Mullite-type Na0.67Al6O9.33 and a discussion of iota-alumina. Issue 12 (September 2020)

• Record Type: Journal Article
• Authors: Lenz, Stephan; Schneider, Hartmut; Fischer, Reinhard X.
• Is Part Of: Journal of the European Ceramic Society. Volume 40:Issue 12(2020)
• Page Start: 4276
• Page End: 4280
• Publication Date: 2020-09
• Subjects: Iota-alumina -- Mullite -- Sol-gel synthesis -- Na-aluminate mullite; Ceramic materials -- Periodicals; Composite materials -- Periodicals; Matériaux céramiques -- Périodiques; Composites -- Périodiques; Ceramic materials; Composite materials; Periodicals; Electronic journals; 666.05
• DOI: 10.1016/j.jeurceramsoc.2020.03.038
• Languages: English
• ISSNs: 0955-2219

Abstract

Abstract: Within the last sixty years, several authors stated that they have synthesized iota-alumina ( ι -Al2 O3 ), the hypothetical endmember of the mullite compositional series Al2 [Al2+2 x Si2−2 x ]O10- x with x = 1. Iota-alumina is supposed to be a silica-free aluminate having a mullite-like X-ray diffraction pattern. However, no detailed information about its crystal structure has been provided. Therefore, we reproduced a typical sol-gel synthesis of ι -Al2 O3 and investigated the obtained material by X-ray powder diffraction (XRPD), EDS analysis, and thermal treatment. Our powder-diffraction diagrams conform to all published XRPD data of iota-alumina, but Rietveld and EDS analyses revealed that the putative ι -Al2 O3 actually is a Na-aluminate mullite (Na0.67 Al6 O9.33 ) with its pseudo-tetragonal metric: a = 7.6778(4) Å, b = 7.6767(3) Å, and c = 2.9163(1) Å. For this reason, we assume that most of the phases designated as iota-alumina in the literature are in fact Na-aluminate mullites. Thus, the existence of ι -Al2 O3 with mullite-type structure is still a matter of debate.