A STUDY OF THE NONSTOICHIOMETRY AND PHYSICAL-PROPERTIES OF THE PEROVSKITE ND1-XCAXFEO3-Y SYSTEM

A series of samples of the perovskite Nd1-xCaxFeO3-y system with the c ompositions x = 0.00, 0.25, 0.50, 0.75, and 1.00 has been prepared at 1150 degrees C under atmospheric air pressure. In the perovskite syste m, the X-ray powder diffraction patterns assign the compositions x = 0 .00 and 0.25 to the orthoferrite-type orthorhombic system, the composi tion x = 0.50 to the cubic system, and the compositions x = 0.75 and 1 .00 to the brownmillerite-type orthorhombic system. The mole ratio of Fe4+ ions to total Fe ions, or tau value, has been determined and iden tified by Mohr salt titration and Mossbauer spectroscopic analysis. Ex cept for the compositions x = 0.00 and 1.00, the samples contained the mixed valence state between Fe3+ and Fe4+, and the composition x = 0. 50 had the maximum tau value. Nonstoichiometric chemical formulas of t he system are formulated from the x, tau, and y values. The Mossbauer spectrum of x = 0.50 exhibits a sharp peak due to rapid electron trans fer between the Fe3+ and Fe4+ ions. The electrical conductivity increa ses with the number of Fe4+ ions because the e(g)-electron hole of the Fe4+ ion acts as a positive hole. Since the Fe4+ ions decrease the ma gnitude of the antiferromagnetic interaction of Fe3+ -O2- -Fe3+, the N eel temperature of the system decreases with increasing values of tau. (C) 1995 Academic Press, Inc.