Comment on "spin-cluster effect and lattice-deformation-induced Kondo effect, spin-glass freezing, and strong phonon scattering in La _0.7Ca_0.3Mn_1-xCr_xO₃ " [J. Appl. Phys. 97, 103908 (2005)]

In a recent paper Wu [J. Appl. Phys. 97, 103908 (2005)] reported on characteristic features of the temperature (T) dependence of electrical resistivity (ρ) in polycrystalline La0.7 Ca0.3 Mn1-x Crx O3 (x=0.05 and 0.1) manganites. In particular, a shallow minimum at T<50 K and coexisting two maxima were observed in ρ (T) curves. Wu claimed that these features appear due to the single-impurity Kondo effect and its competition with the Ruderman-Kittel-Kasuya-Yosida interaction. In this Comment it is shown that such interpretation is very questionable and that a much simpler reason for the appearance of both, the low temperature minimum and splitted maximum on ρ (T) curve, is the well known grain boundary effects.