Negative thermal expansion and antiferromagnetism in the actinide oxypnictide NpFeAsO

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 +/- 0.07 mu(B) aligned along the crystallographic c axis. No magnetic order has been observed on the Fe sublattice, setting an upper limit of about 0.3 mu(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.