Yasuhide Inoue

Features of structural phase transition in La1−xSrFeO3

Abstract The structural phase transition in the insulating phase of La 1− x Sr x FeO 3 has been examined by X-ray powder diffraction. Based on the Rietveld analysis of the obtained profiles, the R 3 c – Pbnm structural transition was confirmed to take place in this oxide system. The interesting feature of the transition is that it is characterized by the appearance of the antiferroelectric displacement of the La/Sr atom, in addition to the change in the rotational–displacement pattern, R 25 x + R 25 y + R 25 z → R 25 x + R 25 y + M 3 z . It is thus understood that the low-temperature Pbnm phase can be identified as an antiferroelectric phase in oxides having a simple perovskite structure.

Structural phase transitions in La1.885Sr0.115CuO4

Abstract The details of crystal structures in La2−xSrxCuO4 with x of about 0.115 have been examined by electron diffraction in a temperature range between room temperature and 15 K. In addition to an HTT-to-LTO transition, a low-temperature transition at 104 K was experimentally found only in x=0115. In electron diffraction patterns, 1 2 1 2 0-type superlattice spots also appear at the transition. An origin of the superlattice spots is simply discussed in relation to a Tc-suppression observed in LaSrCuO.

Features of structural phase transitions in Ca2−xSmxMnO4

Abstract Features of structural transitions in Ca2−xSmxMnO4 have been investigated by means of electron diffraction. When the temperature is lowered from a high-temperature tetragonal (HTT) phase in x = 0.3, there are successive transitions from the HTT phase to a low-temperature orthorhombic (LTO) one at 300 K and further to a low-temperature tetragonal (LTT) one at 100 K. In electron diffraction patterns, satellite spots with q = 1 8 〈110〉 also appear below about 290 K. Features of the satellite spot arc that it is observed in 0.1 ≤ x ≤ 0.35 and its location depends on x.

Crystallographic features of a low-temperature- tetragonal structure in La2-xBaxCuO4

Abstract Crystallographic features of both a low-temperature-tetragonal (LTT) structure and a related structural transition in La 1.875 Ba 0.125 CuO 4 have been examined by means of electron diffraction in a temperature range between room temperature and about 15 K. There exist two types of diffraction spots in electron diffraction patterns taken from La 1.875 Ba 0.125 CuO 4 at 16 K. One is a 100-type forbidden spot which shows that the oxide has the LTT structure. The other is a superlattice spot located at h 2 k 2 0 , the X point. From a calculated band structure [Krakauer et al. , Phys. Rev. B47 , 1002(1993)], it seems that the superlattice spot is an indication of the appearance of a charge density wave in the LTT structure.

Chemical composition of the fine α2 + γ lamellar structure in a Ti-44at.%Al alloy

According to the phase diagram of the Ti-Al alloy system, an {alpha} (hcp) {r_arrow} {alpha}{sub 2} (D0{sub 19}) + {gamma} (L1{sub 0}) eutectoid reaction occurs around 1398 K at a composition of 40at.%Al. Features of the eutectoid reaction are that it involves both the {alpha} {r_arrow} {alpha}{sub 2} ordering and the precipitation of the {gamma} phase and is characterized by the formation of the {alpha}{sub 2} + {gamma} lamellar structure in an equilibrium state. because the lamellar structure consists of two types of ordered structures, the {alpha} {r_arrow} {alpha}{sub 2} + {gamma} reaction attracts their interest from the viewpoint of the structural change involved. The authors examined the features of the structural changes in eutectoid and hypo-eutectoid Ti-Al alloys by transmission electron microscopy. In an effort to clarify the mechanism involved in the formation of the lamellar structure in this alloy, the authors examined the chemical composition of the microstructures present in the samples by means of a transmission electron microscope equipped with an EDX analyzer. This paper presents the experimental data obtained in the present work and briefly discusses the features of the mechanism of lamellar-structure formation in Ti-44at.%Al alloys.

Diffuse scattering in T∗-phase superconducting oxides (I) NdCeSrCuO

Abstract Features of diffuse scattering in La1.8− x Sm x Sr0.2CuO z have been investigated by means of electron diffraction. There exist two types of diffuse scatterings in the T- and T ∗ -structures, which are, respectively, observed as a {100} ∗ planar diffuse scattering and a cylindrical-shape one along c axis in reciprocal space. Because the features of the latter scattering is basically identical to those observed in NdCeSrCuO with the T ∗ -structure, it is understood that the characteristic structural disorder due to static displacements of atoms commonly exists in superconducting oxides with the T ∗ -structure.

Characterization of (Bi, Pb)-Sr-Ca-Cu-O system by analytical electron microscopy

Chemical compositions of superconductors of Bi 1− x Pb x SrCaCu 1.8 O y ( x = 0.1, 0.3, and 0.5) were investigated by analytical electron microscopy (AEM) supplemented by electron microprobe analysis. Samples were prepared by a solid state reaction method under oxygen partial pressures of 0.20 and 0.077 atm. The high- T c phase appeared only in the samples of Bi 1− x Pb x SrCaCu 1.8 O y ( x = 0.1 and 0.3) prepared under an oxygen partial pressure of 0.077 atm. In the samples containing the high- T c phase, stacking structures of 2, 3, and 4 perovskite layers were observed by transmission electron microscopy (TEM). From AEM analysis, it was shown that in order to make the high- T c phase, the substitution ratio of Pb for Bi was about 0.2.

Crystallographic Features of Electronic States in the Highly-Correlated Electronic System Sr1-xSmxMnO3 around x = 0.50

The highly-correlated electron system Sr1-xSmxMnO3 (SSMO) with the simple-perovskite structure has been found to exhibit fascinating electronic states accompanying antiferromagnetic and ferromagnetic orderings. It was, in particular, reported that the electronic state for 0.46 ≤ x ≤ 0.54 was characterized by the coexistence state consisting of the A-type antiferromagnetic and ferromagnetic states. However, the features of the coexistence state in this Sm-content range have not been understood yet. We have thus investigated the crystallographic features of prepared SSMO samples with 0.46 ≤ x ≤ 0.55, mainly by transmission electron microscopy. As a result, all prepared SSMO samples were first confirmed to have the orthorhombic-Pnma structure at 300 K. When the temperature was lowered from 300 K, in the case of x=0.47, the disordered-Pnma state was found to be transformed into an orbital-modulated (OM) state accompanying an incommensurate modulation. The notable feature of the OM state is that the state becomes unstable with increasing Sm contents at 100 K. In other words, the OM state was never changed into the CE-type state with the orbital and charge modulations. In addition, no orbital-ordered state for the A-type antiferromagnetic ordering was also found for 0.46 ≤ x ≤ 0.55.

Features of electronic states in the highly-correlated electronic system Sr1-xNdxMnO3 around x = 0.50

In the highly-correlated electron system Sr1-xNdxMnO3 (SNMO) having the simple perovskite structure, there are interesting electronic states, which are related to degrees of charge, orbital, and spin freedoms for eg electrons in Mn ions. Among these states, in the case of SNMO, the C-, A-and CE-type antiferromagnetic states were reported for 0.20 ≤ x ≤ 0.38, for 0.38 ≤ x ≤ 0.48, and for 0.48 ≤ x ≤ 0.52, respectively. The points to note here are that these antiferromagnetic states are directly associated with corresponding orbital orderings, and that the CE-type state also accompanies charge ordering. Because of these features, we were interested in the (A → CE) state change in SNMO. The crystallographic features of prepared SNMO samples with 0.46 ≤ x ≤ 0.50 have thus been investigated mainly by means of a transmission electron microscope equipped with a low-temperature holder. As a result, the state around 100 K for x = 0.48 was first understood to be identified as the Imma state, which includes a large number of orbital-modulated (OM) regions with an average size of about 10 nm. The feature of such regions is that the orbital modulation has an incommensurate periodicity and a charge modulation is absent in them. On the other hand, the CE-type state having the commensurate orbital and charge modulations was also confirmed to be present for x = 0.50. In addition to these two states, the state around 100 K for x = 0.46 was found to be characterized by the coexistence state consisting of the C-type orbital-ordered state and the Imma states including OM regions. In other words, the presence of the A-type orbital-ordered state could not be confirmed in the temperature range between 300 K and about 100 K for 0.46 ≤ x ≤ 0.50 in this study.

Relaxation phenomenon in the formation of the C-type orbital-ordered state in the simple perovskite manganite Sr1-xNdxMnO3

The formation of the C-type orbital-ordered (COO) state from the disordered cubic (DC) state in Sr1−xNdxMnO3 (SNMO) with the simple perovskite structure has been examined mainly by transmission electron microscopy. As the COO state has tetragonal I4/mcm symmetry, its formation is associated with the cubic-to-tetragonal structural transition. It was found that, when SNMO samples were cooled down from the DC state, the R25-type rotational displacement of MnO6 octahedra was first induced, together with the symmetry change into the tetragonal I4/mcm structure. The C-type orbital ordering then appeared due to the induction of the Jahn–Teller distortion as a response of a lattice system to this orbital ordering. Because no symmetry change occurred in the latter case, the Jahn–Teller distortion can be regarded as a dilatational distortion. One interesting feature is that the appearance of the dilatational Jahn–Teller distortion led to a nanometer-scale banded structure, characterized by an alternating array of t...