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The structure of the (-phase in the Pu-U system has been determined by neutron powder diffraction. The phase crystallizes in space group R3m with 58 atoms in the primitive unit cell and 10 atoms in the asymmetric unit. The structure is characterized by many short bonds and fits the general pattern of the light actinides. Thermal expansion and elastic data were obtained from the diffraction experiments.

We have determined some of the phonon spectral parameters for Pu and Ga in the ''stabilized'' ␦ phase ͑fcc͒ of the Pu-3.6 at. % Ga alloy by measuring the Doppler broadening of neutron resonances as a function of temperature. We find the mean phonon energy ͑first moment of the phonon spectrum͒ of Pu to be ͗h͘ ϭ8.22Ϯ0.12 meV and the second moment to be ͗(h) 2 ͘ϭ(7.2Ϯ0.3)ϫ10 Ϫ5 eV 2 . These moments and the value of the ͑Ϫ1͒st moment determined from Debye-Waller factors from neutron powder diffraction are consistent with a Debye model with a Debye temperature of 127.2Ϯ1.7 K. For Ga in the Pu-Ga alloy, ͗h͘ ϭ16.3Ϯ1.4 meV, which is slightly larger than the m 1/2 -weighted value of 15.3 meV expected from the value for Pu ͑however, the measured value cannot be distinguished from the latter value within experimental error͒. These results show that Pu-3.6 at. % Ga behaves very much like a Debye solid at ambient pressures and low temperature and that the Ga impurity experiences approximately the same ͑or a slightly stiffer͒ force field compared with the Pu it replaces.

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