The structures of the three ferroelectric phases of BaTi03 have been determined by Rietveld refinement using powder diffraction data collected at a spallation neutron source. The correlation between refined atomic displacements and thermal parameters, which has hampered previous structure determinations, has been partially alleviated by using data which extend over a wide range of d spacings. Data collected at a large number of sample temperatures provide information about the temperature dependence of the ferroelectric displacements and changes in the oxygen octahedra which surround the Ti ions. The temperature dependence of the thermal parameters gives atomic Debye-Waller temperatures that are remarkably similar to those for the cations in the high-Tc superconductors. Our results are insensitive to predictions of a soft-mode displacive model; however, values of the anisotropic thermal parameters do not support the order-disorder model suggested by Comes et al. Powder extinction and profile coefficients from the structural refinements show pronounced minima and maxima, respectively, near the phase transitions and provide information about the temperature dependence of the mosaic structure and the strain in the

Many theories published in the last decade propose that either ordered or disordered local moments are present in elemental plutonium at low temperatures. We present new experimental data and review previous experimental results. None of the experiments provide any evidence for ordered or disordered magnetic moments (either static or dynamic) in plutonium at low temperatures, in either the α-or δ-phases. The experiments presented and discussed are magnetic susceptibility, electrical resistivity, NMR, specific heat, and both elastic and inelastic neutron scattering. Many recent calculations correctly predict experimentally observed atomic volumes, including that of δ-Pu. These calculations achieve observed densities by the localization of electrons, which then give rise to magnetic moments. However, localized magnetic moments have never been observed experimentally in Pu. A theory is needed that is in agreement with all the experimental observations. Two theories are discussed that might provide understanding of the ensemble of unusual properties of Pu, including the absence of experimental evidence for localized magnetic moments; an issue that has persisted for over 50 years.PACS index: 75; 75.25 +z; 75.20.En Paper to be submitted to Phys. Rev. B (11-Sept-04) I INTRODUCTIONIt has been known for many years that plutonium lies in the periodic table at a position where it is intermediate between itinerant-and localized-electron behavior. 1The elemental volumes of the 5f elements are shown in comparison to those of the elements in the 3d and 4f series in Fig. 1. The behavior of the early actinides (Th to Np) follows closely the contraction with increasing electron count that is systematically followed in all the d transition-metal series. At the beginning of the series each additional electron contributes to the cohesive energy of the solid, resulting in a decrease of volume until the shell is approximately half full. This characteristic of the early actinides, together with the absence of magnetic order, has been taken as a prima fascia case that the 5f electrons of these early actinide elements are itinerant. On the other hand, for the heavier actinide elements, there is an abrupt (at δ-Pu and Am) jump in the volume and very little change as the electron count is further increased. In comparison with the 4f elements, together with the presence of 2 ordered magnetism in Cm and the elements beyond (those that have been examined), this change in trend has been taken as evidence of localized behavior of the 5f electrons. If we accept this hypothesis, then it focuses a major interest on plutonium. Note that the volume change between α-Pu and Am is almost 50%, a staggering change in volume between two neighboring elements in the periodic table considering that the only change is to add one electron in the 5f shell. (Unlike the lanthanide elements Eu and Yb, which are both divalent in the normally trivalent lanthanide series, there is no indication of a straightforward valence change between Pu and Am)Plutoniu...

We have made time-of-flight neutron diffraction measurements on α-manganese metal. Powder diffraction measurements were made at 14 temperatures between 15 and 305 K, and single crystal measurements were made at 15 and 300 K. We found that the crystal structure of α-Mn is tetragonal below its Néel point of 100 K, with crystal symmetry I4̄2m and magnetic (Shubnikov) symmetry PI4̄21c. In agreement with the earlier results of Yamada et al., there are six independent magnetic atoms, and we found that their moments are weakly temperature dependent. The onset of magnetic order causes slight changes in the atomic positions and in the average atomic elastic constant.

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The phonon density of states and adiabatic sound velocities were measured on fcc-stabilized 242Pu0.95Al0.05. The phonon frequencies and sound velocities decrease considerably (soften) with increasing temperature despite negligible thermal expansion. The frequency softening of the transverse branch along the [111] direction is anomalously large ( approximately 30%) and is very sensitive to alloy composition. The large magnitude of the phonon softening is not observed in any other fcc metals and may arise from an unusual temperature dependence of the electronic structure in this narrow 5f-band metal.