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Low Temperature Crystal Structure Behaviour of Complex Yttrium Aluminium Oxides YAlO_3 and Y_3Al_5O_{12}

100%

Senyshyn A., Vasylechko L.

Acta Physica Polonica A

2013

vol. 124

issue 2

329-335

Crystal structures of two yttrium aluminium oxides, namely YAlO_3 and Y_3Al_5O_{12}, were investigated in the temperature range 3.4-300 K by high-resolution neutron powder diffraction. Neither traces of phase transformations nor discontinuous changes of physical properties were observed. Thermal expansion of yttrium aluminium oxides was evaluated in terms of 1st order Grüneisen approximation, where the Debye temperatures and the Grüneisen parameters have been estimated for both compositions. Anomalies in the thermal expansion of yttrium aluminium perovskite have been observed and modelled using the Einstein oscillator with negative Grüneisen parameter. Extended bond length analysis revealed significant thermally-driven modifications of the aluminium-oxygen framework.

Disorder and Diffusion in Mayenite

61%

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 2013

1 2010

2 Senyshyn A.

1 Berendts S.

1 Boysen H.

1 Hoelzel M.

1 Kaiser-Bischoff I.

1 Lerch M.

1 Vasylechko L.

Search results

Low Temperature Crystal Structure Behaviour of Complex Yttrium Aluminium Oxides YAlO_3 and Y_3Al_5O_{12}

Disorder and Diffusion in Mayenite