  A neptunium analogue 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 analogues, 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-principle 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.
