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Structural phase transitions and mixing behaviour of the Ba-aluminate (BaAl₂O₄)–Sr-aluminate (SrAl₂O₄) solid solution

¹ Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
² University of Oslo, Center for Materials Science, N-0349 Oslo, Norway
³ Department of Earth Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK

^* E-mail: mc43{at}esc.cam.ac.uk

Phase transitions in the BaAl₂O₄–SrAl₂O₄ solid solution have been analysed as a function of temperature and composition using infrared (IR) powder absorption spectroscopy. The improper ferroelectric phase transition P6₃22 P6₃ (2A superstructure) in the BaAl₂O₄ end-member can be detected through a change in slope of the wavenumbers of hard modes at ~450 K. A change in line widths at ~520 K appears to correlate with the development of diffuse intensity in a*-b* planes of electron diffraction patterns reported elsewhere in the literature. The same shift in wavenumber of hard modes is not observed in spectra from samples with compositions corresponding to 60, 80 and 90% of BaAl₂O₄ component, but a change in line widths at ~500 K has been tentatively explained in terms of a different phase transition, from a P6₃22 parent structure to a P6₃ (3A superstructure) product. Strain analysis of published high-temperature lattice parameter data suggests that the hexagonal monoclinic transition in Sr-rich members of the solid solution may consist of two discrete transitions, and a sequence P6₃22 C2 P2₁ is suggested. The second transition could be related to instabilities in the hexagonal solid solution. Autocorrelation analysis of the IR spectra reveals a large positive deviation from linear behaviour across the solid solution, which is interpreted in terms of microscopic strain effects. These microscopic strains are probably responsible for the different transformation behaviour shown by samples with different compositions across the solid solution.

KEYWORDS: phase transitions, Ba-aluminate-Sr-aluminate, IR powder absorption spectroscopy, electron diffraction