Incommensurate/commensurate charge-density-wave states as a source for plutonium metal behavior

Abstract

Physical property behavior of plutonium (Pu) metal phases is like that of an incommensurate charge-density wave (ICDW) system where the CDW influenced distortion modulates the crystal. As incommensurates, the different Pu phases may have to be considered as superspace group structures where there is a one-dimensional modulation of the basic three-dimensional lattice. Certain Pu phases may then be classified into as many as three Bravais classes when considered in (3 + 1) dimensional space. The possible variants in Bravais class, crystals setting and bottom lines, as well as allowable differences in the number of atoms per unit cell, should permit incommensurate materials, as well as Pu phases, to appear in different variants of the basic space group structure on heating and cooling cycles. One should not expect the lower temperature phases, e.g., Pu, to return to their original distorted or modulated structures at constant rate cooling, after being distorted or modulated by CDWs in their higher temperature space group structures. This can explain the hysteresis in phase transitions noted with Pu metal and with incommensurate materials in general.

Chiral symmetry appears to be inherent to the incommensurability of a quasi-one dimensional system. All but one of the reported space group structures for Pu phases have at least a one-dimensional twofold screw axis with a center of symmetry, i.e., they show chiral symmetry. A theory suggests that chiral symmetry must permit the contraction in one or more dimensions noted with most incommensurate materials, as well as with Pu phases.

It is suggested that there is another ICDW Pu phase (αI) below ∼ 60 K, and that the γ-Pu phase (Fddd) must be a composite structure. Other Pu phases appear to be composite structures also. There is evidence for a new phase, or phase mixture, which appears reproducibly between the δ and γ phases only on a cooling cycle. It is infered that this is a reappearance of the δ' phase.

Published dilatometry, internal friction and relative shear modulus results appear to confirm both incommensurate and commensurate CDW states in Pu metal phases. It is suggested that CDWs may be playing a role in f-bonding in Pu metal and that CDWs and valence fluctuations may be manifestations of the same electronic behavior.

Key Words:

• Plutonium
• chiral symmetry
• CDW