The nonstoichiometry and physical properties of the Nd1−xCa1+xFeO4−y system

Abstract

The solid solutions of the Nd1−xCa1+xFeO4−y system for compositions ofx=0.000,0.125, 0.250,0.375, and 0.500 are prepared by drip pyrolysis. XRD analysis shows all the solid solutions are tetragonal I4/mmm. The Fe4+ ratio to the total Fe ions or τ value has a maximum for the compositionx=0.375. From the X-ray powder diffraction analysis and the Mossbauer spectroscopy, the distortion and symmetry change of oxygen octahedra of Fe ions are observed. The structural change of oxygen octahedra of Fe ions strongly affects the physical properties. The solid solution whenx=0.000 shows a weak ferromagnetic behaviour due to the spin canting of the distorted octahedra. The other solid solutions withx=0.125, 0.250, 0.375, and 0.500 show a paramagnetic behaviour over room temperature. The decrease of the magnetic transition temperature is due to the distortion of oxygen octahedra of Fe ions and the existence of the Fe4+ ion. The formation site of oxygen vacancies plays an important role in the conductivity of the Nd1−xCa1+xFeO4−y system. Although the oxygen vacancies in [Nd, Ca]-O layer have little effect on conductivity, the oxygen vacancies in the FeO2 plane of the perovskite layer act as electron trapping sites and thus increase the activation energy.