Solubility Measurements of NaTaO3 in Molten Na2MO4 (M = Mo, W, and S) and Growth of Milli-Order Crystals at High Frequency.

Abstract

Sodium tantalate (NaTaO3) is an attractive functional material for photocatalysis. To understand its physical properties, significant efforts for milli-sized single-crystal growth of NaTaO3 have been made. However, the growth was difficult due to the smaller size in solid-state growth or probable decomposition and melting in melt growth. Recently, we grew milli-order NaTaO3 single crystals in Na2MoO4 flux. However, the reproducibility of the growth was not sufficient and hindered the stable supply of the crystal for physicochemical evaluations and further growth. The poor reproducibility was assumed to be due to the inhomogeneous, unstable growth field in response to the external atmosphere provided by nonoptimal experimental conditions. A saturated solution is considered the most suitable crystal growth field because it has the highest solubility and facilitates crystal growth with suppressed nucleation. Since supersaturation is the driving force for crystal growth, we considered that large crystals could be obtained with high frequency if growth could be controlled in the region where solubility changes rapidly. To compile a guideline for crystal growth under the control of supersaturation, the solubility of NaTaO3 in Na-based fluxes, including Na2MoO4, was studied. Using NaTaO3 molding pellets immersed in molten flux, the solubility curve for NaTaO3 was successfully measured. Based on the solubility, the optimal experimental conditions, that is, the heating temperature, the slow-cooling section, and the amount of flux as a solvent, were determined. Finally, we demonstrated the growth of NaTaO3 in Na2MoO4 flux and achieved milli-order crystals with high frequency. Our findings regarding the solubility of NaTaO3 in molten flux may assist in the stable supply of milli-order single crystals for material evaluation and larger crystal growth.