Magnetic, electrical, and thermal characterization of La0.9Te0.1Mn1−xCoxO3(0 ≤x≤ 1)

Abstract

The structure, magnetizationM, resistivity ρ, thermoelectric powerS, and thermal conductivity κ in La0.9Te0.1Mn1−xCoxO3(0 ≤x≤ 1) have been investigated systematically. The samples withx= 0 andx= 1 have a rhombohedral lattice with space groupR¯3C, while the samples withx= 0.25, 0.50, and 0.75 have an orthorhombic lattice with space groupPbnm. The samples of 0 ≤x≤ 0.75 undergo the paramagnetic–ferromagnetic (PM–FM) phase transition. Based on the temperature dependence of susceptibility, a combination of the high-spin (HS) state for Co2+and the low-spin (LS) state for Co3+can be determined. The metal–insulator transitions (MIT) observed forx= 0 sample are completely suppressed with Co-doping, and ρ(T) displays semiconducting behavior within the measured temperature region forx> 0 samples. Asx⩾ 0.25, the huge magnitude of Seebeck coefficient at low temperatures is observed, which is suggested to originate from the spin-state transition of Co3+ions from intermediate-spin (IS) state or (HS) state to (LS) state and the configurational entropy of charge carriers enhanced by their spin and orbital degeneracy between Co2+and Co3+sites. Particularly,S(T) ofx= 0.50 and 0.75 samples appears an anomalous peak, which is suggested to be related to the contribution of phonon drag. Similar toM(T) and ρ(T), all results ofS(T) are discussed according to the variations of the structure parameters and magnetic exchange interaction caused by Co-doping. In addition, based on the analysis of the temperature dependence ofS(T) and ρ(T), the transport mechanism can be determined in the different temperature region. As to thermal conduction κ(T), the changes of κ with Co-doping is suggested to come from the combined effect due to the suppression of local Mn3+O6Jahn–Teller (JT) lattice distortion because of the substitution of non JT Co3+ions with LS and HS states for JT Mn3+ions, which results in the increase of κ, and the introduction of the disorder due to Co-doping, which contributes to the decrease of κ.