Molten salt synthesis of K₄Nb₆O₁₇, K₂Nb₄O₁₁and KNb₃O₈crystals with needle- or plate-like morphology

Abstract

Here we report on molten salt synthesis of four different compounds in the K 2 O-Na 2 O-Nb 2 O 5 system. The three compounds K 4 Nb 6 O 17 , K 2 Nb 4 O 11 and KNb 3 O 8 , with non-cubic crystal structures, were prepared as single crystalline particles with large aspect ratios. K 4 Nb 6 O 17 was prepared with a platelike morphology, K 2 Nb 4 O 11 a fibre-like and finally KNb 3 O 8 had a plate like morphology. K x Na 1Àx NbO 3 , with a cubic perovskite crystal structure at the synthesis conditions, was only obtained as cube-shaped crystals, which became larger with increasing synthesis temperature. A systematic study of the influence of the processing conditions on the morphology of the product compounds was conducted. The study demonstrated that the morphology of the product phase was controlled by the crystal structure and the connectivity of the NbO 6 -octahedra of the crystal structures. Finally, the crystal structure of K 2 Nb 4 O 11 was determined to be a tungsten bronze type structure with space group P4/mbm.

On the energetics of cation ordering in tungsten-bronze-type oxides

“…The C sites form narrow triangular channels, and are usually vacant, as only a few cations such as Li + and Nb 5+ are small enough for these sites. 8,9 The B1 and B2 sites are symmetrically different While PN and SBN are similar in terms of structure, there are fundamental differences in the behaviour of the two compounds. PN is a classical ferroelectric, while SBN is ferroelectric for barium-rich compositions, but becomes relaxor-like for strontium contents higher than approximately x = 0.6.…”

The contrasting effect of the Ta/Nb ratio in (111)-layered B-site deficient hexagonal perovskite Ba5Nb4−xTaxO15 crystals on visible-light-induced photocatalytic water oxidation activity of their oxynitride derivatives

“…2b-d) as a result of the mixed NbO 6 -and TaO 6 -octahedra, lowering the total interfacial energy and leading to anisotropic growth. As suggested by Madaro et al, 31 the connectivity of octahedra can determine the aspect ratio of the crystals. The ultraviolet-visible (UV-Vis) diffuse reflectance spectra of Ba 5 Nb 4−x Ta x O 15 crystals are plotted in Fig.…”

Epitaxial growth and cationic exchange properties of layered KNb3O8thin films

“…25,28,29 The cation exchange allows to reach new properties consecutive to the reaction between the host matrix Nb 3 and electrochemical performances 25 that are attractive as anode active materials for sodium-ion battery. 33 Several routes have been used to synthesize KNb 3 O 8 as single crystal by Czochralski and Bridgman-type methods, 34 and as powder by solid state chemistry, 5,35 molten salts synthesis, 35,36 hydrothermal synthesis, synthesis. 39 In polycrystalline materials, it was reported that the KNb 3 O 8 particles have an elongated plate-like or wire-like morphology.…”

“…39 In polycrystalline materials, it was reported that the KNb 3 O 8 particles have an elongated plate-like or wire-like morphology. 10,35,40,41 In reason of this particular morphology, wire-like KNb 3 O 8 was used as precursor to obtain by topochemical methods low-dimensional Nb 2 O 5 crystals that are of interest for catalysis. [42][43][44] So far, no synthesis of the KNb 3 O 8 phase as lm or coating has been reported.…”

“…3a-c would correspond to the rst growth mode, and the morphology of the parallelepiped crystals reects the anisotropy of the crystal structure. Madaro et al 35 have synthesized KNb 3 O 8 platelike crystals by molten salt synthesis and shown that morphology of crystals depends both of temperature and salt/ oxide weight ratio. They have evidenced that the plates have the [010]-preferential orientation and that the length of the plates grows along the [001] direction.…”