 °C-1025 °C, 1045 °C-1055 °C, and 1060 °C-1065 °C for a = 0.50 wt%, 0.55 wt%, and 0.60 wt%, respectively. When the heating temperatures were higher than the abovementioned values, large unitary single crystals were not obtained because of the simultaneous growth of many irregularly shaped single crystals from multiple growth nuclei.…”

“…However, as described above, most of the Cu ions were excluded from the crystal grains during the crystal growth process via the RSSCG method, and only a small amount of Cu ions remained in the crystal grains. 23) As described below, oxygen vacancies in the KNN system materials were detected as defect complexes with Cu through ESR analysis. Because a very small amount of Cu ions rather than the density of oxygen vacancies may dominate the formation of defect complexes in the KNN crystal grains, annealing the calcined powder below the temperature at which the RSSCG method occurs is desirable for accurately comparing the concentration of oxygen vacancies among the calcined powders.…”

“…In a previous report, the authors investigated the effect of the amount of added CuO, b, 0.5%-6.0% during the RSSCG method. 23) It was also found that large single crystals are easily grown in the range of 1.5% ⩽ b ⩽ 2.0% under the conditions of x = 1.0% and a = 0.5 wt%. 23) Thus, in this study, upon maintaining b at 1.5 mol%, the crystals were grown by varying the B-site excess ratio over 0.5%-1.75%.…”

“…23) It was also found that large single crystals are easily grown in the range of 1.5% ⩽ b ⩽ 2.0% under the conditions of x = 1.0% and a = 0.5 wt%. 23) Thus, in this study, upon maintaining b at 1.5 mol%, the crystals were grown by varying the B-site excess ratio over 0.5%-1.75%. Different quantities of Bi 2 O 3 were added to the calcined powder at each x to explore the compositions that facilitate the fabrication of large unitary single crystals, thereby allowing the variation of the previously reported combination of x = 1.0% and a = 0.5 wt%.…”

0.98(K0.5Na0.5)NbO3–0.02(Bi0.5Na0.5)(Zr0.85Sn0.15)O3 Single Crystals Grown by the Seed-Free Solid-State Crystal Growth Method and Their Characterization

“…The reason for this poor repeatability is not currently known. The size of the single crystals may be improved by choice and optimization of different sintering additives, as carried out for KNN [32,72,73]. In recent work in our laboratory, it has been found that use of a Ba(Cu 0.13 Nb 0.66 )O 3 additive and a [001]-oriented KTaO 3 seed crystal allowed growth of a large and dense single crystal of (K 0.5 Na 0.5 )NbO 3 , although problems with repeatability persisted [49].…”