Refinement of the real structures of 2 : 1 and 3 : 2 mullite

In a previous investigation (Rahman, S. H., Strothenk, S., Paulmann, C. and Feustel, U., Interpretation of mullite real structure via inter-vacancy co rrelation vectors, J. Eur. Ceram. Soc., 1996, 16, 177-186) we showed that t he real structure of 2:1 mullite can be explained with intervacancy correla tion vectors and a 3D ordering scheme for the oxygen vacancies was derived. This ordering scheme is now refined by varying the frequencies of the most important 3D vectors in videographic 3D simulations, which are tested thro ugh comparison between their Fourier transforms with the experimental diffr action patterns for the planes h01, 0ki, hk1/2, hk1/3. hk1/4 and hk1/6. The best agreement with the experimental patterns of 2:1 mullite was reached b y a 3D simulation that can be described by the following sequence of the mo st frequent correlation vectors Imn (1= a/2, m = b/2, n = c): (310), (111), (022), (201), (330), (401), (131), (130), (042), (113), (060). The experim ental single crystal diffraction patterns of 3:2 and 2:1 mullite generally show similar diffuse scattering phenomena, though the diffuse components of 3:2 mullite are slightly broadened and have weaker relative intensities. E mploying videographic 3D simulations it was confirmed that the real structu re of 3:2 mullite can be characterized via the same correlation vectors. bu t with different sequence and frequencies: (022), (201), (111) (310), (130) (131), (223), (330) (222), (240), (113). With the aid of the above-mention ed frequencies of correlation vectors the real structures of 2:1 and 3:2 mu llite can be completely described and all the complicated diffuse scatterin g phenomena can be explained. Furthermore the dependence on direction of se veral physical properties e.g. thermal heat expansion can be interpreted by means of the vacancy distribution in the real structure. (C) 2001 Elsevier Science Ltd. All rights reserved.