Kazue Nishimoto

Superconductivity and Structural Phase Transitions in Caged Compounds RT2Zn20 (R = La, Pr, T = Ru, Ir)

Electrical resistivity ρ, specific heat C , magnetization M measurements are reported on four compounds, LaRu 2 Zn 20 , PrRu 2 Zn 20 , LaIr 2 Zn 20 , and PrIr 2 Zn 20 , which crystallize in a cubic CeCr 2 Al 20 -type structure. LaRu 2 Zn 20 , LaIr 2 Zn 20 , and PrIr 2 Zn 20 show superconducting transitions at T C = 0.2, 0.6, and ∼0.05 K, respectively, whereas PrRu 2 Zn 20 remains a normal state down to 0.04 K. This is the first observation of superconductivity in the family of RT 2 X 20 (T = transition metal; X = Al and Zn). Furthermore, structural phase transitions manifest themselves at T s = 150, 138, and 200 K for LaRu 2 Zn 20 , PrRu 2 Zn 20 , and LaIr 2 Zn 20 , respectively. No magnetic transition is found in PrRu 2 Zn 20 and PrIr 2 Zn 20 down to 1.8 K. On cooling PrIr 2 Zn 20 below 2 K, the specific heat divided by temperature, C / T , continuously increases and reaches 5 J/(K 2 ·mol) at 0.4 K, suggesting that Pr 4 f 2 electrons are involved in the heavy-fermion state, as observed in a related compo...

Low-temperature superstructures of a series of Cd6M (M = Ca, Y, Sr, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) crystalline approximants

The low-temperature (LT) superstructure and the phase transition temperature have been investigated for a series of Cd6M crystalline approximants by transmission electron microscopy as well as electrical resistivity measurements. Except for M = Lu, Cd6M is found to undergo a phase transition to a monoclinic phase at a low temperature and the transition temperature (Tc) scales well with the size of the M atom. For M = Ca, Y, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er and Tm the LT superstructure is explained by a √2a × a × √2a lattice with the space group C2/c, and for M = Sr and Yb a √2a × 2a × √2a monoclinic lattice with P2/m. On the other hand, no phase transition is observed for M = Lu, indicating that a Cd4 tetrahedron at the cluster center remains disordered down to the lowest temperature, i.e. 16 K. It is shown that the volume inside the Cd20 dodecahedron plays a crucial role in the occurrence of the phase transition, and long-term aging in particular promotes the phase transition for late rare-earth elements such as Ho, Er and Tm, suggesting that the transition is sensitive to and is even hindered by disorder such as atomic vacancies. The absence of the transition for M = Lu is attributed to the highest activation energy for the transition due to the smallest volume inside the Cd20 dodecahedron.

Synthesis and superstructure of Al—Ir, Al—Ir—Pd crystalline approximants and their electrical resistivities

Abstract Chemically ordered Al2.7Ir and Al64.5Ir22Pd13.5 1/0 approximants are prepared by two different methods and characterized by X-ray diffraction and transmission electron microscopy. Al2.7Ir possesses a primitive cubic lattice with a = 15.345(6), whereas Al64.5Ir22Pd13.5 has a face-centered cubic lattice with a = 15.497(9). Examination of the samples prepared by two different methods reveals that the texture of the samples prepared by spark plasma sintering is more homogeneous than those prepared by arc melting. Electrical resistivities of both the compounds exhibit metallic characters, i.e., a positive temperature coefficient of the resistivity (TCR) with a high resistivity value. The results suggest that the TCR may not always be negative for 1/0 approximants with 15 to 16 periodicity but the high residual resistivities are common to all the approximants, which is presumably attributed to the common structural unit, an icosahedral cluster of about 7.7.

A new superlattice structure in the Al2.75Ir and Al2.63Rh 1/0 approximants

We investigate the superstructure of binary Al2.75Ir and Al2.63Rh 1/0 approximants by X-ray diffraction method and transmission electron microscopy. Al2.75Ir possesses a 2a × 2a × 2a C-centred monoclinic lattice with space group C2 or Cm or C2/m. Al2.63Rh possesses a 2a × 2a × 2a face centred lattice with space group F23 or Fm3.

Synthesis and TEM study of Ag–In–(Eu, Ce) ternary approximants

Abstract Ternary Ag–In–(Eu, Ce) 1/1 approximants are synthesized and their structures are studied by transmission electron microscopy (TEM). For both the approximants, superlattice spots are clearly observed at room temperature, and the superstructures of the Ag–In–(Eu, Ce) approximants are found to be similar to those of Cd25Eu4 and Cd37Ce6, respectively. The Ag–In–Eu 1/1 approximant has a double unit cell with space group Fd-3 while the Ag–In–Ce 1/1 approximant has a normal unit cell with space group Pn-3, both at room temperature. The results suggest that the tetrahedron at the center of the icosahedral cluster is ordered in the Ag–In–(Eu, Ce) approximants in the same manner as the corresponding binary approximants Cd6(Eu, Ce). Furthermore, TEM observations at high temperatures reveal that the Ag–In–(Eu, Ce) approximants undergo a phase transition to a bcc lattice at an elevated temperature. The transitions are understood as a consequence of disordering of the tetrahedron located at the center of the icosahedral cluster.

Fast Oxidation of Porous Cu Induced by Nano-Twinning

The fcc lattice of porous Cu prepared by dealloying Al2Cu with HCl aqueous solution exhibits a high density of twinning defects with an average domain size of about 3 nm along the ⟨111⟩ directions. The high density of twinning was verified by X-ray diffraction and qualitatively interpreted by a structural model showing the 5% probability of twinning defect formation. Most of the twinning defects disappeared after annealing at 873 K for 24 h. Twinned Cu reveals much faster oxidation rate in comparison to that without (or with much fewer) twinning defects, as shown by X-ray diffraction and hydrogen differential scanning calorimetry. Using ab initio DFT calculations, we demonstrate that twinning defects in porous Cu are able to form nucleation centers for the growth of Cu2O. The geometry of the V-shaped edges on the twinned {211} surfaces is favorable for formation of the basic structural elements of Cu2O. The fast oxidation of porous Cu prepared by dealloying can thus be explained by the fast formation of the Cu2O nucleation centers and their high density.

In-situ HRTEM observations of growth process of decagonal quasicrystals

Quasicrystals possess quasiperiodicity, where the structure cannot be described simply by the repetition of unit cell like conventional crystals. This fact raises the question of how quasicrystals grow, i.e., what physical mechanism makes the growth of quasicrystals possible. While crystals can grow by copying a unit cell via local atomic interactions, nonlocal structural information seems to be required in the growth of quasicrystals. This problem has attracted much attention ever since the first discovery of a quasicrystal in 1984, and several theoretical growth models [1] have been proposed. However, no experimental studies have so far been reported, and it is still unclear whether these theoretical growth models apply to real quasicrystals. In the present study, we have conducted insitu high-temperature electron microscopic (HRTEM: High-Resolution Transmission Electron Microscopy) observations of the growth process of decagonal quasicrystals to elucidate the growth mechanism. The growth processes of a decagonal quasicrystal of Al70.8Ni19.7Co9.5 were observed by HRTEM in the temperature range 1073-1173K. Tiling patterns with edge length of about 2nm were constructed from a series of HRTEM images. They were analysed in the framework of the projection method. Here, we followed the procedures in our previous work [2]. We have already reported the results of some observations and analyses elsewhere [3]. However, the growth processes of them were on a small scale, and the results were indefinite. Recently, we have succeeded in observing a growth process on a massive scale. In this paper, we present the results of this observation and subsequent analyses, and discuss the growth mechanism of the quasicrystal.

In situ TEM study of the order-disorder phase transition in Cd6M approximants at low temperature

An occurrence of an order-disorder phase transition was first discovered in Cd6Ca and Cd6Yb at 100K and 110K, respectively, which has been attributed to orientational ordering of the Cd4 tetrahedron in the centre of the cluster. A series of Cd6M (M = Ca, Sr, Rare earth metals) compounds [1,2] are 1/1 cubic approximants to the binary Cd5.7Yb and Cd5.7Ca quasicrystals. Also, Cd6M possesses a bcc lattice made of the Tsai-type icosahedral cluster [3, 4]. The Tsai-type icosahedral cluster is composed of four successive shells, which are, from the centre, a Cd4 tetrahedron, a Cd20 dodecahedron, a M12 icosahedron and a Cd30 icosidodecahedron. For most Cd6M (expect for Cd6Eu and Cd6Ce), the Cd4 tetrahedron at the centre is orientationally disordered at room temperature. Among them, Cd6Ca has been investigated in detail and it has been found that Cd6Ca transforms into a lattice with space group C2/c below 100 K [5]. In the present work, we have investigated superlattice structures and structural relationship between the ordered and disordered phases in a variety of Cd6M (M = Y, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Tm, Er and Lu) by in-situ transmission electron microscopy (TEM) at low temperatures. In the cases of Cd6M (M = Y, Pr, Nd, Sm, Gd and Tb), we have observed superlattice reflections at 20 K in the selected area electron diffraction patterns. The superlattice reflections are explained by a C-centred monoclinic lattice with space group C2/c or Cc as well as Cd6Ca. It suggests that the C-monoclinic superlattice structures are commonly formed for Cd6M (M = Ca[5], Y, Pr, Nd, Sm, Gd and Tb). In contrast, for Cd6Ho, Cd6Er, Cd6Tm and Cd6Lu, which have a smallest atomic radius of M, no superlattice reflection has been observed down to 20 K. The results indicated that no order-disorder phase transition occurs in Cd6M (M = Ho, Tm, Er and Lu) and the Cd4 tetrahedron at the centre of the icosahedral cluster remains disordered down to 20 K. The difference in the structure of the low temperature phase is well classified in terms of the atomic radius of M elements, which indicates that the space inside the M12 icosahedron plays a dominant role in the ordered phase formation. Results of other TEM observations will be discussed in details.