We have investigated the structural changes and the microstructures of Ba<inf>1-x</inf>Sr<inf>x</inf>Al<inf>2</inf>O<inf>4</inf> for 0 < x < 0.4 by using transmission electron microscope (TEM) and synchrotron radiation powder X-ray diffraction experiments. The TEM experiments revealed the existence of a structural phase boundary at approximately x = 0.1, at which the superlattice reflection spots at the 1/2 0 0 -type positions change into diffuse streaks along three equivalent <110> directions in the hexagonal structure. In addition, real-space images of Ba<inf>1-x</inf>Sr<inf>x</inf>Al<inf>2</inf>O<inf>4</inf> for 0 < x < 0.4 reveal that BaAl<inf>2</inf>O<inf>4</inf> should be characterized as a modulated structure with triple-q modulation vectors along the <110> directions and on the other hand, Ba<inf>1-x</inf>Sr<inf>x</inf>Al<inf>2</inf>O<inf>4</inf> for 0.1 < x < 0.4 be characterized as an intermediate (precursor) state with a rigid unit mode due to structural instability. These experimental results implied that the partial substitution of Sr<sup>2+</sup> for Ba<sup>2+</sup> should suppress a structural instability due to the AlO<inf>4</inf> tetrahedral network and decrease the structural phase transition temperature.
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