Features of structural transitions in Ba1-xK xBiO3 (BKB) and Ba1-xRbxBiO3 (BRB) have been investiga electron diffraction. The analysis of the patterns shows that the crystal system at room temperature changes with respect by means of to x; the cubic system in x > 0.35 and the monoclinic one in 0.1 < x < 0.35. When the temperature is changed the cube to monoclinic transition takes place in 0.1 < x < 0.35. In addition, the softening of a phonon mode at the R point is observed in the cubic structure of x > 0.35.
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