Effect of annealing conditions on the physical properties of Nd<inf>0.67</inf>Sr<inf>0.33</inf>MnO<inf>3-δ</inf>

A study of the influence of annealing in atmospheres with defined oxygen partial pressures (1.5×10-6 Pa≤PO(2)≤10+5 Pa) on the electrical resistivity ρ(T), magnetic susceptibility χ(T), and heat capacity cp(T) of ceramic samples of Nd0.67Sr0.33MnO3-δ is presented. ρ(T) at 1000 °C shows no appreciable change for 10+2 Pa≤PO(2)≤10+5 Pa. For samples annealed in air a sharp peak is found in cp(T) and in ρ(T) at the ferromagnetic transition (TC≈238 K). The insulator-metal transition is observed below TC as a broad peak in ρ(T). Annealing at lower PO(2) leads to an increase of ρ whereas TC decreases only by ≈2 K. Annealing at PO(2)≤6.3×10-2 Pa suppresses TC and results in spin canting and a vanishing of the peak in cp(T). The specific heat (2-300 K) for samples annealed in air and in Ar/5% H2 is determined and thermodynamic standard values are calculated. The problem of the separation of cp(T) into lattice, electronic, and Mn and Nd magnetic contributions is discussed. cp(T<20 K) is dominated by a Schottky-like anomaly from the Zeeman-split ground-state doublet of Nd3+. A large quasi-linear term is mimicked by a broadening of the Schottky-like anomaly and by magnetic contributions from Mn in oxygen-deficient samples. The results are discussed in the context of oxygen deficiency δ, carrier concentration, defect chemistry, and disorder.