Investigation of the neptunium-zirconium system by X-ray diffraction

X-ray powder diffraction analysis at 25 °C has been used to further characterize the NpZr phase diagram. We have studied a variety of NpZr aggregate compositions obtained by different preparative methods, including arc melting of the components. For the materials at ambient temperature, obtained by either slow cooling or quenching from elevated temperatures, we observed one or more of the following product phases: α-Np, α-Zr, NpZr2 and one new phase. The last was interpreted tentatively to be either Np4Zr or Np6Zr. Calculations based on the X-ray data suggested a limited solubility of Np in α-Zr and an even lower solubility of Zr in α-Np. The X-ray data for the NpZr2 phase showed that it possesses hexagonal symmetry and is isostructural with UZr2 and PuZr2. Annealing of NpZr composites at selected temperatures did not enhance the formation of NpZr2.