Lattice distortions, magnetoresistance and hopping conductivity in LaMnO₃₊δ=Искажение решетки магнитосопротивлением и прыжковая проводимость в LaMnO₃₊δ

Abstract

Structural and transport properties of ceramic LaMnO₃₊δ are investigated for δ = 0-0.154. According to x-ray diffraction measurements at room temperature the crystal structure of this compound varies from orthorhombic (Pbnm) for δ= 0 to rhombohedrally distorted cubic (Pm3m) for δ= 0.065-0.112 and to rhombohedral (R3c) crystal symmetry for δ= 0.125-0.154. These structural modifications are confirmed by the Raman micro-spectroscopy measurements. The resistivity displays in the range δ = 0-0.154 an activated behaviour both above and below the paramagnetic (PM) to ferromagnetic transition temperature, TC. In the field of 8T the relative magnetoresistance, Δρ(B)/ρ(0), reaches at δ= 0.154 the values of -88% near TC and -98% at T ≈20 K. The resistivity of the PM phase of LaMnO3+δ with δ= 0.100-0.154 satisfies the Shklovskii–Efros-like variablerange hopping (VRH) conductivity law between TC ~ 130-160 K and the VRH onset temperature Tv ~ 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 Δ ≈ 0.43-0.48 eV and a rigid gap γ ≈ 0.14-0.17 eV, the latter being connected to formation of small polarons