Magnetic excitations and lattice vibrations in uranium rock-salt structure compounds

The inelastic magnetic and lattice vibrational scattering of neutrons from several uranium pnictides (UN, UAs, and USb) and chalcogenides (US, UTe) have been measured in recent years. The character of the low-temperature magnetic scattering depends on the lattice constant. The compounds with the largest lattice parameters (USb, UTe) exhibit several branches of well defined magnetic excitations. The other compounds exhibit only a broad, featureless frequency response at low temperature, which may be extremely weak, as in new data presented here for US. Nonetheless, intense paramagnetic scattering is observed above the ordering temperature, with a marked 2d anisotropy in all the pnictides. A force-constant analysis of the phonon dispersion relations for these materials has been carried out. We show here that UTe, US, and UAs have negative values of the elastic constant C12, as has been found in certain mixed valence compounds. As expected from the increase in ionic size, the compressibility increases with the lattice parameter in each series; the chalcogenide compressibility is greater than that of the corresponding pnictide.