ISO 25498:2018

INTERNATIONAL ISO
STANDARD 25498
Second edition
2018-03
Microbeam analysis — Analytical
electron microscopy — Selected area
electron diffraction analysis using a
transmission electron microscope
Analyse par microfaisceaux — Microscopie électronique analytique
— Analyse par diffraction par sélection d'aire au moyen d'un
microscope électronique en transmission
Reference number
ISO 25498:2018(E)
©
ISO 2018

---------------------- Page: 1 ----------------------
ISO 25498:2018(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2018
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2018 – All rights reserved

---------------------- Page: 2 ----------------------
ISO 25498:2018(E)

Contents Page
Foreword .iv
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 3
4.1 General . 3
4.2 Spot diffraction pattern . 3
4.3 Kikuchi pattern . 6
4.4 Polycrystalline specimen . 7
5 Reference materials . 7
6 Equipment . 8
7 Specimens . 8
8 Experimental procedure . 8
8.1 Instrument preparation . 8
8.2 Procedure for acquirement of selected area electron diffraction patterns . 9
8.3 Determination of diffraction constant, Lλ .12
9 Measurement and solution of the SAED patterns .13
9.1 Selection of the basic parallelogram .13
9.2 Indexing diffraction spots .15
10 180° ambiguity .16
11 Uncertainty estimation.16
11.1 Factors affecting accuracy .16
11.2 Calibration with a reference material .17
Annex A (informative) Interplanar spacing .18
Annex B (informative) Spot diffraction patterns of single crystals for BCC, FCC and HCP
[ ]
structure 7 .19
Bibliography .38
© ISO 2018 – All rights reserved iii

---------------------- Page: 3 ----------------------
ISO 25498:2018(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: www .iso .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 202, Microbeam analysis, Subcommittee
SC 3, Analytical electron microscopy.
This second edition cancels and replaces the first edition (ISO 25498:2010), which has been technically
revised.
The main changes to the previous edition are as follows:
— the foreword has been revised;
— the introduction has been revised;
— the scope has been revised;
— the figure of Ewald construction has been deleted;
— the terms and definition of terminological entries 3.1, 3.2, 3.10, 3.11, 3.12, 3.13 and 3.14 have
been added;
— the subclause 4.1 has been added;
— Clause 5 has been revised;
— Clauses 4, 6, 7, and 10 and subclauses 8.1.5, 8.1.6, 8.2.1, 8.2.2, 8.2.4, 8.2.7, 8.2.8, 8.2.11 and 9.1.2 have
been editorially revised;
— the subclause 9.2.5 has been added;
— all formulae have been renumbered;
— Annex A has been revised;
— the subclause B.1 has been revised;
iv © ISO 2018 – All rights reserved

---------------------- Page: 4 ----------------------
ISO 25498:2018(E)

— the figures have been modified;
— the bibliography has been updated.
© ISO 2018 – All rights reserved v

---------------------- Page: 5 ----------------------
ISO 25498:2018(E)

Introduction
Electron diffraction techniques are widely used in transmission electron microscopy (TEM) studies.
Applications include phase identification, determination of the crystallographic lattice type and
lattice parameters, crystal orientation and the orientation relationship between two phases, phase
transformations, habit planes and defects, twins and interfaces, as well as studies of preferred crystal
orientations (texture). While several complementary techniques have been developed, for example
microdiffraction, convergent beam diffraction and reflected diffraction, the selected area electron
diffraction (SAED) technique is the most frequently employed.
This technique allows direct analysis of small areas on thin specimens from a variety of crystalline
substances. It is routinely performed on most of TEM in the world. The SAED is also a supplementary
technique for acquisition of high resolution images, microdiffraction or convergent beam diffraction
studies. The information generated is widely applied in the studies for the development of new
materials, improving structure and/or properties of various materials as well as for inspection and
quality control purpose.
The basic principle of the SAED method is described in this document. The experimental procedure
for the acquirement of SAED patterns, indexing of the diffraction patterns and determination of the
diffraction constant are specified. ISO 25498 is intended for use or reference as technical regulation for
transmission electron microscopy.
vi © ISO 2018 – All rights reserved

---------------------- Page: 6 ----------------------
INTERNATIONAL STANDARD ISO 25498:2018(E)
Microbeam analysis — Analytical electron microscopy
— Selected area electron diffraction analysis using a
transmission electron microscope
1 Scope
This document specifies the method of selected area electron diffraction (SAED) analysis using a
transmission electron microscope (TEM) to analyse thin crystalline specimens. This document applies
to test areas of micrometres and sub-micrometres in size. The minimum diameter of the selected area
in a specimen which can be analysed by this method is restricted by the spherical aberration coefficient
of the objective lens of the microscope and approaches several hundred nanometres for a modern TEM.
When the size of an analysed specimen area is smaller than that restriction, this document can also
be used for the analysis procedure. But, because of the effect of spherical aberration, some of the
diffraction information in the pattern can be generated from outside of the area defined by the selected
area aperture. In such cases, the use of microdiffraction (nano-beam diffraction) or convergent beam
electron diffraction, where available, might be preferred.
This document is applicable to the acquisition of SAED patterns from crystalline specimens, indexing
the patterns and calibration of the diffraction constant.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
Miller index
notation system for crystallographic planes and directions in crystals, in which a set of lattice planes or
directions is described by three axes coordinate
3.2
Miller-Bravais index
notation system for crystallographic planes and directions in hexagonal crystals, in which a set of
lattice planes or directions is described by four axes coordinate
3.3
(h k l)
Miller indices (3.1) of a specific set of crystallographic planes
© ISO 2018 – All rights reserved 1

---------------------- Page: 7 ----------------------
ISO 25498:2018(E)

3.4
{hkl}
Miller indices (3.1) which denote a family of crystallographic planes
3.5
[uvw]
Miller indices (3.1) of a specific crystallographic direction or a zone axis
3.6
interplanar spacing
d
hkl
perpendicular distance between consecutive planes of the crystallographic plane set (h k l) (3.3)
3.7
(uvw)*
notation for a set of planes in the reciprocal lattice
Note 1 to entry: The normal of the reciprocal plane (uvw)* is parallel to the crystallographic zone axis [uvw] (3.5).
3.8
reciprocal vector
g
hkl
vector in the reciprocal lattice
Note 1 to entry: The reciprocal vector, g is normal to the crystallographic plane (h k l) (3.3) with its magnitude
hkl,
inversely proportional to interplanar spacing d (3.6).
hkl
3.9
R vector
R
hkl
vector from centre, 000 (the origin), to the diffraction spot, hkl, in a diffraction pattern
Note 1 to entry: See Figure 1.
3.10
camera length
L
effective distance between the specimen and the plane where diffraction pattern is formed
[SOURCE: ISO 15932:2010, 3.7]
3.11
camera constant
Lλ
product of the wavelength of the incident electron wave and camera length (3.10)
Note 1 to entry: Because of the small Bragg angle, the Bragg condition can be given in the first-order
approximation, Rd⋅≅Lλ , where d is the interplanar spacing of plane, (hkl), (3.3) and R (3.9) is the
hkl
hklhkl hkl
distance of the diffraction spot, hkl, from the incident beam.
[SOURCE: ISO 15932:2010, 3.8, modified]
3.12
bright field image
image formed using only the non-scattered beam, selected by observation of the back focal plane of the
objective lens and using the objective aperture to cut out all diffracted beams
[SOURCE: ISO 15932:2010, 5.6]
2 © ISO 2018 – All rights reserved

---------------------- Page: 8 ----------------------
ISO 25498:2018(E)

3.13
dark field image
image formed by a diffracted beam only by using the objective aperture for selection or by collecting
the diffracted beam with an annular dark-field detector
[SOURCE: ISO 15932:2010, 5.6]
3.14
energy-dispersive X-ray spectroscopy
EDS
analytical technique which enables the elemental analysis or chemical characterization of a specimen
by analysing characteristic X-ray emitted by the matter in response to electron irradiation
[SOURCE: ISO 15932:2010, 6.6]
3.15
eucentric position
specimen position at which the image exhibits minimal lateral motion resulting from specimen tilting
4 Principle
4.1 General
When an energetic electron beam is incident upon a thin crystallographic specimen in a transmission
electron microscope, a diffraction pattern will be produced in the back focal plane of the objective lens.
This pattern is magnified by the intermediate and projector lenses, then displayed on a viewing screen
and recorded (see Reference [3]). This pattern can also be displayed on a monitor if the TEM is equipped
with a digital camera system.
4.2 Spot diffraction pattern
The diffraction pattern of a single crystal appears as an array of “spots”, the basic unit of which is
characterized by a parallelogram. A schematic illustration of a spot diffraction pattern is shown in
Figure 1. Each spot corresponds to diffraction from a specific set of crystal lattice planes in the
specimen, denoted by Miller indices (hkl). The vector, R , is defined by the position of the diffracted
hkl
spot, hkl, relative to position on the pattern corresponding to the transmitted beam, i.e. the centre-
spot, 000, of the pattern. It is parallel to the normal of the reflecting plane, (hkl). The magnitude of R
hkl
is inversely proportional to the interplanar spacing, d , of the diffracting plane, (hkl) (see References [4]
hkl
to [9]). In the context of this document, vectors R , R , RR− and
()
hk l hk l hk lh kl
11 1 22 2 22 21 11
RR+ are simplified as R , R , R and R , respectively. The included angle between
() 1 2 2−1 1+2
hk lh kl
22 21 11
vectors, R and R , is denoted by γ*. The basic parallelogram is defined by R and R , where they are the
1 2 1 2
shortest and next shortest in the pattern respectively and not along a common line. The spot, h k l , is
2 2 2
positioned anticlockwise around the centre spot relative to spot, h k l .
1 1 1
Because the centre-spot is often very bright, it is often difficult to determine the exact centre of the
pattern. Therefore, a practical procedure is to establish the magnitude of R by measuring the
hkl
distance between the spots, hkl and hkl on the diffraction pattern and dividing by two,
1
i.e. RR=+ R . Figure 2 shows an example of the SAED pattern where the magnitude of R ,
1
hklh( kl )
hkl
2
1 1 1
R and R is obtained from RR+ , RR+ and RR+ respectively.
2 2−1 () () ()
11 22 21−−21
2 2 2
© ISO 2018 – All rights reserved 3

---------------------- Page: 9 ----------------------
ISO 25498:2018(E)

Key
R vector from 000 to spot, h k l , the shortest vector in the diffraction pattern
1 1 1 1
R vector from 000 to spot, h k l , the next shortest vector
2 2 2 2
NOTE The basic parallelogram is constituted by diffraction spots, h k l , h k l , (h +h , k +k , l +l ), and
1 1 1 2 2 2 1 2 1 2 1 2
central spot, 000.
Figure 1 — Schematic spot diffraction pattern from a single crystal
Key
R vector from 000 to spot, h k l , the shortest vector in the diffraction pattern
1 1 1 1
R vector from 000 to spot, h k l , the next shortest vector
2 2 2 2
NOTE The basic parallelogram is constituted by R and R .
1 2
Figure 2 — Example of SAED spot pattern
4 © ISO 2018 – All rights reserved

---------------------- Page: 10 ----------------------
ISO 25498:2018(E)

The relationship between the interplanar spacing, d , and the magnitude, R for a reflecting plane,
hkl hkl,
(hkl), can be approximately expressed as shown in Formula (1) (see References [7] and [8]):
 3 2
LRλ = ×−dR1 /LR= ×−d 1 Δ (1)
() ()
hklhkl hklhkl hkl
 
8
 
where
3
2
Δ is equal to RL/ ;
()
hkl
8
L is the diffraction camera length and equal to f × M × M ;
o i p
where
f is the focal length, in millimetres, of the objective lens in the microscope;
o
M is the magnification of the intermediate lens;
i
M is the magnification of the projector lenses;
p
Lλ is the camera constant (or diffraction constant) of the transmission electron microscope
operating under the particular set of conditions. This parameter can be determined from the
diffraction pattern of a crystalline specimen of known lattice parameters (see 8.3);
λ is the wavelength, in nanometres, of the incident electron beam which is dependent upon the
accelerating voltage and can be given by Formula (2) (see Reference [4]):
1,226
     λ nm = (2)
()
−6
VV10+ ,97881× 0
()
where V is the accelerating voltage, in volts, of the TEM; the factor in parenthesis is the relativistic
correction.
For most work using a TEM, the value of Δ in Formula (1) is usually smaller than 0,1 % and, hence, a
more simplified Formula (3) may be used, namely
Rd⋅≅Lλ (3)
hklhkl
For the derivation of the above equation, refer to the textbooks (see References [4] to [9]).
The use of Formula (3) requires measuring the length of R . Since, as mentioned earlier, the location of
hkl
the pattern centre may not be easily determined; it is recommended that the distance measurement
taken, 2R , be from the h k l diffracted spot to the hk l spot on the pattern. This is equivalent
1 1 1
hk l 11 1
11 1
to a diameter measurement on the ring pattern from a polycrystalline specimen. To obtain the
interplanar information, the measured distance, 2R ,is halved and Formula (3) applied.
hk l
11 1
If the camera constant is known, the interplanar spacing, d , of plane, (hkl), can be calculated.
hkl
The included angle between any two vectors, R and R , can also be measured on the
hk l hk l
11 1 22 2
diffraction pattern. This is equal to the angle between the corresponding crystallographic planes,
hk l and hk l .
() ()
11 1 22 2
Since diffraction data from a single pattern will provide information on a limited number of the possible
diffracting planes in a specimen area, it is necessary to acquire additional diffraction patterns from the
same area (or from different grains/particles of the same phase). This requires either the tilting of the
specimen or the availability of differently oriented grains or particles of the same phase.
Acquire a second diffraction pattern from another zone axis from the same area by tilting (or tilting and
rotating) the specimen so that the two patterns contain a common spot row (see 8.2.10 and Figure 5).
© ISO 2018 – All rights reserved 5

---------------------- Page: 11 ----------------------
ISO 25498:2018(E)

Index the diffracted spots, and then select three non-planar spots in the two patterns to constitute
a reciprocal lattice, which, if the spots correspond to low values of Miller indices, may define the
primitive unit cell of the crystal lattice. Therefore, crystal lattice parameters can be determined and
the orientation of the grain or particle in the thin specimen can also be calculated.
4.3 Kikuchi pattern
When a specimen area is nearly perfect but not thin enough, Kikuchi lines may occur. They arise from
electrons scattered inelastically through a small angle and suffering only a very small energy loss being
scattered again, this time elastically. This process leads to local variations of the background intensity
in the diffraction pattern and the appearance of Kikuchi lines.
The Kikuchi patterns consist of pairs of parallel bright and dark lines, which are parallel to the
projection of the corresponding reflecting plane, (hkl). The bright (excess) line and dark (defect) line in
the Kikuchi pattern are denoted by K and K , respectively. Therefore, the line pair, K
Bh− kl Dh− kl Bh− kl
and K , will be perpendicular to the vector, R from the corresponding crystallographic plane
Dh− kl hkl
(hkl). Namely they are perpendicular to the reciprocal vector, g , of the plane, (hkl).
hkl
An example of a Kikuchi pattern is given in Figure 3, where the bright line, K , and dark line,
Bh− kl
K , pair is superimposed on the spot pattern. The perpendicular distance, D , between the
Dh− kl Kh− kl
line pair, K and K , is related to the interplanar spacing, d , and camera constant, Lλ, by
Bh− kl Dh− kl hkl
Formula (4).
Dd⋅≅Lλ (4)
Kh− kl hkl
Key
K bright line of Kikuchi pair
B−hkl
K dark line of Kikuchi pair
D−hkl
D distance between the line pair K and K
Kh− kl Bh− kl Dh− kl
+ centre of the direct beam
Figure 3 — Kikuchi pattern from a steel specimen
The distance between the two Kikuchi lines equals to the distance between the diffraction spot, hkl,
and the central spot, 000. The angles between intersecting Kikuchi pairs are the same as the angles
6 © ISO 2018 – All rights reserved

---------------------- Page: 12 ----------------------
ISO 25498:2018(E)

between their corresponding diffraction spots, and can be measured accurately. These angles are also
equal to the angles between the relevant crystallographic planes.
When the specimen is tilted, the diffraction spots only gradually change the brightness, faint or increase
the intensity, but their positions are almost at the same place. Instead, Kikuchi lines are sensitive to
the tilting. Their movement is significant on the viewing screen. Hence, specimen tilting can be guided
by Kikuchi map from one zone axes to another one. The Kikuchi patterns present the real crystal
symmetry of the specimen. They can also be used in establishing crystal orientation with a very high
accuracy (see References [5] and [9]).
The problem is that Kikuchi patterns cannot always be observed in all of the specimens. In most cases,
the SAED studies rely mainly on the spot patterns, though they are not as accurate as Kikuchi patterns.
4.4 Polycrystalline specimen
For randomly oriented aggregates of polycrystals, the diffraction pattern is comprised of a series of
concentric rings centred on the spot, 000, of the direct beam. An example of the pattern from
polycrystalline gold (Au) specimen is given in Figure 4. Each diffracted ring arises from the diffraction
beams from differently oriented crystallographic planes of the form, {hkl}; each of these having an
identical interplanar spacing. From the diameter of each diffraction ring, the corresponding interplanar
spacing, d , can be calculated using Formula (3). Indices of the diffraction rings can be ascribed and
hkl
then the lattice parameters can also be determined. For the method of indexing ring patterns, refer to
that used in X-ray powder diffraction (see Reference [9]).
Figure 4 — Diffraction ring pattern with indices from a polycrystalline Au specimen
5 Reference materials
A reference specimen is required for determining the diffraction constant, Lλ, of the microscope in
electron diffraction studies. In principle, any thin crystalline foil or powder could be considered as
the reference specimen, provided its crystalline structure and lattice parameters have been acquired
accurately and they are certified and stable under irradiation of the electron beam. It should be
ensured that the reference material, which is as thin as electrons can penetrate through it, has the same
crystallographic properties as the bulk material. In addition, a number of sharp diffraction rings or
spots with known indices can be observed. The thickness of the crystal foil or powder grain size should
be consistent with the beam energy and the quality of the diffraction pattern so that clear diffraction
patterns can be observed (when it is too thick, the pattern will lack sharpness).
© ISO 2018 – All rights reserved 7

---------------------- Page: 13 ----------------------
ISO 25498:2018(E)

Reference materials in common use are polycrystalline specimens made from pure gold [which has a
face-centred cubic (FCC) lattice with parameter, a = 0,407 8 nm] or pure aluminium (Al) (FCC structure
with lattice parameter a = 0,404 9 nm). The mass fraction of Au or Al in the reference materials shall be
not less than 99,9 %. The reference specimen shall be prepared by evaporating a small piece of Au or Al
on a grid with a supporting film.
It is also feasible to evaporate a layer of the reference material onto a local surface area of the specimen,
which is to be analysed.
6 Equipment
6.1 Transmission electron microscope (TEM) shall be with double tilt or tilt rotation or double-tilt
rotate specimen holder.
6.2 Recording of SAED patterns and images.
The SAED patterns and images obtained on the transmission electron microscope shall be recorded on
the photographic films or imaging plates or an image sensor built in the digital camera.
When films are used, a darkroom with a negative developing and fixing outfit is required.
6.3 Facilities for specimen preparation.
7 Specimens
7.1 Most specimens are prepared as thin foils (see References [2] and [10]). Such specimens can be
obtained in the form of thin sections from a variety of crystalline substances including metallic and non-
metallic materials. The shape and external size of the specimen should match that of the TEM specimen
holder or, alternatively, it can be held by a support grid.
Fine powders and/or extraction replicas can also be used. These specimens shall be prepared on the
grid with supporting films.
7.2 An applicable area to be tested is in the sizes of micrometre and sub-micrometre. The area is
always defined by selected aperture. The selected area shall be thin enough for the electron beam to pass
through it and diffraction patterns can be observed on the viewing screen.
7.3 The surface of the specimen shall be clean, dry and flat without an oxidizing layer or any
...