Studies of phases in the KNbO 3Nb 2O 5 system by high-resolution electron microscopy and X-ray powder diffraction

Abstract

High-resolution electron microscopy and X-ray powder diffraction studies of specimens in the KNbO 3Nb 2O 5 system have shown that, besides the previously reported layer structures K 4Nb 6O 17 and LKNb 3O 8, there exist three different types of tetragonal tungsten-bronze-related structures as well as a block structure, KNb 13O 33, isostructural with the NaNb 13O 33 phase. The cell parameters and the structural relations to TTB are discussed. The first two TTB-related phases, superstructures of the TTB-type, are characterized by the formula K 2 x Nb A 2− x (NbO) C x Nb 10O 30, where 0 < x ≤ 2. H-KNb 3O 8 ( x = 2), a high-temperature phase, exhibits a new mode of linking pentagonal columns in pairs. Various types of microstructures between the two TTB-related phases will be discussed. The third TTB-related phase can be described as K 10−5 y Nb A yNb 10O 30, where 0.8 < y < 1.25. No pentagonal columns have so far been observed. In specimens prepared at T > 1275 K, the low-temperature modification L-KNb 3O 8, transforms in the solid state via a poorly crystallized phase to TTB-related structures. Some geometrical features of the L-KNb 3O 8, KNb 13O 33 structures and the TTB phase will also be presented.