Local structure and vibrational properties of α-Pu, α-U, and the α-U charge-density wave -: art. no. 184113

The local atomic environment and vibrational properties of atoms in monoclinic pure a-plutonium as well as orthorhombic pure alpha-uranium and its low-temperature charge-density-wave (CDW) modulation are examined by extended x-ray absorption fine structure spectroscopy (EXAFS). Pu L-III-edge and U L-III-edge EXAFS data measured at low temperatures verify the crystal structures of alpha-U and alpha-Pu samples previously determined by x-ray diffraction and neutron scattering. Debye-Waller factors from temperature-dependent EXAFS measurements are fit with a correlated Debye model. The observed Pu-Pu bond correlated Debye temperature of theta(cD)(alpha-Pu)=162 +/- 5 K for the pure alpha-Pu phase agrees with our previous measurement of the correlated Debye temperature of the gallium-containing alpha'-Pu phase in a mixed phase 1.9 at. % Ga-doped alpha'-Pu/delta-Pu alloy. The temperature dependence of the U-U nearest neighbor Debye-Waller factor exhibits a sharp discontinuity in slope near T-CDW=43 K, the transition temperature at which the charge-density wave (CDW) in alpha-U condenses from a soft phonon mode along the (100) direction. Our measurement of the CDW using EXAFS is the first observation of the structure of the CDW in polycrystalline alpha-U. The different temperature dependence of the Debye-Waller factor for TT-CDW, the observed correlated Debye temperature of theta(cD)(alpha-U)=199 +/- 3 K is in good agreement with other measurements of the Debye temperature for polycrystalline alpha-U. CDW structural models fit to the alpha-U EXAFS data support a squared CDW at the lowest temperatures, with a displacement amplitude of epsilon=0.05 +/- 0.02 angstrom.