Theoretical zero-temperature phase diagram for neptunium metal

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Abstract
Electronic structure calculations, based on the density-functional theory with the generalized gradient approximation to the exchange and correlation energy, are used to study the crystallographic properties of neptunium metal under compression. Calculated ground-state properties, such as crystal structure and atomic volume, are found to be in excellent agreement with experiment. The calculated bulk modulus and the first pressure derivative of the bulk modulus are also in accordance with experiment. Our theory predicts that neptunium at low temperature undergoes two crystallographic phase transitions upon compression. First a transition α-Np→β-Np at 0.14 Mbar and second a β-Np→bcc phase transition at 0.57 Mbar. These transitions are accompanied by small volume collapses of the order of 2–3 %. The high-pressure phase (bcc) is also investigated theoretically up to 10 Mbar. A canonical theory for the f-electron contribution to the hcp, fcc, and bcc structures is presented.