Exploiting superspace to clarify vacancy and Al/Si ordering in mullite

IUCrJ
Paul B KlarGotzon Madariaga

Abstract

Synchrotron single-crystal X-ray diffraction has revealed diffuse scattering alongside sharp satellite reflections for different samples of mullite (Al4+2x Si2-2x O10-x ). Structural models have been developed in (3+1)-dimensional superspace that account for vacancy ordering and Al/Si ordering based on harmonic modulation functions. A constraint scheme is presented which explains the crystal-chemical relationships between the split sites of the average structure. The modulation amplitudes of the refinements differ significantly by a factor of ∼3, which is explained in terms of different degrees of ordering, i.e. vacancies follow the same ordering principle in all samples but to different extents. A new approach is applied for the first time to determine Al/Si ordering by combining density functional theory with the modulated volumes of the tetrahedra. The presence of Si-Si diclusters indicates that the mineral classification of mullite needs to be reviewed. A description of the crystal structure of mullite must consider both the chemical composition and the degree of ordering. This is of particular importance for applications such as advanced ceramics, because the physical properties depend on the intrinsic structure of mullite.

References

Sep 18, 2007·Acta Crystallographica. Section B, Structural Science·Zunbeltz IzaolaAlberto García
May 23, 2009·Acta Crystallographica. Section B, Structural Science·Trixie Wagner, Andreas Schönleber
Jan 23, 2015·IUCrJ·Carlos Basílio Pinheiro, Artem M Abakumov
Jun 1, 2015·Acta Crystallographica Section B, Structural Science, Crystal Engineering and Materials·Johannes BirkenstockReinhard X Fischer
May 4, 2016·Journal of Synchrotron Radiation·Vadim DyadkinDmitry Chernyshov
Dec 3, 2016·Acta Crystallographica Section B, Structural Science, Crystal Engineering and Materials·Huifang XuBruce C Noll
Jun 3, 2017·Acta Crystallographica Section B, Structural Science, Crystal Engineering and Materials·Paul B KlarGotzon Madariaga

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Citations

Mar 2, 2019·Acta Crystallographica. Section A, Foundations and Advances·Paul Benjamin KlarGotzon Madariaga

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Methods Mentioned

BETA
electron diffraction
X-ray
transmission electron microscopy
NMR

Software Mentioned

CrysAlisPro
JANA2006

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