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

A series of samples of the Ho1-xCaxCoO3-y (x = 0.00, 0.15, 0.25, 0.75 and 1.00) system have been prepared by the dripping pyrolysis method at 1000 degrees C under an atmospheric pressure. The samples for the compositions of x = 0.00 to x = 0.25 are identified as GdFeO3-type structure with a space group Pbnm. When Ca2+ (r(o) = 1.34 Angstrom) ions are substituted with Ho3+ (r(o) = 1.12 Angstrom) 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 Co3+ ions in that temperature range. Paramagnetic Curie temperatures have negative values which represent an antiferromagnetic interaction in the Co3+ - O2- - Co3+ bond. The samples of x = 0.00, 0.15 and 0.25 contain high spins Co3+ with increasing order. All the compositions of Ho1-xCaxCoO3-y are included in the semi-conducting range. The composition of x = 0.15 with the highest amount of Co4+ ions shows the highest conductivity. The collective Co4+ 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 sigma* and pi* 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 sigma bond of Co-A and pi bond of Co-O2-. The transition of semi-conductor to metal above 1000 K represents the high order transition due to the collective sigma* and pi* bands overlapping. (C) 1997 Elsevier Science Ltd. All rights reserved.