Magnetization and heat capacity of R1/3Sr2/3FeO3-delta (R = La and Pr)

Results of magnetization and heat capacity (C-P) measurements are presented for R1/3Sr2/3FeO3-delta compounds with R = La and Pr. The change in entrop y (DeltaS) near the magnetic transition amounts to only 15% of the theoreti cally expected value for the Fe3+ or Fe5+ spins magnetic ordering. The lowe r value of DeltaS is explained on the basis of reported Mossbauer studies, which revealed a substantial amount of nonmagnetic Fe4+ in the compound acr oss the magnetic transition. The C-P peak height across the magnetic transi tion increases substantially after annealing the samples in flowing oxygen. The symmetry in the C-P transitions points toward a structural phase trans ition coupled with magnetic ordering. The magnitude of DeltaS near the magn etic transition in these compounds depends on the synthesis procedure.