A huge volume of research has resulted from the recent discovery of superconductivity in the RFeAsO series of compounds (where R represents a rare-earth element) characterized by its atomic-like 4f electrons. The replacement of this species by a member of the actinide series presents an exciting opportunity to study the effect of varying electron correlation. The successful substitution with actinides to form the AnFeAsO parent compounds (where An=Np,Pu) has already been reported. For both NpFeAsO and PuFeAsO materials, the anti-ferromagnetic (AF) order occurs at critical temperatures TN , 57 K (Np) and 50 K (Pu). In sharp contrast to the rare-earth analogs, no tetragonal-to-orthorhombic distortion is found. Instead, the negative thermal expansion (-0.2%) – an Invar behavior - is found in NpFeAsO below TN .
Here, we present the results of the first-principles electronic structure calculations for the actinide-based oxypnictides, NpFeAsO and PuFeAsO, and make the comparison with the bulk experimental data. We used the full-potential LAPW method (FP-LAPW) which includes all relativistic effects (scalar-relativistic and spin-orbit coupling), and relativistic version of the rotationally invariant LSDA+U method.