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

Abstract

Abstract Phase transitions in the BaAl 2 O 4 –SrAl 2 O 4 solid solution have been analysed as a function of temperature and composition using infrared (IR) powder absorption spectroscopy. The improper ferroelectric phase transition P 6 3 22 → P 6 3 (2A superstructure) in the BaAl 2 O 4 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 BaAl 2 O 4 component, but a change in line widths at ∽500 K has been tentatively explained in terms of a different phase transition, from a P 6 3 22 parent structure to a P 6 3 (√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 P 6 3 22 → C 2 → P 2 1 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.