Features of structural fluctuations related to the cubic-to-orthorhombic transition in Ba1-xMxBiO3 (M=K, Rb) have been investigated in detail by means of electron diffraction. A fluctuation due to the softening of the R25 mode, which is responsible for the transition, is detected as diffuse scattering at the R point in the reciprocal space of the cubic phase. When x is reduced from x=0.5 to a transition composition in the cubic phase, the fluctuation is enhanced remarkably at lower temperatures. Because the fluctuation of the R25 mode results in those of spontaneous strains, the fluctuations of the A1g and Eg strains are understood to be enhanced near the superconducting transition.
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