Abstract :
Concentration dependent positron annihilation lifetime (PAL) measurements on Cr doped La0.5Pb0.5Mn1−yCryO3 (y=0.075, 0.15, 0.3, 0.35, 0.45) samples showing metal–insulator transitions (MIT) between 178 and 276 K (depending on y) reveal anomalous variation of average and bulk PAL, τav and τB, respectively, around y=0.35. Such anomaly has not, however, been observed from the corresponding magnetic susceptibility and resistivity data. Interestingly, the model parameters (polaron radius, number of ion sites per unit volume) obtained from fitting the high temperature (above MIT, Tp) resistivity data with small polaron hopping model show the signature of the said anomaly around the same concentration. Nonlinear variations of τav and τB support the existence of small polarons (T>Tp) which act as defect centers in such rare-earth manganites. Analysis of X-ray diffraction pattern confirms a change of lattice parameters indicating a structural transformation from rhombohedral (for y≤0.30) to orthorhombic (for y≥0.35) in the present system.
