Effect of Cr3+ substitution at Mn-site on electrical and magnetic properties of charge ordered Bi0.3Pr0.3Ca0.4MnO3 manganites

The effect of Cr3+ substitution at Mn-site on the structure, magnetic and electrical properties of Bi0.3Pr0.3Ca0.4Mn1-xCrxO3 manganites were studied by using X-ray diffraction method, AC susceptibility and electrical transport measurements. All the samples with starting composition of Bi0.3Pr0.3Ca0.4Mn1-xCrxO3 (x = 0, 0.04, 0.08, 0.1, 0.12 and 0.14) were prepared using solid state method. R versus T curve shows that x = 0 sample exhibits a strong insulating behavior while Cr3+substitution at x = 0.08 induced metal-insulator (MI) transition at MI transition temperature (TMI) of 56 K. The substitution for x = 0.08 successfully weakened the hybridization effect which may related to strong ferromagnetic (FM) interaction between Cr3+OMn3+ as well as Mn3+OMn4+. Further substitution of Cr3+ for x = 0.1 increased the TMI to 58 K but decreased to 36 K for x = 0.12. Rietveld Refinement of the X-ray diffraction data showed a decreased of unit cell volume which indicated partial substitution of Cr3+ (0.615 Å) at Mn+3(0.64 Å) site. Susceptibility, χ′ versus temperature, T measurements, showed an increase of ferromagnetic-paramagnetic, FM-PM transition temperature, TC from 66 K (x = 0.04) to 125 K (x = 0.14) which suggested that Cr3+ substitution enhanced the growth of FM phase. Fitting of the experimental data in the metallic region to scattering models suggested scattering involving a combination of electron-electron, electron-magnon, Kondo-like spin-dependent scattering, and electron-phonon interaction are responsible for the observed resistivity behavior. On the other hand, fitting in the insulating region suggests that resistivity behavior obeys the variable range hopping (VRH) model below the charge ordered temperature, TCO for (x = 0) and TMI < T < TθD/2 for x = 0.04-0.14 while at high temperatures involved small polaron hopping, SPH mechanism. Therefore, the enhanced double exchange mechanism (DE) indicates the substitution of Cr3+ increased the delocalization of charge carrier and destabilization of the CO-AFM state. It is suggested that such an increased of DE mechanism was related to the reduction of blocking mechanism which generated by the hybridization effect due to Bi 6s2 lone pair.