Structural and magnetic ordering in La(0.5)Ca(0.5-x)Ba(x)MnO(3) (0 < x ≤ 0.5) manganite.

We report a structural, magnetic and transport study in La(0.5)Ca(0.5-x)Ba(x)MnO(3) (0<x≤0.5) using neutron diffraction, magnetization and resistivity measurements. The samples crystallize in an orthorhombic structure with Pnma space group and no structural transition is observed with Ba doping. Substituting Ca with Ba leads to an increase in average A-site ionic radii, ⟨r(A)⟩ and cation size disorder (σ(2)). Consequently, the nature of magnetic ordering undergoes changes as a function of x. At low temperature for x≤0.1, the CE-type antiferromagnetic spin structure is stabilized, similar to the parent compound. At x = 0.15, the magnetic phase is a mixture of CE-type and A-type spin structure. For x≥0.2, the ferromagnetic phase is stabilized. The signature of orbital ordering in addition to spin and charge ordering is observed in all samples with x≤0.15. T(C), T(CO) and T(N) remain nearly constant for x≤0.2. The transport studies reveal that for samples x≤0.2, adiabatic hopping of small polarons governs the transport behaviour above T(CO) (∼160 K). A parallel combination of small polaron hopping and variable range hopping mechanisms is found to describe resistivity data below T(CO). At x = 0.2, an insulator to metal transition is observed as temperature is lowered, and at x = 0.5 the sample becomes metallic below 280 K. In the metallic regime, resistivity data follows the Bloch-Grüneisen model together with contributions from high frequency optical phonons and magnetic scattering. The experimentally established phase diagram from the present study is compared with theoretical studies in the intermediate electron-phonon interaction regime.