Structural investigation of the "tripled-tetragonal-tungsten-bronze" phases Sr2M10−xO27−y (M = Nb, Ta). (15th September 2019)

• Record Type: Journal Article
• Authors: Felder, Justin B.; Wong-Ng, Winnie; Qabbani, Rania A.; Roth, Robert S.; Toby, Brian H.; Chan, Julia Y.
• Is Part Of: Polyhedron. Volume 170(2019)
• Page Start: 359
• Page End: 363
• Publication Date: 2019-09-15
• Subjects: Tetragonal tungsten bronze -- Ceramics synthesis -- X-ray and neutron diffraction -- Oxides -- Rietveld refinement; Chemistry, Inorganic -- Periodicals; Chimie inorganique -- Périodiques; Organometaalverbindingen; Anorganische chemie; 546.05
• DOI: 10.1016/j.poly.2019.06.003
• Languages: English
• ISSNs: 0277-5387

Abstract

Graphical abstract: Two new tripled tetragonal tungsten bronze (TTTB) phases with the general formula Sr2 M10− x O27− y (M = Nb, Ta) were made via solid-state synthesis. Neutron and X-ray diffraction studies were carried out on the Sr2 Nb10 O27 compound, and the structural model was refined using Rietveld analysis. Abstract: Sr2 Nb10 O27 and Sr2 Ta10− x O27− y exhibit the tripled tetragonal-tungsten-bronze (TTTB)-related superstructures. In contrast to Sr2 Ta10− x O27− y, where an extended solid solution was found, Sr2 Nb10 O27 only forms a narrow range of solid solution, Sr1− x Nb10 O27−z (0 ≤ x ≤ 0.04). Serial X-ray and neutron diffraction studies show that both compounds can be modelled in the orthorhombic Pba 2 space group. Lattice parameters obtained from powder X-ray refinements are a = 12.3887(14) Å, b = 37.103(4) Å, c = 3.866(2) Å, for Sr2 Ta10− x O27− y, and a = 12.3888(2) Å, b = 36.9839(5) Å, and c = 3.94027(3) Å for Sr2 Nb10 O27 . Neutron Rietveld refinements of both structures revealed the possible reason for the differing solid solution behavior. For Sr2 Nb10 O27, all Nb sites are fully occupied, whereas in Sr2 Ta10− x O27− y, three out of the nine crystallographically unique Ta sites were shown to be partially occupied. Charge adjustment is therefore possible by increasing the Sr concentration, leading to a solid solution series and smaller Ta/Sr ratio as compared to Sr2 Nb10 O27 . In the TTTB-related structures, Nb and Ta ions both occupy octahedral and pentagonal bipyramidal coordination sites. Layers of interconnected corner-shared octahedra were found parallel to the ab plane, with 3-, 4- and 5-fold tunnels found in the structure running parallel to the c -axis. Sr ions partially fill the 4- and 5-fold tunnels.