Neutron diffraction study of USb: The ordered state

The elastic magnetic neutron cross section from a single crystal of USb has been measured in the antiferromagnetic state (type-I ordering, TN=241.2 K) for all Bragg reflections with sinθλ=κ4π<0.811 ^-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 5f³ ionic state (U³⁺) with a Γ₈⁽¹⁾ crystal-field ground state. Such a crystal-field ground state implies that the fourth-order crystal-field potential V₄ is negative, which is the opposite sign to that found in the analogous 4f³ neodymium compounds. By measuring the temperature dependence of the magnetic scattering as a function of →κ the scattering vector, we have been able to estimate the magnitude of the crystal-field parameters as V₄≈-300 K, and V₆≈-15 K. The implications of this first unambiguous identification of the electronic ground state in a metallic actinide compound are discussed.