THE FORMATION OF NONEQUILIBRIUM MICRODOMAINS DURING THE OXIDIATION OF SR2COFEO5

The phase Sr//2CoFeO//5// plus //y has been studied by Moessbauer spectroscopy, X-ray powder diffraction, and magnetic susceptibility techniques. Quenching in air from 1200 degree C produces a partially oxidized material which contains small nonequilibrium microdomains of the brownmillerite Sr//2CoFeO//5 with excess oxygen accommodated in the domain walls. Quenching in liquid nitrogen from 1200 degree C produces stoichiometric Sr//2CoFeO//5 with a random distribution of cobalt and iron on the tetrahedral and octahedral sites. Annealing under argon at 800 degree C causes preferential occupation of the tetrahedral sites by cobalt. Both the nitrogen-quenched and the annealed sample are antiferromagnetic at room temperature.