A neptunium analog of the LaFeAsO tetragonal layered compound has been synthesized and characterized by a variety of experimental techniques. The occurrence of long-range magnetic order below a critical temperature T N = 57 K is suggested by anomalies in the temperature-dependent magnetic susceptibility, electrical resistivity, Hall coefficient, and specific-heat curves. Below T N , powder neutron diffraction measurements reveal an antiferromagnetic structure of the Np sublattice, with an ordered magnetic moment of 1.70 ± 0.07μ B aligned along the crystallographic c axis. No magnetic order has been observed on the Fe sublattice, setting an upper limit of about 0.3μ B for the ordered magnetic moment on the iron. High-resolution x-ray powder diffraction measurements exclude the occurrence of lattice transformations down to 5 K, in sharp contrast to the observation of a tetragonal-to-orthorhombic distortion in the rare-earth analogs, which has been associated with the stabilization of a spin-density wave on the iron sublattice. Instead, a significant expansion of the NpFeAsO lattice parameters is observed with decreasing temperature below T N , corresponding to a relative volume change of about 0.2% and to an Invar behavior between 5 and 20 K. First-principles electronic structure calculations based on the local spin density plus Coulomb interaction and the local density plus Hubbard-I approximations provide results in good agreement with the experimental findings.

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Nanoscopic metal fluorides with surface hydroxy groups are of broad interest in heterogeneous catalysis. With both Lewis and Brønsted acid sites on the surface, these catalysts can be applied to a wide range of reactions. Having previously synthesised AlF3‐ and MgF2‐based catalysts, we report a new transition metal fluoride with bi‐acidity. A pre‐dehydration procedure was developed to introduce hydroxy groups to a Lewis acid, FeF3. Subsequently, ternary nanoscopic FeF3‐MgF2 with enhanced porosity was prepared through a one‐step fluorination. The interaction between MgF2 and FeF3 was elucidated. Surface characterisation revealed a remarkable increase in the surface area of FeF3‐MgF2 compared with FeF3. More importantly, medium–strong Lewis and Brønsted acid sites were detected on the FeF3‐MgF2 surface. In line with its bi‐acidity, FeF3‐MgF2 was highly active in the model reaction, the isomerisation of citronellal to isopulegol. Finally, we discuss how the porosity and surface acidity jointly determine the activity of FeF3‐MgF2. Our study demonstrates the feasibility of ternary FeF3‐MgF2 and opens new possibilities to synthesise bi‐acidic fluoride catalysts.

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The magnetic properties and 57Fe Mössbauer spectra of the compounds YFe11TiHx (x = 0, 1) were investigated. The magnetocrystalline anisotropy of YFe11Ti and its hydride was studied by analysing the hard and easy magnetization curves of single crystal samples in the temperature range 4.2–300 K. The spontaneous magnetization was determined in wide temperature range 4.2–650 K. It is established that at T = 4.2 K magnetic anisotropy constants K1 and K2 for YFe11TiH1 single crystal reach values of 25.8 K f.u.−1 and 0.24 K f.u.−1, respectively. The Mössbauer effect spectra of YFe11Ti and its hydride were analysed in terms of a model which takes into account the local environment of Fe atoms on three crystallographic sites (8f, 8j and 8i) and an influence of the random distribution of titanium on the 8i site. Upon hydrogenation both the hyperfine fields and the isomer shifts increase. These results are discussed in terms of the hydrogen-induced unit cell expansion and the electron charge transfer from the conduction band onto the H atoms.