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The crystallographic features of the (α-Mn → α-Mn + β-Samson) reaction in Al-Mg alloys around the composition of 48 at.% Mg have been investigated mainly by transmission electron microscopy to understand the formation of giant atomic clusters in the β-Samson structure. It was confirmed, for instance, that the state of an Al-48 at.% Mg alloy annealed at 573 K for 24 h could be identified as the (α-Mn + β-Samson) coexistence state, and that metastable states were found locally in α-Mn/β-Samson boundary regions. An analysis of electron diffraction patterns obtained from the coexistence state indicated that the orientation relationship of ( 3 ¯ 30)α//( 8 ¯ 80)β-S and [110]α//[110]β-S was established between α-Mn and β-Samson regions, together with a lattice contraction of about 10% along the [001]α direction and an expansion of about 0.4% along the [ 1 ¯ 10]α direction in the (α-Mn → β-Samson) structural change. High-resolution electron micrographs of the coexistence state also suggested that some atoms at the Mg (I) and Mg (II) sites in the distorted α-Mn structure could be converted to those at the Mg (23) site of the β-Samson structure as the center site of the Samson cluster. Furthermore, both the features of giant clusters in the β-Samson structure and their formation from the α-Mn structure were discussed on the basis of the experimental data obtained in this study.

In the highly-correlated electronic system Ca 1- x Pr x MnO 3 having the simple perovskite structure, it has been reported that there exists the C-type orbital-ordered (COO) state accompanying an antiferromagnetic ordering for 0.10 ≤ x ≤ 0.25. According to the previous studies concerning orbital-ordered states in simple perovskite manganites, the COO state was understood to be characterized by a spatial array of (3z 2 -r 2 )-type orbitals for 3d electrons in Mn ions. The notable feature of the COO state in Ca 1- x Pr x MnO 3 is that the state with the monoclinic-P2 1 /m symmetry appears as a result of the structural transition from the disordered state with the orthorhombic-Pnma symmetry. Compared with the COO-state formation from the cubic-Pm $\overline 3$ m state, however, the formation from the disordered-Pnma state has not been understood yet. We have thus examined the crystallographic features of the formation of the COO state in Ca 1- x Pr x MnO 3 , mainly by x-ray powder diffraction and transmission electron microscopy. In the case of x = 0.16, for instance, the COO state was found to appear from the disordered-Pnma state around 90 K on cooling. The notable feature of the formation is that, in the Pnma state just before the COO-state formation, characteristic diffuse scattering appeared around each reflection in electron diffraction patterns, together with the splitting of the 200c reflection in x-ray powder diffraction profiles in the pseudo-cubic notation. Based on these experimental data, it is understood that the formation of the COO state in Ca 1- x Pr x MnO 3 accompanies remarkable fluctuations of the C-type orbital ordering in the disordered-Pnma state.

The charge-exchange-type orbital-ordered (CEOO) state accompanying antiferromagnetic ordering has been reported in the highly-correlated electronic system Ca1-xPrxMnO3 (CPMO) with the simple perovskite structure. The feature of the CEOO state in CPMO is that the state is present in the wide Pr-content range of 0.30 ≤ x ≤ 0.70. Although the Zener-polaron model was proposed for the CEOO state at x = 0.60, the detailed features of the CEOO state, particularly for lower Pr contents, have not been understood sufficiently. We have thus investigated the crystallographic features of the CEOO state in CPMO with 0.40 ≤ x ≤ 0.50, mainly by transmission electron microscopy. It was found that, when the temperature was lowered from the disordered state, incommensurate satellite reflections characterizing the CEOO state appeared in electron diffraction patterns below about 250 K in CPMO. The careful analysis of the reflections indicated that the CEOO state for 0.40 ≤ x ≤ 0.45 was different from that for 0.45 ≤ x ≤ 0.50. Concretely, the former CEOO state accompanied only a transverse lattice modulation, while both transverse and longitudinal modulations with different wave vectors appeared in the latter state. This implies that the state for 0.40 ≤ x ≤ 0.45 can be regarded as an orbital-modulated (OM) state without a charge modulation. It is thus understood that an increase in the Pr content leads to the OM-to-CEOO state change below about 250 K around x = 0.45.

In the Mn-Si alloy system, there is the intermetallic-compound ν phase around Mn-18at.%Si. Its crystal structure called the ν structure has been reported to involve complex coordination polyhedra with coordination numbers (CN’s) 12, 14, and others. To clarify whether one of these polyhedra can be identified as a structural unit or not in the ν structure, we have investigated the crystallographic features of the (R → R + ν) reaction in Mn-15at.%Si alloy samples by transmission electron microscopy. In Mn-15at.%Si samples obtained by cooling from the single R phase at a relatively fast cooling rate, metastable precipitates were found to be present in the R-structure matrix. Their notable feature is that each precipitate involves regions exhibiting characteristic electron diffraction patterns, which indicate the presence of metastable states, in addition to the initial amorphous and final ν states. Based on the careful analysis of these patterns, it was understood that the metastable states were directly associated with the formation of the ν structure, and that the formation started with the appearance of dodecagonal atomic columns. Because a dodecagonal column is characterized by a one-dimensional array of CN14 polyhedra, among the coordination polyhedra involved in the ν structure, a CN14 polyhedron can be identified as its structural unit.

The simple perovskite manganite Sr1-xCexMnO3 (SCMO) has a highly-correlated electronic system with a three-dimensional character. Because the presence of orbital-ordered states of eg electrons can be expected in SCMO, the crystallographic features of SCMO samples with 0.09 ≤ x ≤ 0.20 have been investigated mainly by transmission electron microscopy. In addition to fundamental reflections due to the simple perovskite structure, their electron diffraction patterns at room temperature exhibited both the presence of superlattice reflections at k = ()c in the cubic notation and the splitting of fundamental and superlattice reflections. The careful analysis of these reflections indicated that the superlattice reflections originated from the R25-type rotational displacement of oxygen octahedra about one of the c directions. On the other hand, the splitting of the reflections was found to be due to a {110}c banded structure consisting of two tetragonal bands with different c/a values. Because one of two tetragonal bands had the c/a value of about 1.028, the splitting reflects the introduction of the Jahn-Teller distortion as a response of a lattice system to orbital ordering. It is thus understood that the C-type orbital ordering of eg electrons should be involved in the state at room temperature for 0.09 ≤ x ≤ 0.20 in SCMO.

The structural correspondence among the bcc, ordered-bcc B2, triclinic Al2Fe and approximant Al5Fe2 structures have been examined on the basis of the experimentally obtained orientation relationship of and , where N(hkl) denotes the normal direction of the (hkl) plane. The formation of the triclinic Al2Fe structure from the bcc structure is directly associated with that of Al–Fe covalent bonds; that is, hybridization between Al and Fe atoms. The covalent bond formation results in the appearance of decagonal-like atomic arrangements in the Al2Fe structure, which are similar to those in the approximant Al5Fe2 structure.

The crystallographic features of the simple perovskite oxides La 1 − x Sr x MnO 3 with 0⩽ x ⩽0.08, which are known to be correlated-electron insulators, have been examined by X-ray powder diffraction and transmission electron microscopy. It was confirmed on the basis of the powder diffraction profiles obtained that the orthorhombic-to-monoclinic structural phase transition took place; for instance, at 500 K in the x =0.08 sample. The profiles also indicated that the structural transition accompanied the introduction of the Jahn–Teller distortion and the increase in both the orthorhombic distortion and the A-site-ion displacement. Based on the reciprocal lattices determined from electron diffraction patterns, further, it was found that these distortions and displacement never resulted in the change in the extinction rule of diffraction spots, and that the monoclinic distortion was actually involved in the low-temperature phase. That is, the structural phase transition can basically be regarded as a ferroelastic transition in terms of the lowering of the symmetry.

The crystallographic features of the simple perovskite oxides RTiO3 with R=La, Nd, Sm and Y, which are known to be correlated-electron insulators, have been examined mainly at room temperature by transmission electron microscopy. Electron diffraction patterns indicated that the oxides with R=La and Nd have the space group Pbnm. In electron diffraction patterns of SmTiO3 and YTiO3, on the other hand, the twin splitting of reflections due to the Pbnm structure is seen along one of the 〈1 0 0〉 c directions, where the subscript c denotes the cubic Brillouin zone. That is, it is understood that their crystal system is lowered from an orthorhombic one to a monoclinic one. The analysis of the patterns also suggested that the Jahn–Teller atomic displacement with q =〈 1 2 1 2 0〉 c should be involved in the crystal structure for R=Sm and Y, in addition to the full symmetric Jahn–Teller lattice distortion in the structure for R=La and Nd.

Crystallographic features of La 1− x Sr x MO 3 with M=Cr and Fe in the vicinity of the R 3 c/Pbnm phase boundary have been examined mainly at room temperature by transmission electron microscopy. When the Sr content x increases from x =0, the change in crystal structure from Pbnm to R 3 c was confirmed to occur at room temperature in La 1− x Sr x CrO 3 around x =0.2. In La 1− x Sr x FeO 3 , on the other hand, the incommensurate phase was found to exist between the Pbnm and the R 3 c phases. The analysis of the electron diffraction patterns obtained in the work indicated that the normal structure of the incommensurate structure and the direction of the modulation could, respectively, be identified as the Pbnm structure and one of the 〈3 2 3〉 directions. It was also suggested that the rotational displacement of the oxygen octahedra involved in the Pbnm structure should be modulated in the incommensurate structure.

The new chemical layered structure, referred to as a chemical stripe structure, was found as a metastable state in the (bcc → hcp + C15) metastable phase separation in the Ti-Cr alloys. Note that a metastable separation is here defined as a separation characterized by a positive value of the second derivative of Gibbs free energy with respect ot chemical composition. The experimental data showed that the layered structure had periodicity of 4 × d 002 , about 0.72 nm, and chemical ordering of Ti: Cr = 3: 1, and that a lot of antiphase boundaries with phase shifts of π/2 and π were involved in the structure. The coherent length of the layered structure was estimated to be about 15 nm along the modulated direction. In addition to these features, it was found that the layered structure was formed from the local bct state as another metastable state, not directly from the zone structure consisting of one Cr layer. On the basis of these data, the physical origin of the formation of the chemical layered structure is also discussed.

The crystallographic features of a bcc → hcp + C15 structural change in a Ti-30 mol%Cr alloy have been examined by transmission electron microscopy. When the alloy was annealed at 873 K below the eutectoid temperature, the structural change occurred in the following five steps: bcc → bcc + Zone I → bcc + Zone II → bcc + Zone II + hcp → bcc + hcp + C15 → hcp + C15. The interesting features of the change are that the diffuse w state is present in the metastable bcc matrix, and that in the fourth step the C15 grains with a size of about 2 nm appear in the bcc matrix around the hcp region. As a result of these extremely small C15 grains, the final microstructure, which was obtained from the 5-h-and 100-h-annealed samples, consists of the hcp and C15 grains with an average size of about 100 nm. On the basis of these results, the physical origin of the nm-size polycrystallization in the appearance of the C15 phase is discussed in relation to both the presence of diffuse ω state in the metastable bcc matrix and the formation of the complex coordinated polyhedra in the C15 structure.

Features of the α→α 2 + γ phase separation in a hypo-eutectoid Ti-37 at%Al alloy were investigated by transmission electron microscopy. Note that the eutectoid reaction in the Ti-Al alloy system takes place at 1398 K in an Al concentration of 40 at%. The present data clearly show that in the hypo-eutectoid alloy the α 2 + γ lamellar structure is formed in an equilibrium state, just as in cases of the eutectoid and hyper-eutectoid alloys. A characteristic feature in the hypo-eutectoid alloy is the appearance of the amorphous state as an intermediate state. In other words, the sequence of the formation of the lamellar structure is the same as that in the eutectoid alloy, not in the hyper-eutectoid alloy. It can be therefore concluded that there are two types of the processes for the lamellar-structure formation in the phase separations of the Ti-Al alloy system.

Fundamental concepts of electron diffraction from crystals are introduced, based on the kinematical theory of diffraction. Characteristic features of electron diffraction, such as the Bragg reflection, crystal structure factor, atomic scattering factor, Laue function, Ewald construction, double diffraction effect etc, are described, in comparison with those of the X-ray diffraction.

Features of crystal structures of Al 65 Cu 20 Fe 15-x Mn x alloys have been investigated by transmission electron microscopy. Electron diffraction patterns obtained experimentally show that some alloys exhibit similar diffraction patterns to those of the decagonal quasicrystal, in addition to those taken from the icosahedral quasicrystal as well as the Al 3 Mn and Al 13 Fe 4 crystalline approximants. That is, there are both diffraction patterns showing a crystal period of 1.2 nm along one direction and a pseudo-tenfold one with an electron incidence parallel to it. From high resolution images taken in the present works, it was also found that these patterns originate from a single-state region. These results clearly imply that a structure characterized by these patterns can be identified as a two-dimensional modulated structure, which should be distinct from not only the icosahedral and decagonal quasicrystals but also the crystalline approximants.

Features of the crystal structures at room temperature and structural transitions in both Ba1-x K x BiO3 and Ba1-x Rb x BiO3 with 0.1 < x ≤ 0.5 have been examined in detail by means of electron diffraction. From the analysis of diffraction patterns, it has been confirmed that at room temperature the crystal system changes with respect to x; cubic system for x ≤ 0.35 and orthorhombic for 0.1 < x ≤ 0.35. Furthermore, the cubic to orthorhombic structural transition with changing temperature was found to be due to the condensation of the RX 25 and the RY 25 modes.

Detailed features of the structural disorder in Nd—-Ce—-Sr—-Cu—-O with the T∗-structure have been investigated by diffuse scattering in electron diffraction. There exist three types of characteristic diffuse scatterings: a cylindrical-shape, a spike-shape, and spots in addition to the fundamental spots due to the T∗-structure. The cylindrical-shape and spike-shape scatterings can be concluded to result from both atomic displacements of O(2) atoms along the directions, on planes through the apical positions of the CuO5 pyramids, and the ordering of Nd and Sr atoms. On the other hand, the extra spots are understood to be related to displacements of O(1) or O(3) atoms along the c axis.

Diffuse scattering, reflecting structural disorder in La1.8-x Sm x Sr0.2CuO z , have been investigated by means of electron diffraction. In the T∗-structure, in addition to the fundamental spots due to the T∗-structure there exists cylindrical-shape diffuse scattering due to static displacements of the apical oxygen atoms in the CuO5 pyramids. On the other hand, both thermal diffuse scattering and superlattice spots have been found in diffraction patterns obtained from the T-structure. The thermal diffuse scattering is distributed on the {100}∗ planes of reciprocal space. In addition, the superlattice spots appear in a range of 0.3 ∗ directions. On the basis of the extinction rule for the superlattice spots, the crystal system of the T-structure for 0.3 < x< 0.6 is concluded to be orthorhombic while the system for 0 < x< 0.3 is tetragonal.

Features of structural fluctuations related to the cubic-to-orthorhombic transition in ${\mathrm{Ba}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{M}}_{\mathit{x}}$${\mathrm{BiO}}_{3}$ (M=K, Rb) have been investigated in detail by means of electron diffraction. A fluctuation due to the softening of the ${\mathit{R}}_{25}$ mode, which is responsible for the transition, is detected as diffuse scattering at the R point in the reciprocal space of the cubic phase. When x is reduced from x=0.5 to a transition composition in the cubic phase, the fluctuation is enhanced remarkably at lower temperatures. Because the fluctuation of the ${\mathit{R}}_{25}$ mode results in those of spontaneous strains, the fluctuations of the ${\mathit{A}}_{1\mathit{g}}$ and ${\mathit{E}}_{\mathit{g}}$ strains are understood to be enhanced near the superconducting transition.

Features of temperature-dependent diffuse scattering showing a structural fluctuation in Tl 2 Ba 2 Ca 2 Cu 3 O 10 have been investigated in detail in a temperature range between about 20 K and room temperature by means of electron diffraction. As the temperature is lowered, the intensity of diffuse scattering increases remarkably below about 130 K, which is taken to be the onset temperature of superconductivity

Features of crystal structures and a related microstructure in Ca2MnO4-δ ceramic sample have been investigated by transmission electron microscopy. In electron diffraction patterns at room temperature, there exist h/2 k/2 l-type and h/2 k/2 l/2-type superlattice reflection spots, in addition to fundamental spots due to the tetragonal K2NiF4-type structure. That is, there coexist the Bbcm and I41/acd structures in our samples. In dark field images taken by the superlattice spots, further there are three types of regions arranged along the c axis. A careful analysis indicated that one of the regions has the I41/acd structure and the others are two variants of the Bbcm one. The microstructure was therefore understood to be a layered structure, which is composed of the regions with the I41/acd and Bbcm structures.

Features of both crystal structures and structural fluctuations in superconducting oxides, Ln 2− x Ce x CuO 4− y (Ln Pr, Nd) with 0 ≤ x ≤ 0.20, have been examined in detail by means of electron diffraction. In electron diffraction patterns of Ln 2− x Ce x CuO 4− y (Ln = Pr, Nd), there commonly exists characteristic diffuse scattering in addition to fundamental spots due to the T′-structure. The diffuse scattering is observed as a spike shape along the directions around the fundamental spot. Because of no temperature dependence of the intensity of the scattering, the scattering is understood to be mainly due to static displacements of atoms. In addition, the electron-beam irradiation results in two effects in diffraction patterns. One is characterized by the appearance of superlattice spots in both Nd 2− x Ce x CuO 4− y and Pr 2− x Ce x CuO 4− y . That is, the irradiation produces the modulated structure due to the ordering of oxygen vacancies. The other is the splitting of the fundamental spot in Pr 2− x Ce x CuO 4− y , which is due to a twinning resulting from a structural transition.

Features of the crystal structure of the so-called “S” phase in Ca2RuO4 have been investigated by transmission electron microscopy. Electron diffraction patterns taken from the “S” phase at room temperature show the presence of h/2 k/2 l-type superlattice spots with integer l in addition to fundamental spots due to the K2NiF4-type structure. The extinction rule of the superlattice spots indicates that the crystal structure of the “S” phase at room temperature is basically characterized by the rotation of the RuO6 octahedron about both the [001] and [110] directions. However, experimental data also indicated that the extinction rule of the superlattice spots depends on the location in the sample. It is suggested that it is easy to introduce stacking faults associated with the rotation displacements along the c axis. In addition, no structural change was found to occur in diffraction patterns in the temperature range between 85 K and 422 K. This shows that the magnetic transition near 150 K does not lead to any change in the crystal structure.

Features of modulated structures in La1.5Sr0.5NiO4 and La0.5Sr1.5MnO4 with the K2NiF4-type structure have been investigated in the temperature range between 85K and 400K by transmission electron microscopy. In the case of La1.5Sr0.5NiO4, there exists a modulated structure, which is characterized by streaks along the [001] direction through 1/2 1/2 0-type positions in reciprocal space. Features of electron diffraction patterns from the modulated structure were found to be the same as those reported due to Chen et al. [Phys. Rev. Lett. 71, 2461 (1993)]. This clearly suggests that the streak should be related to the existence of the two-dimensional breathing polaron. In addition to La2-xSrxNiO4, a modulated structure was also found below about 230K in La0.5Sr1.5MnO4. Diffraction patterns from the modulated structure in La0.5Sr1.5MnO4 indicates the presence of superlattice spots at q=l/4 as well as fundamental spots due to the K2NiF4-type structure. Because features of diffraction patterns from La0.5Sr1.5MnO4, are quite different from those La1.5Sr0.5NiO4, it seems that an origin of the modulated structure in La0.5Sr1. 5MnO4 has no relation with the periodic array of the breathing polaron.

Features of microstructures in Sr2RuO4 have been investigated by transmission electron microscopy in a temperature range between 12K and room temperature. In electron diffraction patterns taken from some regions at room temperature, there are both diffuse streaks and superlattice spots as well as fundamental spots due to the K2NiF4-type structure. Layered defects, which is related to the streaks, were also observed in bright field images. In addition, no change was found to occur in diffraction patterns, when a specimen was cooled from room temperature to 12K. This clearly suggests that an anomaly in electrical resistivity along the c axis around 130K is not due to a change in a crystal structure.

Features of low-temperature structural transitions in both L1.875Ba0.125CuO4 and La1.675Ba0.125Nd0.2CuO4 have been investigated by transmission electron microscopy. The low-temperature transitions in both oxides were found to accompany 1/2 1/2 0-type superlattice spots in electron diffraction patterns. A feature of the superlattice spot is that the spot in La-Ba-Nd-Cu-O does not appear at a transition temperature of about 110K, but around 70K below it. In addition, bright- and dark-field images suggest that high- and low-temperature phases in the transition always coexist below the transition temperature.

Features of structural disorder in Tl2Ba2Ca2Cu3O10 have been investigated by means of electron microscopy in a temperature range between about 20K and room temperature. It has been found that the temperature-dependent diffuse scattering is observed as a diffuse streak along the [001] direction through a 1/2 1/2 0-type position in reciprocal space. Important features are that the streak appears around Tc and its intensity increases remarkably on cooling. That is, the structural fluctuation increases in a superconducting state. From the detailed analysis of diffraction patterns, it is understood that the structural fluctuation has a two-dimensional nature and is characterized as a transverse wave with q = 1/2 with a polarization vector parallel to the [001] direction.

Characteristic features of both structural transitions and related fluctuations in La1.875-xSmxSr0.125CuO4 have been investigated by means of electron diffraction at a temperature range between about 20K and about 600K. It has been experimentally found that when the temperature is lowered a structural transition from an HTT phase to an LTO one takes place in 0 ≤ x≤0.2 and HIT→LTO→Pccn→LTT successive transitions occur in 0.3≤x≤0.4. In addition, a structural-fluctuation enhancement indicating the soft phonon mode was also detected in both the HTT-to-LTO and LTO-to-Pccn transitions.

Features of a cubic-to-orthorhombic transition in Ba1-xKxBiO3 with 0.1 directions with the same rotation angles. In addition, a magnitude of a band gap observed in the orthorhombic phase, which exhibits a semiconducting property, can be explained on the basis of the present results; that is, the transition due to the R25 mode.

The details of crystal structures in Sr1−xLaxTiO3 with the simple perovskite structure have been investigated by means of electron diffraction at room temperature. When the Sr content decreases, the crystal structure was found to change as follow; Pm3m (x=0), R3c (0.05≤×

We have investigated microstructures associated with the magnetic and dielectric properties of multiferroic mixed-perovskite compounds of the formula (1-x)BiFeO3-xBaTiO3by transmission electron microscopy (TEM) combined with conventional dielectric and magnetic measurements. TEM observation revealed that there exist complicated ferroelectric (FE) domains 20–30 nm in size in the FE phase of BiFeO3. It was also found that, in anx=0.25 compound, a weak spontaneous magnetization appears in the FE phase at 20 K. From the results of the X-ray and electron diffraction experiments, it is suggested that this weak spontaneous magnetization is closely associated with the decrease in the extent of rhombohedral distortion by a partial substitution of BaTiO3for BiFeO3.

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.

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 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.

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.

Before the discovery of superconductivity in La-Ba-Cu-O by Bednorz and Müller, it has been known that BaPb1−xBixO3 (BPB) exhibits superconductivity below about 13 K. In particular, because Ba1−yKyBiO3 (BKB) was found to be a superconductor with Tc of about 30 K, the Ba-oxides become more important in order to understand the mechanism of superconductivity of the oxide superconductors. Characteristic features for the Ba-Oxides are that their crystal structure is basically the cubic perovskite structure and there is not a Cu-O layer which is responsible for the superconductivity in the oxide superconductors having the Cu-O layer, such as Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O. That is, the O(2p)-[Pb(6s), Bi(6s)] hybridized orbitals would play an important role in the case of the Ba-oxides. In the present work, in order to understand features of the crystal structure of the Ba-oxides, including a structural disorder, the details of the structure have been examined by means of electron diffraction.

Features of modulated structures in Bi{sub 2}Sr{sub 2}Ca{sub 1{minus}{ital x}}Y{sub {ital x}}Cu{sub 2}O{sub {ital y}} have been investigated by means of electron microscopy. In addition to two one-dimensional modulated structures with different periods, a new structure modulated along the {ital a} and {ital b} axes is found in 0.8{le}{ital x}{le}1.0. A high-resolution electron micrograph shows that the new structure is characterized by the periodic array of wavy stripes along the {ital b} axis.