High resolution electron microscopy study in the NaNbO 3Nb 2O 5WO 3 system

Abstract

High resolution electron microscopy studies have been made in the system NaNbO 3Nb 2O 5WO 3 with emphasis on the following two lines in the phase diagram: Na 2− x M 6O 16, 0 < x < 2, and Na 1.50− x M 6.1O 16, 0 < x < 1.5, M = Nb, W. The presence of four structure types, three of which are related to the tetragonal tungsten bronze (TTB) type, has been revealed. The X-ray diffraction patterns only indicated a basic TTB structure in this region. The fourth one, observed in a sample of composition Na 1.40 Nb 6.12O 16, is isotypic with NaNb 6O 15F. The most frequently observed TTB structure is of the Nb 8W 9O 47 type, consisting of three basic TTB unit cells with four pentagonal columns (PCs) in the triple unit. The second most common phase type is NaNb 5WO 16 isotypic with KNb 12O 29F 3 (with a structure comprising two basic TTB unit cells, with four PCs in a square arrangement) which has a doubled content of alkali atoms. A third TTB-related structure observed in Na 1.41Nb 6.01W 0.09O 16 is isotypic with KNb 6O 15F (with two basic TTB unit cells and with two PCs connected by a common octahedron). There are indications that an excess of transition metal stabilizes this phase as well as the NaNb 6O 15F type phase. Twinning, defects, and intergrowth of TTB-related phases are frequently observed phenomena.