Invar effect in Pu-Ga alloys

Abstract

A bstractWe show that the anomalous thermal expansion behaviour of Pu±Ga alloys can be explained by an Invar e ect, which is modelled by assuming the thermal transfer of Pu atoms from a lower-energy higher-volume d 1 state to a higherenergy lower-volume d 2 state. The energy separation of these states is found to be 1400 K from a ®t to the available neutron di raction data on the lattice constants of Pu±Ga alloys. The Invar model enables a precise evaluation of the coe cient of thermal expansion and of the GruÈ neisen constant. The addition of Ga forces a stabilization of the d 2 state and suppresses the negative thermal expansion.

Lattice constants and anisotropic microstrain at low temperature in242Pu–Ga alloys

“…The samples were the same as those used in earlier investigations [8]. The isotopic constitution of the 95% 242 Pu is shown in table 1 compared with that of 'ordinary' 239 Pu.…”

Accelerated dynamics study of vacancy mobility in -plutonium

“…This actually corresponds to the lattice constant of real Pu at a temperature between 650 and 700 K. However, we note that around 2 at.% Ga, the thermal expansion coefficient of ␦ −Pu is nearly zero over its temperature range of stability [14]. If Pu I is allowed to relax isotropically (i.e., all 3 lattice constants are constrained to be equal), the volume is reduced by about 10-15%.…”

Applying the two-state invar model to delta-phase plutonium

“…A complete model for the thermal expansion must include the simultaneous tendencies for expansion and contraction. Details of such a model are given in the literature, 12 and it succeeds in fi tting the observed thermal expansion, as shown in Figure 2.…”

“…12 An energy-level diagram is shown in Figure 1. This energy scheme leads to a tendency for negative thermal expansion as more of the atoms will occupy the higher-energy, smallerradius states at higher temperature.…”