Brownmillerite Ca₁-xSrxFeO2.5(x=0, 0.3, 0.5, 0.7, 1.0)의 결정학적 및 자기적 성질에 관한 연구

다국어 초록 (Multilingual Abstract)

Crystallographic and magnetic properties for Brownmillerite-type oxides Ca_(1-x)Sr_xFeO_(2.5)(x = 0, 0.3, 0.5, 0.7, 1.0) were investigated using x-ray diffraction (XRD) and Mossbauer spectroscopy. Polycrystalline samples were prepared by conventional solid-state reaction method. Information on exact crystalline structures, lattice parameters, bond lengths and bond angles were obtained by refining their XRD profiles using a Rietveld method. The crystal structures were found to be all orthorhombic with space group Pcmn (x = 0, 0.3) and Icmm (x = 0.5, 0.7, 1.0) The lattice parameters increased monotonically with increasing Sr concentration. Both the tetrahedral and the octahedral sites were considerably distorted and elongated along b-axis. While bond lengths and bond angles O-Fe-O tend to increase minutely with the increase of Sr content, bond angles Fe-O-Fe decreased accordingly. The Mossbauer spectra showed two sets of sharp sextets originating from ferric ions occupying the tetrahedral and the octahedral sites under the magnetic transition temperature T_N. Regardless of the compositions x, the electric quadrupole splittings were -0.3 ㎜/s and 0.4 ㎜/s for the octahedral and the tetrahedral site, respectively. Above T_N, the Mossbauer spectra showed the paramagnetic doublets whose electric quadrupole splittings were about 1.6 ㎜/s, irrespective of compositions x. T_N was found to decrease monotonically with the increase of Sr concentration. Ratios of absorption area for the two sites were almost 1:1 up to as high as 0.95T_N for all x. The result of the Debye temperature indicated that the inter-atomic binding force for the Fe atoms in the tetrahedral site was stronger than that for the octahedral site.