Effect of R-site element on crystalline phase and thermal stability of Fe substituted Mn mullite-type oxides: R2(Mn1−xFex)4O10−δ (R = Y, Sm or Bi; x = 0, 0.5, 1). Issue 1 (19th December 2017)

• Record Type: Journal Article
• Authors: Thampy, Sampreetha; Ashburn, Nickolas; Martin, Thomas J.; Li, Chenzhe; Zheng, Yongping; Chan, Julia Y.; Cho, Kyeongjae; Hsu, Julia W. P.
• Is Part Of: RSC advances. Volume 8:Issue 1(2018)
• Page Start: 28
• Page End: 37
• Publication Date: 2017-12-19
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/c7ra09699b
• Languages: English
• ISSNs: 2046-2069

Abstract

Abstract : R-site dependent phase formation and thermal stability of Mn-Fe mullite-type oxides are experimentally and theoretically investigated. Abstract : Combining experimental and theoretical studies, we investigate the role of R-site (R = Y, Sm, Bi) element on the phase formation and thermal stability of R2 (Mn1− x Fe x )4 O10− δ ( x = 0, 0.5, 1) mullite-type oxides. Our results show a distinct R-site dependent phase behavior for mullite-type oxides as Fe is substituted for Mn: 100% mullite-type phase was formed in (Y, Sm, Bi)2 Mn4 O10 ; 55% and 18% of (Y, Sm)2 Mn2 Fe2 O10− δ was found when R = Y and Sm, respectively, for equal Fe and Mn molar concentrations in the reactants, whereas Bi formed 54% O10- and 42% O9-mixed mullite-type phases. Furthermore, when the reactants contain 100% Fe, no mullite-type phase was formed for R = Y and Sm, but a sub-group transition to Bi2 Fe4 O9 O9-phase was found for R = Bi. Thermogravimetric analysis and density functional theory (DFT) calculation results show a decreasing thermal stability in O10-type structure with increasing Fe incorporation; for example, the decomposition temperature is 1142 K for Bi2 Mn2 Fe2 O10− δ vs. 1217 K for Bi2 Mn4 O10 . On the other hand, Bi2 Fe4 O9 O9-type structure is found to be thermally stable up to 1227 K. These findings are explained by electronic structure calculations: (1) as Fe concentration increases, Jahn–Teller distortion results in mid band-gap empty states from unstable Fe 4+ occupied octahedra, which is responsible for the decrease in O10 structure stability; (2) the directional sp orbital hybridization unique to Bi effectively stabilizes the mullite-type structure as Fe replaces Mn.