Structural, transport, thermodynamic, and neutron diffraction studies of layered R₂O₃Fe₂Se₂ (R=Ce, Pr, Nd, and Sm)

We report the structural, transport, and thermodynamic properties of the semiconducting, layered rare-earth iron oxyselenides R₂O ₃Fe₂Se₂ (R=Ce, Pr, Nd, and Sm), which have crystal structures related to the arsenide superconductors except with in-plane oxygens within the Fe square planes. The activation energies are estimated to be between 0.15 and 0.26 eV for these four compounds. The antiferromagnetic Fe sublattice ordering, which appears as an anomaly in the magnetic susceptibility and a sharp peak in the heat capacity, occurs at 87.2, 88.6, 87.7, and 85.3 K, for the Ce to Sm variants, respectively, and the structural analysis suggests a possible correlation between T_N-Fe and the Fe-Se distance, rather than the in-plane Fe-O distance. The R sublattice orders antiferromagnetically at 21.8 K for the Pr variant and 5.8 K for the Sm variant; no R sublattice ordering is observed above 2 K for R=Ce and Nd. The existence of short-range spin correlations above the Fe sublattice ordering temperature is revealed by magnetic entropy analysis, which also suggests that the ordered magnetic states of the R sublattice arise from triply degenerate crystal-electric-field (CEF) states for Pr and doubly degenerate CEF states for Sm. The neutron-scattering measurements performed on Pr₂O₃Fe₂Se ₂ reveals that the ordered Fe sublattice has the same magnetic structure as the one in La₂O₃Fe₂Se₂, with a propagation vector of k=(0.5,0,0.5) and an ordered magnetic moment of 2.23(3)μ_B/Fe at 5 K.