Kwang Hyun Ryu

Influence of the technological conditions on the luminescence of Eu3+ ions in Sr2SnO4

Different synthesis conditions have been used to prepare the stoichiometric compounds of the Sr2−xEuxSnO4 with x=0.00, 0.01, 0.02 and 0.03. As the 5D0→7F0 transition is forbidden by a selection rule of the Judd–Ofelt theory, the J=0→J=0 level gains some intensity from the mixing of J states in the Sr2−xEuxKxSnO4 and Sr1−xEuxSnO4 samples. For Sr2−xEuxSnO4 samples prepared under a slightly reducing atmosphere, the J=0→J=0 transition is split into two peaks, which confirms the substitution of Eu3+ ions for different crystallographic sites.

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.

Structural, magnetic, and electrical properties of nonstoichiometric perovskite Ho1−xCaxCoO3−y system

Abstract A series of samples of the Ho 1− x Ca x CoO 3− y ( x = 0.00, 0.15,0.25, 0.75 and 1.00) system have been prepared by the dripping pyrolysis method at 1000 °C under an atmospheric pressure. The samples for the compositions of x = 0.00 to x = 0.25 are identified as GdFeO 3 -type structure with a space group Pbnm . When Ca 2+ (r o = 1.34 A ) ions are substituted with Ho 3+ ( r 0 = 1.12 A ) ions, structural distortions decrease. All samples exhibit paramagnetic behaviour in the temperature range from 300 to 900 K. There is not any spin state change of Co 3+ ions in that temperature range. Paramagnetic Curie temperatures have negative values which represent an antiferromagnetic interaction in the Co 3+  O 2− Co 3+ bond. The samples of x = 0.00,0.15 and 0.25 contain high spins Co 3+ with increasing order. All the compositions of Ho 1− x ,Ca x CoO 3− y are included in the semi-conducting range. The composition of x = 0.15 with the highest amount of Co 4+ ions shows the highest conductivity. The collective Co 4+ ions favor low spin states which act as positive holes. The linearity in the CoOCo bond enhances covalency that can decrease the band gap between α ∗ and π ∗ bands. Activation energies for the electrical conduction of the samples are relatively higher than those of other cobaltates due to small size of A-site ions. The small size ions of A-site broaden the band gap as a result of the competing σ bond of CoA and π bond of CoO 2− . The transition of semi-conductor to metal above 1000 K represents the high order transition due to the collective σ ∗ and π ∗ bands overlapping.