Vibrational properties of Ga-stabilizedδ−Puby extended x-ray absorption fine structure

Abstract

Dated: Phys. Rev. B in press as of April 15, 2002) Temperature dependent extended x-ray absorption fine structure (EXAFS) spectra were measured for a 3.3 at% Ga stabilized Pu alloy over the range T = 20 -300 K. EXAFS data were acquired at both the Ga K-edge and the Pu LIII-edge. Curve-fits were performed to the first shell interactions to obtain pair-distance distribution widths, σ, as a function of temperature. The temperature dependence of σ(T ) was accurately modeled using a correlated-Debye model for the lattice vibrational properties, suggesting Debye-like behavior in this material. Using this formalism, we obtain pair-specific correlated-Debye temperatures, ΘcD, of 110.7 ±1.7 K and 202.6 ±3.7 K, for the Pu-Pu and Ga-Pu pairs, respectively. The result for the Pu-ΘcD value compares well with previous vibrational studies on δ-Pu. In addition, our results represent the first unambiguous determination of Ga-specific vibrational properties in PuGa alloys, i.e, ΘcD for the Ga-Pu pair. Because the Debye temperature can be related to a measure of the lattice stiffness, these results indicate the Ga-Pu bonds are significantly stronger than the Pu-Pu bonds. This effect has important implications for lattice stabilization mechanisms in these alloys.

Temperature-dependent defect properties from ion-irradiation in Pu(Ga)

“…The transition to the ␦-phase is at T = 593 K, while the melting point for Pu is 913 K. At two-thirds the melting point, we estimate an equilibrium vacancy population of at least ∼10 −4 . Indeed, given the low value of the EXAFS Debye-Waller factor measured for Pu-Pu bonds in stabilized ␦-PuGa [27], a higher concentration of vacancies (a lower formation enthalpy) might even be expected. Thus, it is reasonable, but remains to be proven, that a small concentration of equilibrium vacancies gives rise to a small concentration of the local spin-sites that stabilize the ␦-phase of pure plutonium.…”

“…Atomic relaxation to minimize the forces on atoms in Pu 32 and Pu 31 Ga supercells calculated with the full-potential LMTO method accounts for only 1.04% of this contraction [26]. Recently, Allan et al have measured elementally specific Debye temperatures in Pu(Ga) [27], and determined that the lattice stiffness for Pu-Ga is approximately two times greater than for Pu-Pu. The nature of the structure surrounding the Ga sites and the dynamical properties of the Pu lattice suggests that the Ga sites (along with the surrounding Pu atoms) are consistent with idea of a localized Kondo impurity.…”

The magic of plutonium: 5f electrons and phase instability

“…As mentioned previously, several investigators have probed the local structure of Pu-Ga alloys using XAFS. [52][53][54][55] These XAFS results show that the Pu-Ga bond is 0.13 Å, or 3.7 pct (the experimental measurements vary from 3.5 to 4 pct), shorter than the Pu-Pu bond in dilute alloys. Scheuer and Lengeler [65] and Massalski [31] point out that local distortions in substitutional alloys may bear little or no relation to macroscopic distortions of the unit cell.…”

“…[51] He found that dilute additions of elements such as gallium did not favor the ␦ phase even if he allowed for inward relaxation of the plutonium atoms to isolated gallium atoms, as observed experimentally using X-ray absorption fine structure (XAFS). [52][53][54][55] He suggests that the stabilization may instead result from a cooperative effect from neighboring gallium atoms, rather than the simple sum of the effects of individual gallium atoms.…”

First-principles calculations of stability of δ-Pu–Am alloys

“…These four stabilizers can be divided into two groups: (i) elements with atomic size smaller than the size of the ␦-Pu atoms (Ga and Al) and (ii) elements with atomic size larger than that of the ␦-Pu atoms (Ce and Am). Numerous experimental and theoretical studies have been performed on Pu-Ga and Pu-Al systems [2][3][4][5][6][7][8] suggesting that Ga (Al) atoms stabilize ␦-Pu through hybridization between 4p (3p) states of IIIB metal and 5f states of Pu and thus inducing delocalization of 5f (Pu) electrons. The delocalization is believed to manifest itself through the observed negative deviation from the Vegard's law in these systems due to a decrease of the ␦-Pu radius.…”