Synthesis and structure-property relationship of mullite-type oxides

Abstract

Mullite-type oxides draw intensive research attention because of their crystal structures, magnetic and multiferroic properties. Diverse chemical compositions in mullite-type oxides can offer plenty of new phases with novel crystal-chemico-physical properties The main objective of this work is to study the structure-property relationships of mullite-type oxides as function of chemical composition. Within the scope of this cumulative dissertation, the O10-mullite-phases were synthesized and studied. To understand whether the lone electron pair (LEP) containing cation play a role for structural stability in O10-phases, a systematic substitution of Bi3+ by rare-earth cations Nd3+, Sm3+ and Eu3+ in the Bi2Mn4O10 compound [1] was carried out (Chapter 3). Rietveld refinement of powder X-ray diffraction data indicated that all polycrystalline samples crystalize in orthorhombic space group Pbam, where the edge-sharing MnO6 octahedra form an infinite chain along the c-axis and interconnected by edge-sharing MnO5 distorted tetragonal pyramids. The R3+cation (R = Bi, Nd, Sm and Eu) sits in the channels between the polyhedral chains, forming distorted RO8 polyhedra. To measure the strength of stereochemical activity of LEP-cation, Wang-Liebau eccentricity (WLE) parameter was calculated from the structural data, showing a linear decrease of WLE value in (Bi1-xRx)2Mn4O10 (R = Nd, Sm and Eu). Whereas substitution of transition metal cations in different polyhedral sites by non-transition metal cations can offer a plenty of phases of this O10-mullite-type family, mullite-type compounds with R-cations and Al3+/Ge4+ were still missing though the ionic radii and Al/Ge-O bond distances in a given coordination are quite similar. As such, synthesis and characterization of RAlGeO5 (R =Y, Sm-Lu) phases [2] were studied (Chapter 4). A rare inversion of Al/Ge between octahedral and pyramidal sites have been observed for these O10-phases, and the inversion parameter found to be between 0.22(1) and 0.30(1) for different R-cations. Selective vibrational features at higher wavenumber regions further complement the inversion of Al/Ge between two coordination sites. Due to centrosymmetric structure of the mullite-type Bi2Mn4O10 compound the microscopic origin of the multiferroicity requires convincing explanation in terms of complex interplay between spin-ordering, highly polarizable Bi3+ with stereo-chemically active LEP, Mn3+/Mn4+ charge-ordering and geometric distortions of the MnOy coordination polyhedra. As such, mullite-type NdMnTiO5 [3] was synthesized followed by multi-tool characterizations (chapter 5). The Mn3+ and Ti4+ cations were observed to be located in the octahedral and pyramidal site, respectively. The higher decomposition temperature of NdMnTiO5, compared to other RMn2O5 phases, was explained in terms of higher bond strength of Ti-O than those of Mn-O. Temperature-dependent DC magnetic susceptibility suggested paramagnetic to antiferromagnetic phase transition (TN) at 43(1) K. Inverse susceptibility in the paramagnetic region above 120 K followed the Curie-Weiss law, resulting in a magnetic moment of 6.33(1) μB per formula unit. Neutron diffraction data collected at 7.5 K revealed that the magnetic moments of Nd3+ and Mn3+ in NdMnTiO5 are incommensurately ordered with a propagation vector k = (0, 0.238, 0.117). References [1] K. Ghosh, M.M. Murshed, T.M. Gesing; Synthesis and characterization of (Bi1−xRx)2Mn4O10: structural, spectroscopic and thermogravimetric analyses for R = Nd, Sm and Eu; Journal of Materials Science 54(21) (2019) 13651-13659. Doi:10.1007/s10853-019-03852-7 [2] K. Ghosh, M.M. Murshed, M. Fischer, T.M. Gesing; Aluminum to germanium inversion in mullite-type RAlGeO5: characterization of a rare phenomenon for R = Y, Sm – Lu; Journal of the American Ceramic Society 105 (2022) 728-741. Doi:10.1111/jace.18085 [3] K. Ghosh, M.M. Murshed, T. Frederichs, T.M. Gesing, structural; vibrational, thermal and magnetic properties of NdMnTiO5, Journal of the American Ceramic Society (Accepted 2021). DOI: 10.1111/jace.18261