Negative thermal expansion and antiferromagnetism in the actinide oxypnictide NpFeAsO

Abstract

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 ${T}_{\mathrm{N}}$ $=$ 57 K is suggested by anomalies in the temperature-dependent magnetic susceptibility, electrical resistivity, Hall coefficient, and specific-heat curves. Below ${T}_{\mathrm{N}}$, powder neutron diffraction measurements reveal an antiferromagnetic structure of the Np sublattice, with an ordered magnetic moment of 1.70 $\ifmmode\pm\else\textpm\fi{}$ 0.07${\ensuremath{\mu}}_{B}$ aligned along the crystallographic $c$ axis. No magnetic order has been observed on the Fe sublattice, setting an upper limit of about 0.3${\ensuremath{\mu}}_{B}$ 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 ${T}_{\mathrm{N}}$, 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.