5 Electron diffraction analysis of nanostructured
materials
5.1 Modulated superstructures of Tl-based copper oxides
In addition to the basic layer structures of Tl-based copper oxides, modulated structures accompanied with the satellite spots have been observed [1–4]. Figure 5.1 shows
electron diffraction patterns of Tl2 Ba2 CuO6 taken along the various directions of the
crystal [5]. In addition to the fundamental reflections, sharp satellite spots are observed in Figures 5.1 (a)–(c), which indicate the modulated superstructure. The electron diffraction pattern of Figure 5.8 (c) is obtained by 18° rotating Figure 5.1 (d) along
the c-axis. The Tl2 Ba2 CuO6 has both tetragonal and orthorhombic structures, and the
modulated structure is observed in the orthorhombic phase. The fundamental structure with the modulated structure is distorted a little, and the indices are those of an
orthorhombic unit cell (a = 0.545 nm, b = 0.549 nm, c = 2.318 nm) as observed in
the electron diffraction pattern of Figure 5.8 (a). The fundamental lattice has a twin
a*
a*
200
b*
220
220
b*
000
000
020
a*
b*
c*
c*
0010
311
000
620
000
220
Fig. 5.1: Electron diffraction patterns of Tl2 Ba2 CuO6 taken with the incident beam parallel to the
̄
̄ directions. The electron diffraction pattern (b) showing the
(a) [001], (b) [001], (c) [130],
and (d) [110]
modulated structure and its twinning.
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88 | 5 Electron diffraction analysis of nanostructured materials
structure with a twin plane of {110} as observed in Figure 5.1 (b). The superstructure
reflections are observed along the [130], and the modulated wave vector was determined as q = [±0.07 0.22 1] = 1/6.2 ⟨1 3 0⟩, i.e. the modulation is incommensurate.
Figure 5.2 (a) is an HREM image of Tl2 Ba2 CuO6 taken along c-axis. An enlarged image of Figure 5.2 (a) is shown in Figure 5.2 (b). In addition to the fundamental lattice
fringes, dark and bright contrasts with a distance of ∼ 1.2 nm and their twin relations
can be seen. This HREM images show that the direction of the modulated structure is
near [130]. Although twinning of the modulated structure appears on both {110} and
near {100} planes, the twinning of fundamental lattice appears only on {110} planes,
as indicated by the arrows in Figures 5.2 (a) and (b). Figure 5.2 (c) is a high-resolution
1.2 nm
Tl
2.3 nm
2.4 nm
(b)
(a)
(b)
c
[130]
Fig. 5.2: HREM image of Tl2 Ba2 CuO6 taken along c-axis. (b) Enlarged image of (a). (c) HREM image
̄ direction.
with the incident beam parallel to the [310]
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5.1 Modulated superstructures of Tl-based copper oxides
| 89
̄
incidence. Dark and bright contrasts
image of Tl2 Ba2 CuO6 2201 taken with the [130]
with a distance of ∼ 2.4 nm are observed, and the modulated region (a) and the nonmodulated region (b) are clearly distinguishable. By careful observation of the modulation contrast, changes in both the darkness and position of Tl atoms can be seen,
as indicated by arrows in the region (a) of Figure 5.2 (c). This indicates that the origin
of modulated structure would exist on the Tl–O planes.
From detailed composition analysis of the Tl-2201 phase, the orthorhombic with
modulated structure and the tetragonal without the modulation had the composition
of Tl1.7 Ba2 CuO5.7 and Tl1.6 Ba2 CuO5.6 , respectively. The modulated structure has 0.3 oxygen and 0.3 Tl deficiencies per unit cell. The electron diffraction and high-resolution
observation showed the 6.2 times superstructure along the [130] direction. These results indicated that the modulation would be due to the atomic ordering of oxygen and
Tl in the Tl–O layers along the [130] with a period of 6.2 times. If the oxygen and Tl
deficiencies are assumed along the [130] direction, the deficiencies are calculated as
0.36 per the unit cell, which agree well with the composition analysis. Therefore, the
modulated superstructure is believed to be due to Tl and oxygen vacancies in the Tl–
O layers along the [130] with a period of 6.2 times (∼ 2.4 nm). On the other hand, the
tetragonal phase had more atomic deficiencies randomly, and did not show the modulated structure.
Electron diffraction patterns of Tl2 BaSrCuO6 taken with the incident beam parallel to the [001] and [010] directions are shown in Figure 5.3 (a) and (b), respectively.
When Sr atoms are doped at the Ba sites, the fundamental structure has a tetragonal
structure. Satellite reflections due to a modulated structure are observed, which are
weak and diffuse compared to the Tl-2201 phase. In addition, the modulation wave
vector was changed as q = ⟨1/6 0 1⟩.
For Tl2 Ba2 CaCu2 O8 (Tl-2212) [1, 2] and Tl2 Ba2 Ca2 Cu3 O10 (Tl-2223) [1], weak diffuse scatterings were also observed, and the modulation wave vector is determined to be q = ⟨1/6 0 1⟩ as observed in Figure 5.3 (c). This modulation shows twodimensional character, and the symmetry of the fundamental lattice remains tetragonal (a = 0.385 nm, c = 2.92 nm). An HREM image corresponding to Figure 5.10 (c) is
shown in Figure 5.3 (d). Modulation contrast with a distance of ∼ 2.3 nm is observed
in the Tl–O layer along the a-axis. Models for the modulated superstructures were
reported as follows: short-range ordering due to displacements of Tl and O in the Tl–O
planes [2], extra oxygen in the Tl–O planes [1], a partial substitution of Tl3+ by Tl+ [1, 3],
and the mutual substitution of Tl and Ca atoms [4, 6]. The compositional analysis of
the present samples showed that the Tl-2212 and Tl-2223 phases had compositions of
Tl1.7 Ba2 Ca1.3 Cu2 O8 and Tl1.7 Ba2 Ca2.3 Cu3 O10 , respectively. This implies that the excess
0.3 Ca atoms are doped at the Tl sites per the unit cell. The electron diffraction and
HREM observation showed six times superstructure along the a-axis. If the Tl atoms
are substituted by Ca atoms with a period of six times along the axis, the substitution
atoms are 0.33 per the unit cell, which agreed well with the measured composition of
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90 | 5 Electron diffraction analysis of nanostructured materials
c*
a*
110
200
000
200
a*
000
c*
000
2.9 nm
2.3 nm
200
c
a
020
010
000
100
000
200
Fig. 5.3: Electron diffraction patterns of Tl2 BaSrCuO6 taken with the incident beam parallel to
the (a) [001] and (b) [010] directions. (c) Electron diffraction pattern and (d) HREM image of
Tl2 Ba2 CaCu2O8 taken with the incident beam parallel to the [010] direction. Electron diffraction
patterns of (e) TlBa2 CaCu2O7 and (f) TlBa2 Ca2 Cu3 O9 taken along the c-axis.
the samples. Therefore, the modulated superstructure is believed to be due to the Tl
substitution by Ca atoms along the a-axis with a period of six times (∼ 2.3 nm).
Figure 5.3 (e) and (f) are electron diffraction patterns of TlBa2CaCu2 O7 (Tl-1212) and
TlBa2 Ca2 Cu3 O9 (Tl-1223) taken along the c-axis, respectively. Weak, diffuse satellite
scatterings are observed as indicated by arrows, and the observed modulation wave
vector is approximately q = ⟨0.28 0 0.5⟩ [5]. Almost the same incommensurate diffuse
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| 91
5.2 Modulate structures of lanthanoid-based copper oxides
Table 5.1: Oxygen deficiencies 𝛿 of Tl-based superconductors calculated from electron diffractions of
Figure 5.3 and iodimetric measurements of oxygen contents.
Structure
TlBa2 CaCu2 O7−𝛿
TlBa2 Ca2 Cu3 O9−𝛿
TlBa2 Ca3 Cu4 O11−𝛿
Reflection at x 0 0.5
Averaged 𝛿
Iodimetric measurement
Averaged 𝛿
0.24–0.29
0.26–0.31
0.25–0.32
0.27
0.29
0.29
0.279–0.309
0.277–0.311
0.254–0.326
0.29
0.29
0.29
scattering was observed for the TlBa2 Ca3 Cu4 O11 (Tl-1234) phase. As the modulation
shows a two-dimensional character, the symmetry of the fundamental lattice remains
tetragonal. The diffuse scattering becomes stronger as the oxygen loss is increased
and also the Tc increases. Oxygen deficiencies 𝛿 of Tl-based superconductors calculated from electron diffractions of Figure 5.3 and iodimetric measurements of oxygen
contents are summarized in Table 5.1. From the compositional analysis for the Tl-1212,
1223, and 1234 phases, the atomic ratios of Tl:Ba:Ca:Cu were determined to be 1:2:1:2,
1:2:2:3, and 1:2:3:4 (stoichiometry), respectively. However, 0.29 oxygen atoms are deficient per unit cell, as summarized in Table 5.1, which implies that the oxygen vacant
positions are the same for these structures. In this work, oxygen atoms in the Tl–O
layers would be deficient, and the measured modulation from the electron diffraction
patterns are summarized in Table 5.1. As listed in Table 5.1, assumed oxygen vacancies in the Tl–O layers measured by electron diffraction agreed well with the measured oxygen vacancies by iodimetric measurements. Therefore, it is believed that the
modulation superstructure would be due to oxygen vacancy ordering in the Tl–O layer
along the a-axis with a period of 3.45 times (= 0.29−1 ) and two times along c-axis. The
period of 3.45 times is incommensurate, which implies the mixture of 3- and 4-times
superstructures. In fact, modulations with periods of 3.1–4.2 times (= 0.32−1 − 0.24−1 )
are observed for electron diffraction patterns, as listed in Table 5.1.
5.2 Modulate structures of lanthanoid-based copper oxides
Various types of lanthanoid-based copper oxides have been reported, and electrondoped Nd2 − x Cex CuO4 superconductors were discovered [7, 8]. In order to clarify the
microstructures, single crystals of Ln2 CuO4 prepared with various heat treatments are
investigated by means of high-resolution electron microscopy and electron diffraction.
Single crystals of Ln2 CuO4 (Ln = Pr, Nd, Sm) were grown by the traveling-solventfloating-zone technique using an infrared-heating furnace [9, 10]. To reduce oxygen
content, parts of the Ln2 CuO4 (Ln = Pr, Nd, Sm) samples were annealed at 1100°C for
18 h in air and quenched in liquid nitrogen. The rests of the samples were annealed at
400°C in air for 38–140 h to saturate oxygen content in the crystals. SmLa0.75 Sr0.25 CuO4
was also synthesized from a mixture of La2 O3 , Sm2 O3 , CuO, and SrCO3 [11]. Mixed
powder was first calcined at 950°C in air for 10 h, then pressed into pellets, and fi-
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92 | 5 Electron diffraction analysis of nanostructured materials
nally sintered at 1130°C in air for 15 h. The pellets were quenched to room temperature
in air, and subsequently annealed at 550°C in the atmosphere with various oxygen
pressures.
Modulated superstructures are also observed in the lanthanoid-based copper
oxides [12–14]. The domains of superlattice with sizes of 5–60 nm in diameter were
observed, and the smaller domains (5–10 nm) are observed around a large one, and
seem to grow into larger ones (40–60 nm). A representative superstructure domain
in Nd2 CuO4 is shown in Figure 5.4 (a), and Figure 5.4 (b) is an electron diffraction
pattern of Figure 5.4 (a). Sharp satellite reflections with a wave vector q = ⟨1/4 1/4 0⟩
are observed in Figure 5.4 (b). In Figure 5.4 (a), repeated dark and bright contrasts
separated at a distance of 1.1 nm (≃ 2√2 × a) are observed in the [110] direction. The
high-resolution image and the diffraction pattern reveal that the basic lattice spacing
̄
of the superlattice lengthen as much as 101.5% and 100.3% in the [110] and [110]
directions, respectively, as compared with the fundamental lattice. Therefore, the
contrast due to strain field is observed around the domain. Two-directional superlattice domain is also observed as shown in a HREM image in Figure 5.4 (c), and an
110
000
110
110
000
110
Fig. 5.4: (a) HREM image and (b) electron diffraction pattern of a single domain of the superlattice
in Nd2 CuO4 , taken with the [001] incidence. (c) HREM image and (d) electron diffraction pattern of
superlattice domains along two directions.
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5.2 Modulate structures of lanthanoid-based copper oxides
|
93
electron diffraction pattern of the superlattice domains along two directions is shown
in Figure 5.4 (d), which also indicates a modulation wave vector of q = ⟨1/4 1/4 0⟩.
It can be considered that such domain structure is due to nonuniformity of oxygen
content in the specimens.
Various types of modulated superstructures were observed in the Ln2 CuO4 ,
as summarized in Table 5.2. Figures 5.5 (a)–(d) are electron diffraction patterns of
̄ direction.
Nd2 CuO4 , Pr2 CuO4 , Pr1.85 Ce0.15 CuO4 , and Sm2 CuO4 , taken along the [110]
004
224
004
112
000
110
004
004
000
110
000
110
000
110
224
112
004
000
101
101
200
000
010
Fig. 5.5: Electron diffraction patterns of (a) Nd2 CuO4 , (b) Pr2 CuO4 , (c) Pr1.85 Ce0.15 CuO4 , and (d)
̄ incidence. Electron diffraction patterns of Pr2 CuO4 taken along the
Sm2 CuO4 , taken with the [110]
̄
(e) [010] and (f) [111] directions.
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94 | 5 Electron diffraction analysis of nanostructured materials
Table 5.2: Summary of modulated structures of Ln2 CuO4 .
Structure
Modulation wave vector q
Nd2 CuO4
Sm2 CuO4
Pr2 CuO4
Nd1.85 Ce0.15 CuO4
Sm1.85 Ce0.15 CuO4
Pr1.85 Ce0.15 CuO4
⟨1/4 1/4 0⟩
⟨1/4 1/4 0⟩, ⟨1/2 1/2 1⟩
⟨1/4.2 1/4.2 3/4.2⟩, ⟨1/4 1/4 1/2⟩, ⟨1/2 0 1/2⟩
⟨1/4 1/4 0⟩
⟨1/4 1/4 0⟩, ⟨1/2 1/2 1⟩
⟨1/3 1/3 0⟩
For the Pr2 CuO4 , Pr1.85 Ce0.15 CuO4 and Sm2 CuO4 crystals, satellite reflections at 0.24
0.24 0.72, 1/3 1/3 0 and 1/2 1/2 1 are observed, as shown in Figure 5.5 (b), (c), and (d),
respectively. Figures 5.5 (e) and (f) are electron diffraction patterns of Pr2 CuO4 taken
̄ directions, respectively, which also indicates weak diffuse
along the [010] and [111]
scattering and sharp satellite reflections at 1/2 0 1/2 and 1/4 1/4 1/2, respectively. A
superstructure have been observed and characterized for Nd2 − x Cex CuO4 by a wave
vector q = [1/4 1/4 0] [9], and was suggested that the modulation is due to ordering
of oxygen vacancy and/or Ce. However, the satellite reflections were not observed in
the diffraction patterns of Nd2 CuO4 and Pr2 CuO4 quenched from 1100°C. The result
indicates that the appearance of superstructures is sensitive to the oxygen content
and unrelated to ordering of Ce atoms. Since neutron diffraction study shows the
deficiency of oxygen in Cu–O planes, it can be supposed that the superlattices are due
to ordering of oxygen atoms in the Cu–O planes.
5.3 Oxygen ordering in YBa2 Cu3 O7−x
The crystal structure of YBa2 Cu3 O7 is based on a triple perovskite structure and is characterized by the ordering of oxygen vacancies, that is, the oxygen positions on the Y
atom layer and between two Ba atoms are vacant. In addition, oxygen orderings in the
Cu–O basal planes were observed [15]. Bulk samples of YBa2 Cu3 O7−x superconductors were prepared by mixing BaCO3 , Y2 O3 , and CuO powders with the composition of
YBa2 Cu3 O7−x phase. The mixture pellets were calcined at 930°C for 12 h in air and then
cooled slowly in a furnace. After crushing the pellets to form powders, the process was
repeated once more. The obtained pellets were reheated at 500–900°C and quenched
into liquid nitrogen, and subsequently annealed at 500–300°C in the vacuum seal. The
pellets were also annealed in a flowing N2 gas at various temperatures to control the
oxygen contents. The oxygen contents were investigated by iodimetric measurements
and mass change.
As a consequence of the tetragonal-to-orthorhombic phase transition of
YBa2 Cu3 O7−x at ∼ 600°C [16], twin boundaries are often observed. Figure 5.6 (a) and (b)
are TEM image and lattice image of YBa2 Cu3 O7−x taken with the incident beam parallel
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5.3 Oxygen ordering in YBa2 Cu3 O7−x
|
95
to the c-axis. Twin boundaries (TB) are coherent and indicated by arrows. The twin
boundaries show distinct contrast in the TEM image, and the existence of boundaries is evident from kinks of the lattice fringes. In the oxygen-deficient YBa2 Cu3 O7−x
compounds, oxygen vacancy ordering was observed. Figures 5.6 (c) and (d) are electron diffraction patterns of YBa2 Cu3 O6.68 taken with the incident beam parallel to the
c-axis and b-axis, respectively. The electron diffraction pattern shows orthorhombic
TB
a*
b*
TB
TW
TB
003
110
000
100
b*
000
a*
a
b
b
a
TB
Fig. 5.6: (a) TEM image and (b) lattice image of YBa2 Cu3 O7−x taken with the incident beam parallel to the c-axis. Twin boundaries (TB) are indicated by arrows. Electron diffraction patterns of
YBa2 Cu3 O6.68 taken with the incident beam parallel to the (c) c-axis and (d) b-axis. (e) Fourier transform of (b). (f) Filtered inverse Fourier transform of (e).
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96 | 5 Electron diffraction analysis of nanostructured materials
structure with a- and b-axes and a twin structure with a {110} twin plane. Both electron diffraction patterns in Figure 5.6 (c) and (d) show diffuse satellite reflections at
1/2 0 0 along the a-axis. This indicates the existence of modulated superstructure with
a modulation wave vector q = ⟨1/2 0 0⟩, which would be due to ordering of oxygen
vacancies on the basal Cu–O planes. Figure 5.6 (e) is a Fourier transform of HREM
image of Figure 5.6 (b), and filtered inverse Fourier transform of Figure 5.6 (e) is shown
in Figure 5.6 (f), in which linear bright stripes with the distance of 2a are observed
a*
b*
b*
010
b*
000
000
100
a*
a*
a*
b*
b*
010
b*
100
000
000
a*
a*
a*
b*
b*
000
a*
Fig. 5.7: Electron diffraction patterns of (a) YBa2 Cu3 O6.80 , (b) YBa2 Cu3 O6.47 , (c) YBa2 Cu3 O6.23 and
(d) YBa2 Cu3 O6.29 taken along the c-axis. (e) HREM lattice image and (f) electron diffraction pattern of
YBa2 Cu3 O6.47 taken along the c-axis.
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5.3 Oxygen ordering in YBa2 Cu3 O7−x
| 97
Table 5.3: Summary of modulated structures of YBa2 Cu3 Oy .
Quenched and annealed in a sealed tube
y Tc / K Observed structure
6.80
6.68
6.60
6.47
6.29
86
49
48
4
–
Annealed in N2
Observed structure
y
Diffuse streaks along a-axis
⟨1/2 0 0⟩
Diffuse streaks along a-axis
⟨1/3 0 0⟩, ⟨1/2 0 0⟩
⟨1/3 0 0⟩, ⟨1/4 0 0⟩
6.91
6.74
6.50
6.23
6.00
Perfect orthorhombic
⟨0 1/3 0⟩, ⟨1/2 0 0⟩
⟨1/2 0 0⟩
⟨1/2 0 0⟩, ⟨0 1/3 0⟩
Perfect tetragonal
along a-axis. The twin boundary can be clearly seen at a glancing view parallel to the
a- or b-axis in Figure 5.6 (f).
Electron diffraction patterns of oxygen deficient YBa2 Cu3 O6.80 , YBa2 Cu3 O6.47 ,
YBa2 Cu3 O6.23 and YBa2 Cu3 O6.29 taken along the c-axis are shown in Figure 5.7 (a)–(d),
respectively. In Figure 5.7 (a), weak diffuse streaks are observed along a-axis, which
would indicate short range ordering of oxygen atoms. In Figure 5.7 (b) and (c), superstructures with a modulation wave vector q = ⟨1/3 0 0⟩ and ⟨0 1/3 0⟩, which indicates
the superstructures are formed along the a-axis and b-axis of orthorhombic cell, respectively. In addition, a superstructure with a modulation wave vector q = ⟨1/4 0 0⟩
is observed along the a-axis, as shown in Figure 5.7 (d). These modulated structures
are summarized as listed in Table 5.3 [17].
Figure 5.7 (e) and (f) is a lattice image and an electron diffraction pattern of
YBa2 Cu3 O6.47 taken with the [001] incidence. Satellite peaks with a modulation wave
vector q = ⟨1/3 0 0⟩ are observed together with q = ⟨1/2 0 0⟩ in the diffraction pattern.
Linear bright stripes with the distance of 3a are observed along the two principal
lattice directions in the HREM image of Figure 5.7 (e).
OV
Cu
O
Cu
O
O6
O7
O6.66
b
O6.5
a
O6.33
c
b
O6.33
a
O6.25
OV
(a)
(b)
Fig. 5.8: Models for ordered arrangement of oxygen vacancies in YBa2 Cu3 O7−x . (a) Two-times model
along the a-axis. (b) Oxygen ordering models of basal planes (Cu–O) of the YBa2 Cu3 O7−x .
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98 | 5 Electron diffraction analysis of nanostructured materials
From these observations, a model for the ordered arrangement of oxygen vacancies is proposed, as shown in Figure 5.8 (a). The fundamental unit cell is orthorhombic,
with a dimension of 2a × b × c. Other oxygen ordering models of basal planes (Cu–O) of
the YBa2 Cu3 O7−x are also proposed as shown in Figure 5.8 (b), which depends on the
oxygen content. These phases would correspond to the ortho-II phase and ortho-III
phases [18–20], which results in the changes of Tc , and the control of oxygen atoms in
the oxide crystals is important.
5.4 Structures of Bi-based copper oxides
Bi-based copper oxides with Ag are expected for wire application. A spray-dried
aqueous solution of nitrates with atomic ratio Bi1.5 Pb0.5 Sr2 Ca2 Cu3 was calcined at
650°C for 15 h, resulting in a mixture of oxides [21–23]. The resulting precursor powder
with a grain size of 3 μm was mixed with 30 vol% Ag whiskers with a diameter of 20–
50 μm and a length of a few hundred micrometers. The Ag whiskers were synthesized
via an electrochemical reduction of a Ag nitrate solution by a copper wire at pH 2.
The mixture was pressed into bars and sintered at 853°C for 170 h in air to obtain the
Bi-2223/Ag composites.
Ag
Bi2223
c
b
220
111
000
0010
000
020
Fig. 5.9: (a) TEM image of (Bi,Pb)2 Sr2 Ca2 Cu3 Ox /Ag whisker interface in a sintered composite. Electron diffraction of (b) Ag and (c) (Bi,Pb)2 Sr2 Ca2 Cu3 Ox .
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| 99
5.4 Structures of Bi-based copper oxides
Figure 5.9 (a) is a TEM image of the Bi-2223/Ag whisker interface in a sintered composite. A thin layer with a different contrast is observed at the Bi-2223/Ag interface.
Electron diffractions of Ag and Bi-2223 phase are also shown in Figures 5.9 (b) and
̄ of the face-centered cubic
(c), respectively. Figure 5.9 (b) is observed along the [110]
Ag crystal. The diffraction pattern of Bi-2223 phase in Figure 5.9 (c) was taken along
the a-axis, which indicates a modulated structure with a modulation wave vector of
q ∼ ⟨0 1/4 0⟩. The origin of the modulated superstructure would be metal atom displacements in the crystal [24–26].
Figure 5.10 (a) is a TEM image of the Bi-2223/Ag whisker composite with the Agrich phase. Lattice fringes of c-planes of Bi-2223 phase are observed, and an amorphous/nanocrystalline (AM-NC) structure is observed at the Ag/Bi-2223 interface. The
white areas are the result of preferential ion milling, probably of amorphous phases,
which are more easily removed compared to Bi-2223. The superconducting phase is
oriented with the c-axis perpendicular to the interface. An EDX spectrum of the AMNC phase in Figure 5.10 (a) is shown in Figure 5.10 (b), which indicates a composition
of Ag2 Bi2.4 Pb0.6 Sr2 Ca0.8 Cu4.3 , and an increase of Ag, Bi, Pb, and Cu concentration in
the amorphous phase. An enlarged HREM image and an electron diffraction pattern
AM-NC
AM-NC
Cu
AM-NC
Bi2223
Bi
Intensity [A. U.]
Bi2223
O
Sr
Ag
Ag Ca
Ca
Bi2223
c
0
AM-NC
1
10 nm
[110]
2
3
4
5
Energy [keV]
c*
c
Bi2223
[110]
0010
Bi
000
220
AM-NC
10 nm
Fig. 5.10: (a) TEM image of the Bi-2223/Ag whisker composite with the Ag-rich phase. (b) EDX spectrum of the AM-NC phase in (a). (c) HREM and (d) electron diffraction pattern at the interface.
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100 | 5 Electron diffraction analysis of nanostructured materials
at the interface are shown in Figure 5.10 (c) and (d), respectively. The Bi-2223/AM-NC
interface exhibits small steps of half or one unit cell of Bi-2223, and such intermediary phase was also observed [22]. The diffraction pattern exhibits [110] incident of the
Bi-2223 crystal, and the observed streak along the c-axis indicates that there is a small
amount of the Bi-2212 or Bi-2234 phase to form an intergrowth structure. A diffuse ring
is also observed as indicated by arrows, which exhibits the amorphous-like structure
of AM-NC phase. The influence of Ag on the yield in Bi-2223 synthesis can be explained
in terms of the shift in the incongruent melting point.
5.5 Twin structures in BN nanoparticles
Chemical vapor-deposited boron nitride (CVD-BN) has been used in various practical
fields, such as crucibles for semiconductor materials, high-temperature jigs, and insulators, due to its high purity, high density, and chemical inertness. The effects of
deposition temperature and total gas pressure on the crystal structure, density, and
microstructure of CVD-BN have been studied. Structures of BN are hexagonal (h-BN,
ABAB stacking), cubic (c-BN, ABCABC stacking), wurzite type (w-BN, ABAB stacking), amorphous (a-BN), turbostratic (t-BN), and rhombohedral (r-BN, ABCABC stack-
N
B
B
N
h-BN
r-BN
B
N
B
N
c-BN
w-BN
Fig. 5.11: Structure models of h-BN, r-BN, w-BN, and c-BN.
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5.5 Twin structures in BN nanoparticles
| 101
ing) was also reported [27, 28], as shown in Figure 5.11. The r-BN is expected as a
starting material for c-BN, with high hardness and thermal conductivity next to diamond because of the same periodicity in the stacking ABCABC in crystallographic
layers, and r-BN can be directly converted into c-BN by shock compression and high
static pressure [29]. Formation and atomic structures of CVD-BN with rhombohedral
113
110
102
101
003
000
600 nm
113
102
111
011
000
70 nm
c*
102
003
101
000
202
a*
200 nm
Fig. 5.12: (a) TEM image of CVD-BN synthesized from BCl3 –NH3 –H2 gas system at a deposition temperature of 1600°C and a total gas pressure of 3 Torr. (c) TEM image of r-BN nanoparticle. (e) TEM
image of r-BN nanoparticle perpendicular to the c-axis. (b, d, f) Electron diffraction patterns of (a, c,
e), respectively. The diffraction patterns were taken in the large area (a), along (c) [211], and (e) [010]
of r-BN nanoparticles.
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102 | 5 Electron diffraction analysis of nanostructured materials
and hexagonal structures were investigated, and the nanostructures of c-BN converted from r-BN were also investigated. CVD-BN plates with a rhombohedral structure
were synthesized from BCl3 –NH3 –H2 gas system at 1600°C and a total gas pressure
of 3–5 Torr on the graphite substrates in CVD apparatus (Tachibana Riko CVD-250T4) [30, 31]. BCl3 (purity, 99.9%) and NH3 (purity, 99.95%) gases were used as starting
materials and H2 (99.999%) for dilution. These gases were introduced separately into
the CVD reactor near the substrate. The gas-flow rates were kept constant at 90 sccm
N B
NB
N B
c
c
a
a
c*
c*
003
104
101
000
c
a
c*
c*
110
{000}
{003}
113
000
003
Fig. 5.13: (a) HREM image of r-BN. (b) Enlarged HREM image of r-BN after Fourier filtering. (c) HREM
image of microtwin. (d) Electron diffraction pattern of twinned r-BN nanoparticle taken along [010].
(e) TEM image of twinned r-BN nanoparticle. (f) Electron diffraction pattern of (e), taken along [110].
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5.5 Twin structures in BN nanoparticles |
103
for NH3 , 140 sccm for BCl3 , and 670 sccm for H2 . The growth rate was 1.7 mm/min.
The lattice parameters of the deposited r-BN, as determined through XRD analysis,
were a = 0.2506 ± 0.0004 nm and c = 1.003 ± 0.002 nm, which would indicate B/N
1:1. These BN plates were thinned to 0.1 mm with emery papers, and then punched to
disks 2.3 mm in diameter with a supersonic wave cutter. The disks were polished with
a dimple grinder to < 50 mm in thickness and thinned by argon ion milling at an accel-
c
b
d
{101}
a
113
101
101
012
012
111
000
111
000
113
101
012
101
111
000
000
012
111
Fig. 5.14: (a) TEM image of r-BN nanoparticle with twin structures. (b) HREM image of twin boundary
at region d in (a) after Fourier filtering. (c–f) Electron diffraction pattern of twin boundary at regions
a–d in (a), respectively, taken along [121].
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104 | 5 Electron diffraction analysis of nanostructured materials
erating voltage of 3–5 kV. c-BN powder was produced from r-BN at 1800–2200°C and
6–7 GPa in an octahedral anvil-type device.
Figure 5.12 (a) is a TEM image of CVD-BN synthesized from BCl3 –NH3 –H2 gas system at a deposition temperature of 1600°C and a total gas pressure of 3 Torr. A considerable number of particles are observed in the sample. It should be noted that
only those particles which satisfy certain diffraction conditions are visible in the image, which was confirmed by tilting the crystal. An electron diffraction pattern of Figure 5.12 (a), taken from the wide area (1 μm), is shown in Figure 5.12 (b). The electron
diffraction pattern of Figure 5.12 (b) shows many diffraction spots attributed to the
particles in addition to the Debye–Scherrer rings from the t-BN matrix. The rings are
indexed as 003, 101, 102, 110, and 113 of r-BN. An enlarged image and an electron diffraction pattern of a r-BN nanoparticle is shown in Figure 5.12 (c) and (d), respectively.
The reflections of Figure 5.12 (d) are indexed as r-BN along the [211] direction. A TEM
image and an electron diffraction pattern of r-BN nanoparticle, taken along [010], are
shown in Figure 5.12 (e) and (f), respectively. Streaks along the c∗ -axis are observed,
which are due to the microtwin and stacking faults of {001}.
A HREM image of r-BN in Figure 5.12 (e) is shown in Figure 5.13 (a). Figure 5.13 (b)
is an enlarged HREM image of r-BN after Fourier filtering. White dots correspond to
311
220
111
000
111
200
111
000
Fig. 5.15: (a) TEM image and (b) electron diffraction pattern of c-BN nanoparticles synthesized from
r-BN. (c) TEM image and (d) electron diffraction pattern of c-BN nanoparticle taken along [011].
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5.5 Twin structures in BN nanoparticles |
105
BN atomic pair, as illustrated in Figure 5.13 (b). HREM image of microtwin is shown in
Figure 5.13 (c), which agree with the streaks along c∗ -axis in electron diffraction pattern of Figure 5.12 (f). Figure 5.13 (d) is an electron diffraction pattern of twinned r-BN
nanoparticle taken along [010], which indicates a {101} twin structure of r-BN. A TEM
image of r-BN nanoparticle is shown in Figure 5.14 (e), and a twin boundary is indic-
{101}
{101}
{113}
{113}
{001}
{111}
Fig. 5.16: Atomic structure models of (a) {101}; (b) {101}; (c) {113}; (d) {113}; (e) {001} twin structures of
r-BN along [010], [211], [110], [211], and [010], respectively. (f) Atomic structure model of {111} twin
structure of c-BN along [011]. Circles indicate BN atomic columns along the projection model. Unit
cells and twin boundaries are indicated by solid and dotted lines, respectively.
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106 | 5 Electron diffraction analysis of nanostructured materials
̄
ated by arrows. An electron diffraction pattern of Figure 5.13 (e), taken along the [110]
direction, is shown in Figure 5.13 (f), which indicates a {113} twin structure of r-BN.
A TEM image of r-BN nanoparticle with twin structures is shown in Figure 5.14 (a),
and three twin boundaries are indicated by arrows. A HREM image of twin boundary
at region A in Figure 5.14 (a) is shown in Figure 5.14 (b), which indicates the mirror
relation at the boundary. Figure 5.14 (c)–(f) shows electron diffraction pattern of twin
boundary at regions A–D in Figure 5.14 (a), respectively. All diffraction patterns are
taken along [211]. A {101} twin structure is observed in Figure 5.14 (d) and (f), and a
c*
c*
111
003
012
104
101
101
000
000
c*
c*
111
012
110
113
000
000
003
101
101
111
200
003
111
000
000
102
Fig. 5.17: (a–f) Calculated electron diffraction patterns of twin structures of Figure 5.16 (a–f) of r-BN
and c-BN, respectively.
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Bibliography
|
107
{113} twin structure is observed in Figure 5.14 (e). These twin structures are often observed in the r-BN nanoparticles.
A TEM image and an electron diffraction pattern of c-BN nanoparticles synthesized from r-BN are shown in Figure 5.15 (a) and (b), respectively. Debye–Scherrer rings
indexed as 111, 220, and 311 of c-BN are observed in Figure 5.15 (b). A TEM image and
electron diffraction pattern of c-BN nanoparticle taken along [011] are shown in Figure 5.15 (c) and (d), respectively. The electron diffraction pattern shows {111} twin structure of c-BN.
Based on the above observation, atomic structure models of four kinds of twin
structure are proposed, as shown in Figure 5.16. The structures have mirror relation
at the twin boundaries. The {101} twin structures of Figure 5.16 (a) and (d) are completely the same model from different directions, and the {113} twin structures of Figure 5.16 (b) and (e) are also the same.
Calculated electron diffraction patterns of twin structures of Figure 5.16 (a)–(f) are
shown in Figure 5.17 (a)–(f), respectively. Figure 5.17 (a)–(d) and (f) agree well with
observed diffraction patterns of Figure 5.13 (d), 5.14 (f), 5.13 (f), 5.14 (e), and 5.15 (d),
respectively, which confirms the proposed atomic models of twin structures. The {001}
twin structure of Figure 5.17 (e) does not agree well with Figure 5.12 (f), which is due
to microtwins and stacking faults of h-BN and r-BN, as observed in Figure 5.13 (c) and
streaks along the c∗ -axis in Figure 5.12 (f).
Although only {112} twin plane was found in h-BN [27], three kinds of {101}, {113},
and {001} twin planes were found in r-BN [28]. The {113} plane is consistent with {112}
planes of h-BN, and the {001} twins of r-BN is due to the crystallographic characteristics such as ABCABC stackings. The {101} twins were often observed in r-BN, which
would be due to low interfacial energy in r-BN.
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