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Evolution of Disordering in SiC upon Sintering; Phase Analysis of SiC by Rietveld Method with Application of Neutron and X-Ray Diffraction

100%

Pałosz B., Boysen H., Schneider J., Schulz H.

Acta Physica Polonica A

1993

vol. 83

issue 1

95-106

Neutron and X-ray diffraction patterns of α and β powders as well as of sintered SiC were analysed by a multiphase Rietveld method. It is shown that structural models combined of large period polytypes can be used to approximate the disordering of these polytype structures. The hexagonality of the samples could be terminated with reproducibility 1-2% using different combinations of large-period polytypes. It follows that the usual classification into α and β SiC is an oversimplification. The polytype behaviour of SiC powders and the role of twinning of cubic layer stackings is discussed. Distribution functions of stacking sequences of different length in α and β phases are derived.

Disorder and Diffusion in Mayenite

76%

Boysen H., Kaiser-Bischoff I., Lerch M., Berendts S., Hoelzel M., Senyshyn A.

issue 1

38-41

Mayenite, Ca_{12}Al_{14}O_{33}, has attracted enormous attention for novel technological applications after the discovery of its high oxygen ionic conductivity. The crystal structure consists of a calcium-aluminate framework, comprising 32 of the 33 oxygen anions. The remaining oxygen is distributed over 1/6 of large cages within the framework. The true structure is heavily disordered and usually non-stoichiometric due to the presence of extra anions and is presented for four samples: pure oxygen mayenite (O-mayenite), partly (O/N-mayenite) and fully (N-mayenite) exchanged by nitrogen and doped with iron (Fe-mayenite). All samples were investigated by neutron powder diffraction up to 1050°C. Data were analysed by the Rietveld method and by difference Fourier methods. As prepared O-mayenite contains O_{2}^{-} and OH^{-}, N- and O/N-mayenite also NH_{2}^{-}, NH^{2-} and N^{3-}, while Fe-mayenite was free of extra radicals. In O- and N-mayenite the extra species are lost under vacuum conditions above ca. 700°C, whereas O/N-mayenite retained NH_{2}^{-} up to high temperatures. Diffusion of oxygen proceeds via a jump-like interstitialcy process involving exchange of the "free" oxygen with framework oxygen, coupled to relaxations of Ca ions. In O/N-mayenite NH_{2}^{-} diffuses via interstitial process. In Fe-mayenite encaged oxygen is "invisible" due to extreme delocalisation or loss.

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2 Acta Physica Polonica A

1 2010

1 1993

2 Boysen H.

1 Berendts S.

1 Hoelzel M.

1 Kaiser-Bischoff I.

1 Lerch M.

1 Pałosz B.

1 Schneider J.

1 Schulz H.

1 Senyshyn A.

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Evolution of Disordering in SiC upon Sintering; Phase Analysis of SiC by Rietveld Method with Application of Neutron and X-Ray Diffraction

Disorder and Diffusion in Mayenite