Interplay of structure and transport properties of sodium-doped lanthanum manganite

The crystal structure, magnetic and electrical transport properties of the sodium-doped lanthanum manganites La_1-xNa_xMnO₃ (0.07⩽x⩽0.40) have been studied in detail using x-ray powder diffraction, atomic absorption spectroscopy, a SQUID (superconducting quantum interference device) magnetometer and the four-probe resistivity measurement technique. A rhombohedrally distorted perovskite structure has been observed in the range 0.07⩽x⩽0.20. Both the lattice parameter and unit-cell volume decrease with increase in the Na content. A ferromagnetic-to-paramagnetic phase transition associated with a metal-insulator transition is observed for all the La_1-xNa_xMnO₃ compounds. There is a systematic change in both the Mn-O-Mn bond angle and the tolerance factor with Na content. The compositional variation of the magnetic and metal-insulator transition temperatures is explained as due to the distortion of the MnO₆ octahedron and increase in the tolerance factor that controls the hopping interaction. In the metallic region a ρ~AT² behaviour is observed due to the magnon excitation effect. The resistivity shows a field-dependent minimum at low temperature that has been explained as due to the intergrain transport phenomenon.