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A theoretical study of the crystallographic structures in neptunium

The total energy has been calculated as a function of volume for various crystal structures in neptunium metal. Around the equilibrium volume the experimentally observed low-temperature structure, alpha -Np, is found to have the lowest energy. As a function of compression we predict Np to undergo a crystallographic phase transition from alpha -Np to sec. The zero-temperature calculations also show that at volumes close to the alpha -Np to BCC transition the BCT structure is very close in energy, and high-pressure experiments performed at elevated temperatures on Np may allow observation of this structure as well.