Selective Phase Control of Dopant-Free Potassium Sodium Niobate Perovskites in Solution

Abstract

As one of the perovskite families, potassium sodium niobates (K 1−x Na x )NbO 3 (KNN) have been gaining tremendous attention due to their various functional properties which can be largely determined by their crystallographic phase and composition. However, a selective evolution of different phases for KNN with controlled composition can be difficult to achieve, especially in solution chemical synthesis because of its strong tendency to stabilize into orthorhombic phase at conventional synthetic temperature. We herein developed a facile solution approach to control the phase and composition of dopant-free KNN particles selectively through the modification of reaction parameters. A conventional hydrothermal synthesis method yielded orthorhombic KNN particles, while the monoclinic phase, which has never been observed in a bulk counterpart, was kinetically generated by the compositional modification of an intermediate phase under a high-intensity ultrasound irradiation. Cubic KNN particles were stabilized when ethylene glycol was used as a co-solvent together with deionized water through bonding between ethylene glycol molecules and the surface of the KNN. Composite-structured piezoelectric harvesters were fabricated using each phase of KNN particles and the β-phase poly(vinylidene fluoride-co-trifluoroethylene) polymer. Maximum output power was found for the harvester containing orthorhombic KNN particles. This facile synthetic methodology could pave a new pathway for fabricating numerous phasecontrolled materials.

Hydrothermal synthesis of potassium–sodium niobate powders

“…The ICDD database of the three discrete niobate phases (NaNbO 3 , K 0.5 Na 0.5 NbO 3 , and KNbO 3 ) are also included in Figure 2 to point out the peak positions. The samples R1 (K + /Na + = 1), R2 (K + /Na + = 2), and R4 (K + /Na + = 4) had XRD‐detachable phase‐pure NaNbO 3 , which was probably caused by the faster reaction between sodium and niobium in comparison with the one between potassium and niobium, as previously seen 30,37 . The higher diffusivity of sodium ions compared to that of potassium was reported for the plausible explanation 47,48 .…”

“…It was previously demonstrated that the monoclinic KNN‐hydrate phase composes of two octahedron layers and one layer of water akin to bismuth‐layered structures (see Figure 7). 67,76 Such layered arrangement allows the formation of primary crystals via the path () 37,41–43,59,67,74 and subsequent growth along with the layer directions 76 $$\begin{equation}3{\rm{N}}{{\rm{b}}_2}{{\rm{O}}_5} + 8{\rm{O}}{{\rm{H}}^ - } \to {\rm{N}}{{\rm{b}}_6}{\rm{O}}_{19}^{8 - } + 4{{\rm{H}}_{\rm{2}}}{\rm{O}}\end{equation}$$ $$\begin{eqnarray} && {\rm{N}}{{\rm{b}}_6}{\rm{O}}_{19}^{8 - } + \left( {8 - 8x} \right){{\rm{K}}^ + } +\, 8x{\rm{N}}{{\rm{a}}^ + } + n{{\rm{H}}_{\rm{2}}}{\rm{O}} \nonumber\\ && \quad\quad\to {{\rm{K}}_{8 - 8x}}{\rm{N}}{{\rm{a}}_{8x}}{\rm{N}}{{\rm{b}}_{\rm{6}}}{{\rm{O}}_{19}}.n{{\rm{H}}_{\rm{2}}}{\rm{O}}\end{eqnarray}$$ …”

“…The samples R1 (K + /Na + = 1), R2 (K + /Na + = 2), and R4 (K + /Na + = 4) had XRD-detachable phase-pure NaNbO 3 , which was probably caused by the faster reaction between sodium and niobium in comparison with the one between potassium and niobium, as previously seen. 30,37 The higher diffusivity of sodium ions compared to that of potassium was reported for the plausible explanation. 47,48 Besides, Gibbs-free energy for NaNbO 3 (−59 J.mol −1 ) is more negative than that of the KNbO 3 (−49 J.mol −1 ).…”

“…Nevertheless, dissimilar reaction routes were proposed for the KNN formation; while some of them ignored the formation of intermediate hydrate products, 41,49,73 few others mentioned them. 37,38,42,51,52,59 It is generally accepted that in the alkaline medium through the path (I), Nb 2 O 5 (depicted by using Vesta in Figure 7) dissolves into hexaniobate ion Nb 6 O 8− 19 (Lindqvist ion), consisting of six edge-sharing NbO 6 octahedra as demonstrated in Figure 7. 41,69,70,72 The stability of such an may be obtained with the presence of Na + ions.…”

“…42,67 It was previously demonstrated that the monoclinic KNN-hydrate phase composes of two octahedron layers and one layer of water akin to bismuth-layered structures (see Figure 7). 67,76 Such layered arrangement allows the formation of primary crystals via the path (III) 37,[41][42][43]59,67,74 and subsequent growth along with the layer directions. It should be underlined that the structural characterization of K x Na 1−x NbO 3 is compelling as it is emphasized in Figure 2 (the shift (∼ 0.1 • ) between three separate niobate phases and striking similarity of the peak positions).…”