Metallic and semi-conducting resistivity behaviour of La 0.7Ca 0.3? x K x MnO 3 ( x?=?0.05, 0.1) manganites

摘要

The temperature dependence of electrical resistivity, ρ , of ceramic La~(0.7)Ca~(0.3? x )K~( x )MnO~(3)( x ?=?0.05, 0.1) is investigated in metallic and semi-conducting phase. The metallic resistivity is attributed to be caused by electron–phonon, electron–electron and electron–magnon scattering. Substitutions affect average mass and ionic radii of A –site resulting in an increase in Debye temperature θ ~( D )attributed to hardening of lattice with K doping. The optical phonon modes shift gradually to lower mode frequencies leading to phonon softening. Estimated resistivity compared with reported metallic resistivity, accordingly ρ ~(diff.)?=?[ ρ ~(exp.)???{ ρ ~(0)?+? ρ ~(e?ph)(= ρ ~(ac)?+? ρ ~(op))}], infers electron–electron and electron–magnon dependence over most of the temperature range. Semi-conducting nature is discussed with variable range hopping and small polaron conduction model. The decrease in activation energies and increase in density of states at the Fermi level with enhanced Ca doping is consistently explained by cationic disorder and Mn valence.