Effect of cation site-disorder on the structure and magneto-transport properties of Ln5/8M3/8MnO 3 manganites

Abstract

Five members of Ln 5/8 M 3/8MnO 3 series with A-cation size variance ( σ 2 ) ranging between 3×10 −4 and 71×10 −4 Å 2, and the same A-cation size 〈 r A 〉 = 1.2025 Å , have been synthesized by the ceramic method. The five manganites are single phase and they crystallize in the Pnma perovskite superstructure. The five compositions display ferromagnetic–paramagnetic transitions at temperatures ranging between 130 and 270 K, for the highest and lowest variance sample, respectively. The samples with smaller variances show sharp magnetization transitions and the samples with the larger variances display broad transitions. These transitions have also been studied by differential scanning calorimetry, DSC, and some enthalpy changes are reported. The resistivity study indicates that all samples display the expected metal-to-insulator transitions at temperatures ranging between 140 and 270 K. The samples have been analysed at room temperature by ultra-high-resolution synchrotron powder diffraction and the structural and microstructural features are reported. Furthermore, Nd 5/8Sr 0.255Ca 0.12MnO 3 ( σ 2 = 40 × 1 0 - 4 Å 2 ) and Sm 0.225Nd 0.4Sr 0.308Ca 0.067MnO 3 ( σ 2 = 53 × 1 0 - 4 Å 2 ) samples have also been studied by synchrotron powder diffraction at 140 K, below the transition temperatures. Both samples are found to be single phase above and below the transition by ultra-high-resolution synchrotron powder diffraction. The microstructure of the samples has been investigated through Williamson–Hall plots. Sample broadenings are markedly anisotropic and strongly dominated by microstrains with average values of the Δ d/ d term close to 14×10 −4. A direct correlation is found between the microstrain values and the widths of the magnetization transitions.