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 La1−xNaxMnO3+y and La1−xBaxMnO3. In particular, the ρ(T) dependences show the shallow minimum at Tmin∼35K and ∼40K for Na- and Ba-doped samples, respectively. Application of the external H up to 8T leads to marked flattening of such minimum and to notable lowering of the Tmin 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 5–50K. 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.