Kwon Sun Roh

Nonstoichiometry and physical properties of the perovskite BaxLa1−xFeO3−y system

A series of samples in the BaxLa1−xFeO3−ysystem (x=0.00, 0.25, 0.50, 0.75, and 1.00) have been prepared at 1200°C under an atmospheric air pressure. The solid solutions of the system were analysed from the X-ray diffraction spectra and thermal analyses. X-ray diffraction studies assigned the compositions of the x=0.00 and 1.00 to the orthorhombic system and the compositions of the x=0.25, 0.50, and 0.75 to the cubic system. The reduced lattice volume increased with the x value in the system. The mole ratios of the Fe4+ ion in the solid solutions, or τ values, were determined by Mohr salt analyses and the non-stoichiometric chemical formulae of the system were formulated from the x, τ, and y values. From the results of the Mössbauer spectroscopy, the coordination and the magnetic property of the iron ion have been discussed. The electrical conductivities were measured as a function of temperature under atmospheric air pressure. The activation energy was minimum at the composition of x=0.50. The conduction mechanism can be explained by the hopping model between the mixed valences of the Fe3+ and Fe4+ ions.

Nonmetallic spin glass behavior by spin frustration at low temperature in nonstoichiometric BaSn1−xFexO3−y system

Abstract The solid solutions of BaSn1−xFexO3−y (x=0.00–1.00) have been synthesized and investigated as materials being expected to show the spin glass behavior by spin frustration at low temperature. The Fe ions substituted in place of Sn ions could have the mixed valence states of Fe3+ and Fe4+ ions. The formation of Fe4+ ion began at x=0.50, which has been identified by the potentiometric titration and the Mossbauer resonance spectra. Magnetic susceptibilities after the zero field cooling (ZFC) and field cooling (FC) processes have shown the irreversibility at the compositions of x=0.50, 0.75 and 1.00. This result has indicated the spin frustration between Fe3+OFe4+ ferromagnetic coupling and Fe3+OFe3+ or Fe4+OFe4+ antiferromagnetic coupling on the magnetic domain boundaries. The field dependence of magnetization at 5 K has shown that there is no net magnetization at zero field in the ZFC process; on the other hand, the remnant magnetization exists in the FC process. In the ZFC process, the magnetization can be relaxed to an equilibrium resulting in the zero magnetization. In the FC process, however, the ferromagnetic couplings incompatible with antiferromagnetic couplings are frozen on the boundaries and kept on, which shows the spin glass behavior.

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

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 Mössbauer 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.