Lattice distortions, magnetoresistance and hopping conductivity in LaMnO3+δ

Structural and transport properties of ceramic LaMnO3+delta are investigated for delta = 0-0.154. According to x-ray diffraction measurements at room temperature the crystal structure of this compound varies from orthorhombic (Pbnm) for delta = 0 to rhombohedrally distorted cubic (Pm3m) for delta = 0.065-0.112 and to rhombohedral (R (3) over barc) crystal symmetry for 8 = 0.125-0.154. These structural modifications are confirmed by the Raman micro-spectroscopy measurements. The resistivity displays in the range 6 = 0-0.154 an activated behaviour both above and below the paramagnetic (PM) to ferromagnetic transition temperature, T-C. In the field of 8 T the relative magnetoresistance, Deltarho(B)/rho(0), reaches at delta = 0.154 the values of -88% near T-C and -98% at T approximate to 20 K. The resistivity of the PM phase of LaMnO3+delta with delta = 0.100-0.154 satisfies the Shklovskii-Efros-like variable-range hopping (VRH) conductivity law between T-C similar to 130-160 K and the VRH onset temperature T-v similar to 250-270 K. The resistivity is governed by a complex energy dependence of the density of the localized states near the Fermi level, comprising a soft Coulomb gap Delta approximate to 0.43-0.48 eV and a rigid gap gamma approximate to 0.14-0.17 eV, the latter being connected to formation of small polarons.