Physical properties and magnetic structure of the layered oxyselenide La₂ O₃ Mn₂ Se₂

We report the characterization of the layered rare-earth manganese oxyselenide La₂ O₃ Mn₂ Se₂. The susceptibility data show a broad maximum near 350 K, indicating the existence of two-dimensional (2D) short-range ordering in this compound. A sharp feature associated with a short-range to long-range antiferromagnetic phase transition is seen at 163 K. A very small heat-capacity anomaly is detected around 163 K, indicating that most of the magnetic entropy is lost during the 2D ordering process. Both crystal and magnetic structures were studied by neutron powder diffraction at 300, 200, 150, 100, and 6 K. The structure was refined in space group I4/mmm with a=4.13939 (3) Å and c=18.8511 (2) Å at ambient temperature. No structural distortion was detected. The resulted magnetic structure is G-type with a propagation vector of k= (0,0,0) and an ordered magnetic moment of 4.147 (28) μ_B /Mn along c is found at 6 K. Warren peak shape analysis of the neutron-diffraction data near 22°is employed to characterize the increase in correlation length in the 2D magnetic state on approaching the three-dimensional ordering transition.