Structural and physical characterization of NpPt2In7

A new neptunium intermetallic compound, NpPt2In7, has been synthesized in polycrystalline form and characterized by several macroscopic techniques. A Rietveld analysis of its powder X-ray diffraction pattern shows that NpPt2In7 crystallizes in a tetragonal lattice with I4/mmm symmetry and lattice parameters a=4.58471(3) Å, c=21.5065(3) Å. Magnetic susceptibility, electrical resistivity, Hall effect, and heat capacity measurements indicate a metallic character and the occurrence of antiferromagnetic order below a Néel temperature TN  = 23 K. The transition is exceptionally robust and TN decreases by 0.2 K under a magnetic field of 9 T. A modified Curie-Weiss fit of the high-temperature magnetic susceptibility curve χ(T) gives an effective magnetic moment μeff=2.54μB close to the value expected for trivalent Np. Low temperature heat capacity measurements give a reduced Sommerfeld linear coefficient close to 25 mJ mol−1K−2 and a Debye Temperature ΘD = 181 K. First principles, correlated-band electronic structure calculations suggest that the neptunium magnetic moment in NpPt2In7 is localized and that a quasi-two-dimensional antiferromagnetic structure could result from the competition of very weak interlayer interactions leading to very anisotropic properties.