Neutron diffraction study of USb: The ordered state

Abstract

The elastic magnetic neutron cross section from a single crystal of USb has been measured in the antiferromagnetic state (type-I ordering, ${T}_{N}=241.2$ K) for all Bragg reflections with $\frac{sin\ensuremath{\theta}}{\ensuremath{\lambda}}=\frac{\ensuremath{\kappa}}{4\ensuremath{\pi}}<0.811$ ${\mathrm{\AA{}}}^{\ensuremath{-}1}$. By using the tensor-operator method, we have calculated the theoretical cross sections from a number of possible ground-state configurations and compared them with experiment. Excellent agreement is obtained for one model only; a $5{f}^{3}$ ionic state (${\mathrm{U}}^{3+}$) with a ${\ensuremath{\Gamma}}_{8}^{(1)}$ crystal-field ground state. Such a crystal-field ground state implies that the fourth-order crystal-field potential ${V}_{4}$ is negative, which is the opposite sign to that found in the analogous $4{f}^{3}$ neodymium compounds. By measuring the temperature dependence of the magnetic scattering as a function of $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\kappa}}$ the scattering vector, we have been able to estimate the magnitude of the crystal-field parameters as ${V}_{4}\ensuremath{\approx}\ensuremath{-}300$ K, and ${V}_{6}\ensuremath{\approx}\ensuremath{-}15$ K. The implications of this first unambiguous identification of the electronic ground state in a metallic actinide compound are discussed.