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Oxygen deficiency induced suppression of JT-distortion and stabilization of chargeordering in La0.2Sr0.8MnO3-d

Structural phase transition studies employing low-temperature transmission electron microscopy, and low-temperature x-ray diffraction have been carried out on nearly stoichiometric (δ=0.01) and off-stoichiometric (δ=0.12) versions of La0.2Sr0.8MnO3-δ manganite. The nearly stoichiometric La0.2Sr0.8MnO3-δ under goes a cubic to Jahn-Teller distorted C-type antiferromagnetic tetragonal phase-transition at 260K. For off-stoichiometric La0.2Sr0.8MnO3-δ the Jahn-Teller distorted tetragonal phase transition gets totally suppressed and the basic perovskite lattice remains cubic but shows a charge ordered phase. Stabilization of charge ordered phase in the absence of cooperative Jahn-Teller distortion has been attributed to coulomb-repulsion and Hunds coupling energy. The off-stoichiometric sample shows characteristically different physical properties. This has been realized through transport, magnetic and calorimetric measurements. A smooth crossover from variable range hopping transport to power-law dependence of resistivity (ρ) on temperature (T) i.e. ρ=C.T-α has been realized. This has been attributed to multistep inelastic tunneling through channels involving localized states around oxygen vacancy sites.