33 research outputs found
Dodecagonal quasicrystal in Mn-based quaternary alloys containing Cr, Ni and Si
A dodecagonal quasicrystal showing 12-fold symmetry forms in Mn-rich
quaternary alloys containing 5.5 or 7.5 at.% Cr, 5.0 at.% Ni and 17.5 at.% Si.
After annealing at 700 C for 130 h, the quasicrystal precipitated in a matrix
of \b{eta}-Mn type crystalline phase. The shape of the quasicrystal is
needle-like having a length of several tens of micrometers. Electron
diffraction as well as powder X-ray diffraction experiments have revealed the
following characteristics of the quasicrystal; diffraction symmetry 12/mmm,
presence of systematic extinction for h1h2h2h1h5 type reflections with odd h5
index, and then five-dimensional space group P126/mmc. Indexing of the
reflections indicated that the dimension of the common edge in the equilateral
triangle-square tiling is 4.560 {\AA}, and the periodicity is 4.626 {\AA} along
the 12-fold axis. This is the first example of the dodecagonal quasicrystal
synthesized by ordinary metallurgical method in 3-d transition-metal alloys.Comment: 9 pages, 1 tabl and 4 figure
Icosahedral quasicrystal and 1/1 cubic approximant in Au-Al-Yb alloys
A P-type icosahedral quasicrystal is formed in Au-Al-Yb alloy of which
6-dimensional lattice parameter a6D = 7.448 {\AA}. The composition of the
quasicrystal was analyzed to be Au51Al34Yb15. This quasicrystal is formed in
as-cast alloys, and is regarded as metastable because of decomposition into
other crystalline phases by annealing at 700 \degree C. Among Tsai-type
quasicrystals, this quasicrystal is situated just between Zn-Sc group with
smaller a6D and larger Cd-Yb group. Intermediate valence of Yb recently
observed in this quasicrystal may be due to this unique situation, namely
smaller major component Au-Al than in Cd-Yb.
The predominant phase in the annealed specimen is a 1/1 cubic approximant
with lattice parameter a = 14.500 {\AA} belonging to the space group Im-3 .
This phase is stable at the composition Au51Al35Yb14 at 700 \degree C. Rietveld
structural analysis indicated that the crystal structure is understood as
periodic arrangement of Tsai-type clusters each including four Au-Al atoms at
their centers. Chemical ordering of Au and Al is characteristics of this
approximant.Comment: 20 pages including 1 table and 7 figure
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
New group of stable icosahedral quasicrystals: structural properties and formation conditions
Structural studies on the icosahedral quasicrystals in Zn-Mg-Sc, Cu-Ga-Mg-Sc,
and Zn-Mg-Ti alloys as well as their corresponding 1/1 cubic approximants, have
revealed that these quasicrystals belong to a new structural group similar to
Cd-based quasicrystals. This group is characterized by a triple-shell
icosahedral cluster different from both Mackay- and Bergman-types. The presence
of the atomic cluster has been deduced from the structure model of the
approximant crystal, Zn17Sc3, in which the clusters are embedded in a periodic
network of so-called "glue atoms". Density measurement suggested the presence
of at least 2.7 Zn atoms in the first shell of the cluster in this approximant.
The substitutional relationship in these three quasicrystals indicates the
important role of Hume-Rothery rule for the formation of this type of
quasicrystal. The occurrence of a P-type icosahedral quasicrystal in Zn-Mg-Yb
alloy is also reported.Comment: 11 pages, 1 table, 6 figure
Neutron scattering study on spin correlations and fluctuations in the transition-metal-based magnetic quasicrystal Zn-Fe-Sc
Spin correlations and fluctuations in the 3d-transition-metal-based
icosahedral quasicrystal Zn-Fe-Sc have been investigated by neutron scattering
using polycrystalline samples. Magnetic diffuse scattering has been observed in
the elastic experiment at low temperatures, indicating development of static
short-range-spin correlations. In addition, the inelastic scattering experiment
detects a -independent quasielastic signal ascribed to single-site
relaxational spin fluctuations. Above the macroscopic freezing temperature
K, the spin relaxation rate shows Arrhenius-type behavior,
indicating thermally activated relaxation process. In contrast, the relaxation
rate remains finite even at the lowest temperature, suggesting a certain
quantum origin for the spin fluctuations below .Comment: To be published in Phys. Rev.