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

Results of magnetization and heat capacity (C_P) measurements are presented for R_1/3Sr_2/3FeO_3-δ compounds with R=La and Pr. The change in entropy (ΔS) near the magnetic transition amounts to only 15% of the theoretically expected value for the Fe³⁺ or Fe⁵⁺ spins magnetic ordering. The lower value of ΔS is explained on the basis of reported Mossbauer studies, which revealed a substantial amount of nonmagnetic Fe⁴⁺ in the compound across the magnetic transition. The C_P peak height across the magnetic transition increases substantially after annealing the samples in flowing oxygen. The symmetry in the C_P transitions points toward a structural phase transition coupled with magnetic ordering. The magnitude of PS near the magnetic transition in these compounds depends on the synthesis procedure