CDW-gaps, soft-phonons and physical transitions in bulk single crystal of alpha-Uranium

Abstract

Abstract. An overview of three decades of studies of the low temperature structural behaviour of the bulk of alpha-Uranium (α-U) is presented. Combining high-resolution elastic and inelastic neutron scattering made it possible to observe and study, down to 5 K, the structural and lattice dynamical aspects of a series of three charge-density waves (CDWs) transition (43, 37 and 22 K). However at lower temperatures several physical quantities show anomalies such as a controversial superconductivity transition. This stresses the need for extra structural data down to very low temperatures. While this would be a challenge with neutrons due to fission sample heating, a new opportunity is offered by low temperature Laue diffraction with hard Xrays. For example a full locking of the periodic lattice distortion (PLD) can be expected related to the evolution of the CDW and/or the appearance of a SDW, this might help understand the physical behaviour of the material and clarify the role of sample impurities. Diffraction and charge-density-wave transitionsThe low-temperature phase of uranium metal (see [1] for a review) is called the alpha phase (α-U) and is stable below 935 K. We consider here the bulk material at ambient pressure and low temperature. Others put the focus on thin film [2] or epitaxial [3] samples that exhibit a substantially different behavior e.g. due to intrinsic strain, dominating surface effect, skin effect, etc. Thus the comparison is hazardous and a global discussion of both bulk and film results is out of our scope.α-U crystallizes in the orthorhombic system with space group Cmcm (A20) and exhibits a series of three periodic lattice distortions (PLDs) at low temperature. In the case of a metal these are known to be responsible for the lowering of the energy of the electron states thus opening a gap at the Fermi level. At ambient pressure a modulation wave vector (q) with respect to the equilibrium phase appears below T 0 ~ 43 K (: α 1 with <q x , q y , q z >), changes to (: α 2 with <1/2, q y , q z >) at 37 K and then to (: α 3 with <1/2, 1/6, q z >) at 22 K. α-U is the only element, so far, which exhibits such a series of transitions. These true PLDs with ± q components were observed for the first time on the neutron Lauediffractometer S42 on α-U single crystals first and then on polycrystals [4,5]. The single crystals were produced by Fisher [6] using a grain coarsening method. In fact, white-beam neutron diffraction was

On the ground state of the U-Mo system

“…The latter has a lower limit composition of 93.75 %at U and a contraction of b-parameter with increasing Mo content. This raises the question of whether the metastable α'-phase is related or not with the new discovered α-U 15 Mo one due to their composition and structural similitude, but this possibility it is not studied in this work limited to T = 0K.…”

“…In the particular region of low solute concentration, structures were introduced by hand to simulate the presence of one or two impurities in a matrix with the structure corresponding to the pure elements phases. Therefore, at the Mo-rich side, a 3 × 3 × 3 supercell with the pure Mo-bcc structure was considered as a base for the compositions UMo 53 and U 2 Mo 52 ; and at the U-rich side, the orthorhombic α-U phase structure was used as the building block of a 1 × 2 × 2 supercell for U 15 Mo and U 14 Mo 2 compositions. Details of these two structures will be given below.…”

“…It should be noticed that the presence of Mo atoms in the cell certainly affects the movement of U atoms that participate in the CDW in pure U. The displacements are lower in U 15 Mo than in U 14 Mo 2 , being the collective movements different in both structures due to the interaction between U atoms and the nearby Mo ones. The energy gain in U 15 Mo due to minimization of internal coordinates is ∆E=0.…”

“…The last one has a primitive cell with volume of ∼ 6000Å 3 [13,14] . In addition, several physical quantities at lower temperatures show anomalies such as a controversial superconductivity transition (T SC = 0.78 to 0.48 K) [15,16] .…”

“…The last one has a primitive cell with volume of ∼ 6000 Å3 [13,14] . In addition, several physical quantities at lower temperatures show anomalies such as a controversial superconductivity transition (T SC = 0.78 to 0.48 K) [15,16] . Metastable phases based on the orthorhombic αU structure are observed in quenched samples with Mo content below 12 at.%.…”