From T ′-Type to Channel-like Anion Ordering: Structure, Thermal Stability, and Magnetic Properties of the Ruddlesden–Popper Oxyfluorides Pr2CuO3F2 and Nd2CuO3F2

Abstract

In this contribution, we report on the oxyfluorides Pr2CuO3F2 and Nd2CuO3F2. Both compounds are obtained from fluorination of the corresponding oxides Ln2CuO4 (Ln = Pr and Nd) with polytetrafluoroethylene (PTFE) as a fluorine source. The oxyfluorides show a monoclinic distorted version of the channel-like anion-ordered structure of La2NiO3F2, while both precursor oxides crystallize in the 2D-layered T′-type structure. By in situ X-ray diffraction (XRD), optimized synthesis parameters were obtained, and a one-step transition from the oxide to the oxyfluoride was identified. The absence of reaction intermediates is assumed to be related to the square-planar coordination of Cu in the oxides, which allows for the direct incorporation of fluorine on the empty apical octahedral positions. Both oxyfluorides are metastable with thermal decomposition above 470 °C as found by additional in situ XRD investigations. From magnetic susceptibility measurements, an antiferromagnetic transition with an ordering temperature TN ∼ 240 K was derived for both compounds. The presence of a magnetic hysteresis at 5 and 2 K was attributed to spin canting and the contribution of Pr3+ and Nd3+ moments and further investigated by specific heat measurements. The presented results highlight the importance of an appropriate fluorination reagent for the oxyfluoride formation, as well as the strong influence of anionic substitution on the structural and physical properties of such compounds.