Structural changes and microstructures in stuffed tridymite-type compounds Ba1−xSrxAl2O4

Crystal structures and microstructures in Ba 1− x Sr x Al 2 O 4 solid solutions between the end members of BaAl 2 O 4 and SrAl 2 O 4 have been carefully investigated by powder X-ray diffraction, electron diffraction and transmission electron microscopy (TEM) imaging experiments. With the help of fast Fourier transform (FFT) calculation, high-resolution TEM images suggested that diffuse streaks along three equivalent 〈110〉 directions in the (001) plane, which appear in the P 6 3 structure of Ba 1− x Sr x Al 2 O 4 for x = 0.4, originate from the large structural fluctuation of the AlO 4 tetrahedral network. On the other hand, the monoclinic P 2 1 structure in Ba 1− x Sr x Al 2 O 4 with x = 0.7 was found to consist of a modulated structure with . The present experimental results reveal that a structural phase boundary exists at approximately x = 0.6 between the P 6 3 structure with a large structural fluctuation and a monoclinic P 2 1 phase with the single- q modulated structure.