Abstract
A neutron diffraction investigation ofpolycrystalline NpC has confirmed the measurements of magnetic susceptibility which suggested that the compound is antiferromagnetic between 310° and 220°K, and ferromagnetic below 220°K. Neptunium carbide has the NaCl type structure (a = 4.992 A ̊ ) ; the antiferromagnetic structure consists of ferromagnetic layers perpendicular to [001] coupled antiferromagnetically with the spin direction along the [001] axis. The change in ordered moment in the antiferromagnetic region below 310°K does not follow a Brillouin function. The maximum moment in the antiferromagnetic state is 1.3 ± 0.1 μ B and the magnetic form factor for the neptunium atom has been determined at low angles. The transition to ferromagnetism is probably first order, and the initial moment at 218°K is 1.6 μ B . The moment increases to a maximum value of 2.1 ± 0.1 μ B at ∼150°K and remains constant to 6°K.
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