Comment on "Evidence for the immobile bipolaron formation in the paramagnetic state of the magnetoresistive manganites"

Recently, Zhao et al. [Phys. Rev. B 62, R11 949 (2000)] reported that bipolarons are formed in the high-temperature paramagnetic phase (above the metal-insulator transition temperature Tp) in oxygen-isotope-diffused epitaxial thin films of La0.75Ca0.25MnO3 and Nd0.75Sr0.25MnO3. But many earlier observations indicated the formation of small polarons in the said temperature range in different manganites. We have confirmed that the transport properties in the high-temperature phase (T>Tp) of the above manganites can be well explained by a small-polaron hopping mechanism and the consideration of bipolaron formation is inadequate. We have presented an explanation of the O-isotope effect on the transport properties of the same manganite sample studies by Zhao et al.