Mullite-type Na0.67Al6O9.33 and a discussion of iota-alumina

Abstract Within the last sixty years, several authors stated that they have synthesized iota-alumina (ι-Al2O3), the hypothetical endmember of the mullite compositional series Al2[Al2+2xSi2−2x]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-Al2O3 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-Al2O3 actually is a Na-aluminate mullite (Na0.67Al6O9.33) with its pseudo-tetragonal metric: a = 7.6778(4) A, b = 7.6767(3) A, and c = 2.9163(1) A. 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-Al2O3 with mullite-type structure is still a matter of debate.