80 research outputs found
Interpretation of some Yb-based valence-fluctuating crystals as approximants to a dodecagonal quasicrystal
The hexagonal ZrNiAl-type (space group: P-62m) and the tetragonal
Mo2FeB2-type (space group: P4/mbm) structures, which are frequently formed in
the same Yb-based alloys and exhibit physical properties related to
valence-fluctuation, can be regarded as approximants of a hypothetical
dodecagonal quasicrystal. Using Pd-Sn-Yb system as an example, a model of
quasicrystal structure has been constructed, of which 5-dimensional crystal
(space group: P12/mmm, aDD=5.66 {\AA} and c=3.72 {\AA}) consists of four types
of acceptance regions located at the following crystallographic sites; Yb
[00000], Pd[1/3 0 1/3 0 1/2], Pd[1/3 1/3 1/3 1/3 0] and Sn[1/2 00 1/2 1/2]. In
the 3-dimensional space, the quasicrystal is composed of three types of
columns, of which c-projections correspond to a square, an equilateral triangle
and a 3-fold hexagon. They are fragments of two known crystals, the hexagonal
{\alpha}-YbPdSn and the tetragonal Yb2Pd2Sn structures. The model of the
hypothetical quasicrystal may be applicable as a platform to treat in a unified
manner the heavy fermion properties in the two types of Yb-based crystals.Comment: 19 pages, 6 figure
Study of the structure and physical properties of quasicrystals using large scale facilities
International audienceQuasicrystals have been puzzling scientists since their discovery. In this article we review some of the recent advances in this field and show how the use of large scale facilities has brought in decisive information for the understanding of their structure and physical properties. (C) 2011 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved
Phononic filter effect of rattling phonons in the thermoelectric clathrate BaGeNi
One of the key requirements for good thermoelectric materials is a low
lattice thermal conductivity. Here we present a combined neutron scattering and
theoretical investigation of the lattice dynamics in the type I clathrate
system Ba-Ge-Ni, which fulfills this requirement. We observe a strong
hybridization between phonons of the Ba guest atoms and acoustic phonons of the
Ge-Ni host structure over a wide region of the Brillouin zone which is in
contrast with the frequently adopted picture of isolated Ba atoms in Ge-Ni host
cages. It occurs without a strong decrease of the acoustic phonon lifetime
which contradicts the usual assumption of strong anharmonic phonon--phonon
scattering processes. Within the framework of ab-intio density functional
theory calculations we interpret these hybridizations as a series of an
ti-crossings which act as a low pass filter, preventing the propagation of
acoustic phonons. To highlight the effect of such a phononic low pass filter on
the thermal transport, we compute the contribution of acoustic phonons to the
thermal conductivity of BaGeNi and compare it to those of pure
Ge and a Ge empty-cage model system.Comment: 10 pages, 10 figure
Structure of smectic defect cores: an X-ray study of 8CB liquid crystal ultra-thin films
We study the structure of very thin liquid crystal films frustrated by
antagonistic anchorings in the smectic phase. In a cylindrical geometry, the
structure is dominated by the defects for film thicknesses smaller than 150 nm
and the detailed topology of the defects cores can be revealed by x-ray
diffraction. They appear to be split in half tube-shaped Rotating Grain
Boundaries (RGB). We determine the RGB spatial extension and evaluate its
energy per unit line. Both are significantly larger than the ones usually
proposed in the literatureComment: 4 page
Antiferromagnetic order and the structural order-disorder transition in the Cd6Ho quasicrystal approximant
International audienceIt has generally been accepted that the orientational ordering of the Cd4 tetrahedron within the Cd6 R quasicrystal approximants is kinetically inhibited for R = Ho, Er, Tm and Lu by steric constraints. Our high-resolution X-ray scattering measurements of the Cd6Ho quasicrystal approximant, however, reveal an abrupt (first-order) transition to a monoclinic structure below T S = 178 K for samples that have 'aged' at room temperature for approximately one year, reopening this question. Using X-ray resonant magnetic scattering at the Ho L 3-edge we have elucidated the nature of the antiferromagnetic ordering below T N = 8.5 K in Cd6Ho. The magnetic Bragg peaks are found at the charge forbidden H + K + L = 2n + 1 positions, referenced to the high-temperature body-centred cubic structure. In general terms, this corresponds to antiferromagnetic arrangements of the Ho moments on adjacent clusters in the unit cell as previously found for Cd6Tb
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