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, $T_N=241.2\mathrm{}$ K) for all Bragg reflections with $\frac{sin\theta }{\lambda }=\frac{\kappa }{4\pi }<0.811\mathrm{}$ ${\mathrm{\AA }}^{-1\mathrm{}}$. 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 $5f^3$ ionic state (${\mathrm{U}}^{3+}$) with a ${\Gamma }_8^{\mathrm{}\left(1\right)\mathrm{}}$ 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 $4f^3$ neodymium compounds. By measuring the temperature dependence of the magnetic scattering as a function of $\overrightarrow{\kappa }$ the scattering vector, we have been able to estimate the magnitude of the crystal-field parameters as $V_4\approx -300\mathrm{}$ K, and $V_6\approx -15\mathrm{}$ K. The implications of this first unambiguous identification of the electronic ground state in a metallic actinide compound are discussed.