A THEORETICAL-STUDY OF THE CRYSTALLOGRAPHIC STRUCTURES IN NEPTUNIUM

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