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H. Lehnert, Hans Boysen, J. Schneider, Friedrich Frey, D. Hohlwein, P. G. Radaelli, H. Ehrenberg

A powder diffraction study of the phase transition in LaAlO₃

The structure of LaAlO₃ has been investigated around the phase transition at T_c ≈ 800 K by neutron powder diffraction in vacuum and by X-ray powder diffraction under nitrogen atmosphere as well as by a very high resolution synchrotron experiment in air. The results were analysed in frame of the Landau theory using the fluctuation-dissipation theorem to relate the susceptibility to the atomic displacement parameters. The room temperature structure is a rhombohedrally distorted perovskite structure, space group R-3c , which undergoes a transition to the ideal perovskite structure, space group Pm-3m , at high temperatures. The order parameter is a rotation of the O₆-octahedron described by one x(O)-parameter. This parameter and the spontaneous strain (c/a-√6), as well as the relevant atomic displacement parameter U^op₁₁(O) in the order parameter system, show a critical behaviour in agreement with a second order phase transition. Although the critical exponents of the order parameter and strain show the expected coupling behaviour, there is a striking difference of the transition temperature: the metric becomes cubic roughly 30 K below the proper T_c. This is related to spontaneous formation of domains imposing the average cubic symmetry via internal stresses.

Zeitschrift für Kristallographie, Oldenbourg Wissenschaftsverlag

Print ISSN: 0044-2968
Volume: 215, 09/2000
Pages: 536

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