Comment on 'Structural properties and electrical resistivity of Na-substituted lanthanum manganites: La _1-x Na _x MnO _3+y (x = 0.1, 0.125 and 0.15)' and 'Structure and electrical resistivity of La _1-x Ba _x MnO ₃ (0. 25 ≤ x ≤ 0.35) perovskites'

In the recent papers (Varshney et al. [1] and Mansuri et al. [2]) the authors reported data on structure and electrical transport, i.e., resistivity (ρ) versus temperature (T) and magnetic field (H) on polycrystalline manganite systems La _1-x Na _x MnO _3+y and La _1-x Ba _x MnO ₃ . In particular, the ρ(T) dependences show the shallow minimum at T _min ∼ 35 K and ∼40 K for Na- and Ba-doped samples, respectively. Application of the external H up to 8 T leads to marked flattening of such minimum and to notable lowering of the T _min on ρ(T,H) curves. The authors attributed this low temperature anomaly to competitive quantum Kondo effect and three-dimensional electron-electron interaction, using the results of numerical fittings of ρ versus T and H dependences in the interval of about 5-50 K. It is suggested in the comment that such a claim seems to be doubtful due to the fundamental inapplicability of this approach for analysis of low temperature conductivity in polycrystalline manganites. It is noted that a natural reason for the both, the low temperature minimum on ρ(T) curve and its evolution upon magnetic field application, is the well known grain boundary effects.