Growth and Structure of Potassium Tantalum-Niobium Oxide Crystals by Seed-Free Solid-State Growth

The solid solution structures of potassium tantalum niobate (i.e., KTa_1–xNb_xO₃, KTN) crystals are investigated, which are formed by the solid solution of potassium tantalate (KTaO₃) and potassium niobate (KNbO₃). The KTN crystals exhibit superior piezoelectric, ferroelectric and electro-optic properties. However, a difficulty in preparing large-sized, high-quality KTN crystals restricts their practical applications. Some studies indicate that the Curie temperature of potassium tantalum niobate single crystals with a composition of KTa_0.63Nb_0.37O₃ is near room temperature. Currently, single crystals of solid solutions are mostly grown by a melt method. The seedless solid-phase crystal growth technique is a method for growing single crystals in the solid-phase without the need for artificially introduced seed crystals. This method offers some advantages such as low cost, simplicity in process, and short growth cycles, providing a novel approach to crystal growth and demonstrating an application potential. This paper was to investigate the growth of potassium tantalum niobate single crystals by a seedless solid-phase technique, to expand the application scope of crystal growth and obtain an effective pathway for preparing potassium tantalum niobate single crystals. In addition, the mechanism of the transformation from KTN ceramic preforming to single crystals and delving into the this transformation was also discussed.

High-purity K₂CO₃ (99.0%, in mass, the same below), Nb₂O₅ (99.5%), Li₂CO₃ (98.0%), Bi₂O₃ (99.0%), and Ta₂O₅ (99.5%) were selected as raw materials, and 0.3% (in mole fraction) of LiBiO₃ was used as a flux. In the preparation by a seedless solid-phase method, the raw materials with anhydrous ethanol were placed in a nylon jar with zirconia balls as grinding media. The mixture was then ground in a ball mill for 24 h. After drying, dried powdered material was pre-fired at 750 ℃ for 10 h. After pre-firing, the powdered material was further ground for 24 h. The dried powdered material was sieved, and then pressed into a circular shaped preform with a diameter of 20 mm and a thickness of approximately 2–3 mm at 100 MPa. The circular green preform was sintered in a box-type resistance furnace at 1168–1198 ℃ for 21 h to ensure that the green preform could be fully sintered and converted into the desired crystal structure. After sintering, the furnace temperature was slowly reduced to room temperature at a rate of 0.5 ℃/min. The crystal structure of the samples was analyzed using X-ray diffraction (XRD). The microstructure of the single crystals and ceramics was determined by scanning electron microscopy (SEM), and the elemental composition of the samples was characterized by energy-dispersive spectroscopy (EDS).

The KTN single crystals grow in a layered mode, with a close contact between the crystals and the polycrystalline regions, featuring flat interfaces and regular geometric shapes. In the growth process, solute molecules or ions arrange in layered stacks on the crystal surface under the action of driving forces. There are differences in growth rates due to the different driving forces required for growth in various directions, leading to the formation of stepped structures on the crystal surface. The single crystals gradually engulf and integrate with the dispersed micro-grains in the ceramic region to achieve their growth. However, crystal surface energy exists during the actual growth process. The crystal planes with a higher surface energy have more intense attraction and binding capabilities for atoms, resulting in faster growth rates. In the growth of KTN crystals, the surface energy and growth rates of the crystal planes (100) and (110) jointly determine the main morphology of the crystals. The growth direction of the crystals follows a directional pattern of layered arrangement, and the crystals follow a two-dimensional layered accumulation growth mechanism in the radial dimension. The growth of KTN strip-shaped crystals is achieved through two-dimensional layered accumulation. In the solid-phase growth of KTN crystals, the crystals grown by two-dimensional layered accumulation create a "shell–core" structure with different coexistence interface geometries. The large KTN crystals grown by two-dimensional layered accumulation are enclosed within the "shell" of the fine grain region, resulting in their "shell–core" coexistence. The XRD patterns of the natural surface of the KTN bulk sample indicate a preferred orientation of the crystal plane (210). The addition of K, Bi, and Li elements promotes the formation of the liquid phase, which facilitates the atom migration and thereby drives the continuous growth of the crystals.

The strip-shaped KTN single crystals were prepared as LiBiO₃ was used as a flux, and the single crystallization of ceramic preforms in this system via the seed-free solid-state crystal growth was explored. The obtained KTN single crystals grew in a two-dimensional layered stacking manner and possessed a cubic perovskite structure. A "shell-core" structure was formed within the strip-shaped crystals, consisting of outer layers that were short and fine, and inner layers that were long and coarse. The exposed surface of the crystals was the crystal plane (210). Based on the theory of two-dimensional crystal growth, liquid-phase-assisted mechanisms, and the anisotropy of interfacial energy and interfacial migration, the mechanism of seed-free solid-state crystal growth of KTN crystals was analyzed. This mechanism was the promotion of ordered expansion of the crystals on a two-dimensional plane through the auxiliary action of the liquid phase. Also, the differences in interfacial energy and the anisotropic nature of their migration without the need for seed crystal guidance could intervene the direction and morphology of crystal growth, thereby achieving the seedless solid-phase growth of KTN crystals.