BRITISH STANDARD

BS EN
13925-2:2003

Non-destructive
testing — X-ray
diffraction from
polycrystalline and
amorphous materials —
Part 2: Procedures

The European Standard EN 13925-2:2003 has the status of a
British Standard

Confirmed
December 2008
ICS 19.100

12&23<,1*:,7+287%6,3(50,66,21(;&(37$63(50,77('%<&23<5,*+7/$:


BS EN 13925-2:2003

National foreword
This British Standard is the official English language version of
EN 13925-2:2003.
The UK participation in its preparation was entrusted to Technical Committee
WEE/46, Non-destructive testing, which has the responsibility to:
—

aid enquirers to understand the text;

—

present to the responsible international/European committee any
enquiries on the interpretation, or proposals for change, and keep the
UK interests informed;

—

monitor related international and European developments and
promulgate them in the UK.

A list of organizations represented on this committee can be obtained on
request to its secretary.
Cross-references
The British Standards which implement international or European
publications referred to in this document may be found in the BSI Catalogue
under the section entitled “International Standards Correspondence Index”, or
by using the “Search” facility of the BSI Electronic Catalogue or of British
Standards Online.
This publication does not purport to include all the necessary provisions of a
contract. Users are responsible for its correct application.
Compliance with a British Standard does not of itself confer immunity
from legal obligations.

This British Standard was
published under the authority
of the Standards Policy and
Strategy Committee on

20 March 2003

Summary of pages
This document comprises a front cover, an inside front cover, the EN title page,
pages 2 to 23 and a back cover.
The BSI copyright date displayed in this document indicates when the
document was last issued.

Amendments issued since publication
Amd. No.
© BSI 20 March 2003

ISBN 0 580 41462 0

Date

Comments


EUROPEAN STANDARD

EN 13925-2

NORME EUROPÉENNE
EUROPÄISCHE NORM

March 2003

ICS 19.100


English version

Non-destructive testing - X-ray diffraction from polycrystalline
and amorphous materials - Part 2: Procedures
Essais non destructifs - Diffraction des rayons X appliquée
aux matériaux polycristallins et amorphes - Partie 2:
Procédures

Zerstörungsfreie Prüfung - Röntgendiffraktometrie von
polykristallinen und amorphen Materialien - Teil 2:
Verfahrensabläufe

This European Standard was approved by CEN on 29 November 2002.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the Management Centre or to any CEN member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CEN member into its own language and notified to the Management Centre has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece,
Hungary, Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands, Norway, Portugal, Slovakia, Spain, Sweden, Switzerland and United
Kingdom.

EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG

Management Centre: rue de Stassart, 36

© 2003 CEN


All rights of exploitation in any form and by any means reserved
worldwide for CEN national Members.

B-1050 Brussels

Ref. No. EN 13925-2:2003 E


EN 13925-2:2003 (E)

Contents
page
Foreword......................................................................................................................................................................3
Introduction .................................................................................................................................................................4
1

Scope ..............................................................................................................................................................4

2

Normative references ....................................................................................................................................5

3

Terms and definitions....................................................................................................................................5

4
4.1
4.2

4.3
4.4
4.5

Specimen preparation ...................................................................................................................................5
General preparation .......................................................................................................................................5
Block specimens............................................................................................................................................8
Powder specimens ........................................................................................................................................9
Analysis of small quantities of sample......................................................................................................11
Reactive samples and non-ambient conditions .......................................................................................11

5
5.1
5.2
5.3

Data collection..............................................................................................................................................12
General considerations ...............................................................................................................................12
Angular range and mode of data collection..............................................................................................12
Parameters relevant to the quality of collected data................................................................................12

6
6.1
6.2
6.3
6.4
6.5
6.6
6.7


Data processing and analysis ....................................................................................................................13
Background ..................................................................................................................................................13
Peak searching.............................................................................................................................................13
Pattern decomposition into individual line profiles including background subtraction......................14
Phase identification .....................................................................................................................................15
Indexing ........................................................................................................................................................15
Lattice parameter refinement......................................................................................................................15
Other types of analysis ...............................................................................................................................16

Annex A (informative) Relationship between the XRPD standards ...................................................................17
Annex B (informative) Example of Report Form .....................................................................................................18
Annex C (informative) Scheme of a typical procedure for XRPD measurements ...............................................19
Annex D (informative) Some analytical functions used for profile fitting .........................................................20
Annex E (informative) Some methods for testing the internal consistency of XRPD data..............................21
E.1

General..........................................................................................................................................................21

E.2

Figures of Merit for FWHMs and intensities..............................................................................................21

E.3

Figures of Merit for line positions and lattice parameters ......................................................................22

Bibliography ..............................................................................................................................................................23

2



EN 13925-2:2003 (E)

Foreword
This document (EN 13925-2:2003) has been prepared by Technical Committee CEN/TC 138 "Non destructive
testing", the secretariat of which is held by AFNOR.
This European Standard shall be given the status of a national standard, either by publication of an identical text or
by endorsement, at the latest by September 2003, and conflicting national standards shall be withdrawn at the
latest by September 2003.
This European Standard about “Non destructive testing - X-ray diffraction from polycrystalline and amorphous
material” is composed of:
•

EN 13925-1 General principles;

•

EN 13925-2 Procedures;

•

prEN 13925-3

Instruments;

•

prEN 13925-4

Reference materials.


In order to explain the relationships between the topics described in the different standards, a diagram illustrating
typical operations involved in XRPD analysis is given in annex A.
Annexes A to E are informative.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to implement this European Standard: Austria, Belgium, Czech Republic, Denmark, Finland,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands, Norway, Portugal,
Slovakia, Spain, Sweden, Switzerland and the United Kingdom.

3


EN 13925-2:2003 (E)

Introduction
X-ray powder diffraction (XRPD) is a powerful Non Destructive Testing (NDT) method for determining a range of
physical and chemical characteristics of materials. These include the type and quantities of phases present, the
crystallographic unit cell and structure, crystallographic texture, macrostress, crystallite size and microstrain, and
the electron radial distribution function.
This standard aims to describe the general aspects of the XRPD technique and its applications but not to define a
specific or detailed standard for each field of application or type of analysis.
The main purposes of the standard are therefore:
• to provide practical guidance, unified concepts and terminology for use of the XRPD technique in the area of
Non Destructive Testing with general information about its capabilities and limitations of relevance to
laboratories working at different levels of sophistication, from routine testing to research;
• to provide a basis for Quality Assurance in XRPD laboratories allowing performance testing and monitoring of
instruments as well as the comparison of results from different instruments;
• to provide a general basis (without imposing specifications) for further specific NDT product standards and
related Quality Assurance applications, with aspects common to most fields of application.
In order to make the standard immediately usable in a wide range of laboratories and applications, diffractometers

with Bragg-Brentano geometry are considered in more details than the diffractometers using other geometries.

Radiation Protection. Exposure of any part of the human body to X-rays can be injurious to health. It is therefore
essential that whenever X-ray equipment is used, adequate precautions should be taken to protect the operator
and any other person in the vicinity. Recommended practice for radiation protection as well as limits for the levels
of X-radiation exposure are those established by national legislation in each country. If there are no official
regulations or recommendations in a country, the latest recommendations of the International Commission on
Radiological Protection should be applied.

1

Scope

This European Standard specifies the basic procedures applied in the X-ray Powder Diffraction (XRPD) method.
Many of these procedures are common to most types of diffractometer used and types of analysis mentioned in
EN 13925-1. In the interests of clarity and immediate usability more details are given for procedures using
instruments with Bragg-Brentano geometry and application to phase identification. Aspects of specimen
preparation and data quality assessment are included, but the standard remains non-exhaustive. It is anticipated
that particular standards will address specific fields of application in more details.

4


EN 13925-2:2003 (E)

2

Normative references

This European Standard incorporates by dated or undated reference, provisions from other publications. These

normative references are cited at the appropriate places in the text, and the publications are listed hereafter. For
dated references, subsequent amendments to or revisions of any of these publications apply to this European
Standard only when incorporated in it by amendment or revision. For undated references the latest edition of the
publication referred to applies (including amendments).
EN 13925-1:2003, Non-destructive testing — X-ray diffraction from polycrystalline and amorphous materials —
Part 1: General principles.
prEN 13925-3, Non-destructive testing — X-ray diffraction from polycrystalline and amorphous materials — Part 3:
Instruments.

3

Terms and definitions

For the purposes of this European Standard, the general terms and definitions concerning X-ray powder
diffraction1) apply.

4

Specimen preparation

The sample treatment and specimen preparation shall be adapted to the nature of the sample and the type of
analysis in order to optimise the quality of the data to be collected [1], [2]. As explained in clause 5 of
EN 13925-1:2003, the term ‘powder’ when used in crystallography, does not strictly correspond to the common
usage.

4.1

General preparation

4.1.1


Lateral specimen size

When theta-compensating variable apertures are used, the surface area of the specimen irradiated by the beam
can be kept constant (but not the volume from which diffraction is measured). The specimen shall always intercept
the whole incident beam to avoid a loss of diffracted intensity. This can be checked, for example, by initially
investigating the range of angles to be measured, after replacement of the specimen with a fluorescent screen of
the same dimensions. Alternatively, the length of the specimen surface irradiated by the X-ray beam can be
calculated using the equation:
Irradiated length (mm) = R α / sinθ

(1)

where
R

is the radius of the goniometer, in millimetres;

α

is the divergence angle of the beam, in radians;

θ

is half the diffraction angle 2θ, in degrees or radians.

In practice, with fixed aperture slits, the incident beam at low 2θ angles often spreads beyond the specimen
surface. The corresponding diffracted intensities can be approximately corrected by comparing them with data
recorded in the same angular domain but using a fixed slit of smaller aperture.


1

) a European draft standard (WI 00138078 "Non-destructive testing – X-ray powder diffraction – Terminology") is in
preparation

5


EN 13925-2:2003 (E)

When a small amount of a powder is to be examined, the diffracted intensity can be maximised by preparing it to
form a thin layer with a large area intercepting the incident X-ray beam.
When specimens with low attenuation are investigated on diffractometers working in a reflection geometry, it should
also be taken into account that, at low diffraction angles, the incident and diffracted X-rays may propagate a
considerable distance in a direction nearly parallel to the specimen surface. Thus the optimum lateral sample size
might be considerably larger than the area of intersection of the incident X-ray beam with the specimen surface.
4.1.2

Effect of specimen displacement

A specimen surface that is offset with reference to the Bragg-Brentano goniometer 2θ rotation axis, results in a line
)
shift by an angle, in radians of2

∆ (2θ ) = 2θ obs − 2θ theo = −2∆h cos θ / R

(2)

where
∆(2θ)

is the shift (in radians) in the theoretical line position to align it with the observed position. It is positive
when the specimen surface is displaced towards the source and detector;
2θobs and 2θtheo

are the observed diffraction angle and the diffraction angle calculated from the Bragg law;

∆h is the specimen surface displacement (in millimetres) measured along the bisector of the angle between
the incident beam and the diffracted beam. It is positive if the specimen surface moves away from the X-ray
source and the detector.

R

is the radius of the goniometer (in millimetres).

This is illustrated schematically in Figure 1.

2)This equation is similar to that given by Wilson [3].

6


EN 13925-2:2003 (E)

Key
1
2

Source
Bisector


3 Detector
The symbols are defined in equation (2)

Figure 1 — Relationships between the specimen displacement and the diffraction line position
Specimen displacements smaller than 20 µm are difficult to avoid. For example, with a goniometer of 200 mm
radius, this offset would result in a maximum angular error of 0,01°(2θ).
Use of an appropriate internal standard allows the detection and correction of this effect simultaneously with that
arising from specimen transparency.
4.1.3

Effects of specimen thickness and transparency

When the XRPD method is applied in a reflection geometry it is often preferable to work with specimens of ‘infinite’
thickness. This means that, for a given mass attenuation and apparent density of the specimen and a given range
of diffraction angles, the diffracted intensity from the back of the specimen is negligible.
To ensure that the diffracted intensity is at least 99,9% of the maximum attainable by increasing the specimen
thickness, the thickness shall be at least [4]:

t = 3.45 sin θ / (µ ' ρ ')

(3)

where
t

is the thickness, expressed in centimetres;

ρ' is the specimen density, the mass of the specimen divided by its volume including voids expressed in
grams per cubic centimetre;
µ' is the weighted sum of the mass attenuation coefficients (often referred to as the mass absorption

coefficient) expressed in square centimetres per gram [5]. It is additive for the mass attenuation coefficients
of the constituent elements of the material when weighted by their fractional concentration, e.g.:
7


EN 13925-2:2003 (E)

µ' = Σci µ'i

(4)

where

µ'i is the mass attenuation coefficient of the i element, expressed in square centimetres per gram;
th

th

ci is the fractional concentration by weight of the i element.
For specimens with low attenuation (such as organic compounds) a large fraction of the diffracted intensity appears
to originate from a position below the surface resulting in line shifts and changes in line widths and shapes. This
effect, referred to as the transparency effect, is large for thick specimens with low attenuation. The line shift (in
radians) due to the transparency of a thick specimen is given by the relationship (see footnote 1):

∆ (2θ ) = 2θ obs − 2θ theo = −

1
sin 2θ
2µ ' ρ ' R


(5)

where
∆(2θ)

is the shift (in radians) in the theoretical line position to align it with the observed position;

2θobs and 2θtheo
R

are the observed diffraction angle and the diffraction angle calculated from the Bragg law;

is the radius (in centimetres) of the goniometer.

For such materials, the specimen should be as thin as possible (consistent with acceptable diffraction intensities) to
give accurate measurement of line position. It is advisable to use a non-diffracting specimen substrate (also called
a low background holder), e.g. a plate of mono-crystal silicon cut parallel to the {510} lattice planes. In the case of
thin specimens with low attenuation accurate measurements of line positions can be made with focusing
diffractometer configurations in either transmission or reflection geometry. Accurate measurements of line positions
on thick specimens with low attenuation are preferably made using diffractometers with parallel beam optics. This
helps to reduce the effects of specimen thickness.
Use of an appropriate internal standard allows the detection and correction of this effect simultaneously with that
arising from specimen displacement.
NOTE

4.2
4.2.1

"centimetre " and "gram" are the units commonly used in tables of attenuation coefficient and density.


Block specimens
Surface preparation

The specimen surface shall be sufficiently flat for the purpose of the measurement to be made, e.g. when using
Bragg-Brentano geometry, surface roughness can result in displacement, broadening and reduced intensity of
diffraction lines. Mechanical, electrolytic or chemical polishing can be carried out to obtain a flat surface or to study
an area in depth or free of disturbances arising from the initial preparation.
Mechanical polishing can cause various changes in the material (strain hardening, phase changes, etc.). This
altered layer shall be removed by adequate chemical or electrochemical polishing.
4.2.2

Mounting and specimen holder

Block specimens are mounted either directly into the stage of a diffractometer if the size and shape are suitable or
mounted into a specimen holder that is itself mounted on the diffractometer stage. Care has to be taken in either
method to ensure that the specimen surface is aligned with all the relevant rotational axes of the goniometer,
including additional rotations that might be used for applications such as texture or macrostress measurement. In
the case of Bragg-Brentano geometry, the specimen surface has to be aligned with the goniometer axis and be
symmetrically oriented between the incident and diffracted beams.
8


EN 13925-2:2003 (E)

4.2.3

Additional precautions

When examining block specimens, the possibility of depth-dependent inhomogeneity has to be recognised. It can
result in a diffraction pattern with varying relative contributions from the different components as 2θ is varied. X-ray

opaque masking is sometimes used to limit the irradiated surface on large specimens but care has to be taken to
ensure that the mask does not contribute to the diffraction pattern. A preferred alternative, where practical, is to
mask the X-ray beam to limit the area irradiated or from which diffracted X-ray are detected.

4.3

Powder specimens

4.3.1

Sampling of multi-phase powders

Prior to carrying out an XRPD investigation of an unknown powder, there shall be proper sampling followed by
appropriate specimen preparation. In the case of a multi-phase powder, the unknown powder might be
inhomogeneous on a microscopic or even a macroscopic scale due to differences in the properties of the individual
components such as the density, size and shape of the particles, state of agglomeration, etc. To provide
representative and reproducible results of an XRPD analysis, it may be necessary to homogenise an amount of the
unknown powder that is much larger than the quantity needed for the specimen size.
In cases where maximum reliability of the XRPD results is required, statistical methods for homogeneity testing
shall be applied. For sampling techniques see e.g. the BCR guidelines of the European Commission [6].
4.3.2

Milling and sieving

Milling and sieving may be required to increase the number of crystallites in the specimen or to minimise microabsorption effects between particles of different composition and size. The issue of crystallite size is dealt with in
this sub-clause in more details.
The number of crystallites of each individual crystalline phase in the irradiated specimen volume shall be sufficient
to assure a desired level of reproducibility for the collected data. This problem is often denoted as "crystallite
statistic". For Cu Kα radiation and quartz specimens measured with Bragg-Brentano geometry, a maximum
crystallite size of 10 µm has been found to achieve reproducibility of diffraction line intensity within 2 % to 3 %

[7 (p. 365 ff)].
Based on this figure and the relationship given in the same work, the mean relative deviation, Um, in diffracted
intensity may be roughly estimated by:
Um = 60 (µ' ρ l )

3 ½

(6)

where
ρ is the crystal density, expressed in grams per cubic centimetre;
µ' is the mass attenuation coefficient, expressed in square centimetres per gram;
l

is the crystallite dimension, expressed in centimetres.

Values of Um up to about 10%, arising from larger crystallites, often give satisfactory data for phase identification.
Smaller values of Um (and hence smaller crystallite sizes) are necessary for quantitative analysis where a higher
level of reproducibility is needed. However excessive milling, giving crystallite dimensions below about 0,5 µm, may
cause line broadening and significant changes to the intrinsic characteristics of the specimen, such as:


sample contamination by particles abraded from the milling instruments (e.g. mortar, pestle, balls etc.);



partial amorphisation of the near-surface region of the sample particles;




transition to different polymorphic crystallographic forms;
9


EN 13925-2:2003 (E)



chemical decomposition (e.g. loss of structurally bound CO2 or H2O);



the introduction of lattice deformation;



solid-state reactions.

The type and intensity of milling to be applied depends both on the hardness of the material and on the XRPD
investigation to be carried out (e.g. phase identification, quantitative analysis, structure refinement, refinement of
lattice parameters). Manual milling in a mortar made of agate, mullite or corundum is sometimes sufficient. More
complex milling techniques often have to be applied, including mechanical milling, wet milling, ultrasonic treatment,
etc. Specific problems may arise if the sample is a mixture of phases with significantly different hardnesses.
Sieving and/or sedimentation are sometimes the only effective methods allowing isolation of particles of a specific
size. To achieve reliable diffraction intensities, sieving is often necessary as an adjunct to milling. However, great
care shall be taken when milling and sieving are performed on multiphase samples as the risk of changing the
initial mixture composition is high. The technique shall only be applied to such mixtures when necessary and the
analyst shall be fully aware of the risks involved.
4.3.3


Preparing a mixture from individual powders

In XRPD characterisation of powders it is often necessary to prepare mixtures of different powders. When
preparing such mixtures, the particle sizes and size distributions shall be taken into account and the powders mixed
intimately. For rigorous work the following recommendations shall be considered:
a)

before the powders are mixed quantitative information on the particle size distributions of the individual
powders should be known;

b)

if a powder is too coarse and its particle size has to be reduced by milling, where possible, the milling shall be
performed before preparing the mixture;

c)

the powders should be mixed intimately. Homogeneity on the scale of individual particles should be achieved
and agglomerates of the individual substances destroyed if possible;

d)

re-segregation of the mixed powders has to be avoided during all steps involved in the preparation, storing and
transport of the mixture and of the XRPD specimen;

e)

the homogeneity of representative mixtures should be checked by optical and/or electron microscopy.

Procedures for specific analysis involving powder mixing should be validated by demonstrating that the mixing

procedures are adequate, e.g. by showing that the analysis results can be reproduced upon further mixing and
milling, or upon repeating the procedures, and that there has been no degradation of the component phases.
4.3.4

Mounting of powder specimens

The following three standard sample mount techniques are often applied in connection with diffractometer
configurations working in reflection geometry:
a)

‘front loading’: filling the powder into the opening of the specimen holder from the front and levelling the
specimen surface that will be directly exposed to the X-ray beam with a flat surface;

b)

‘back filling’: filling the powder into the opening of the specimen holder from the side opposite that will be
directly exposed to the X-ray beam;

c)

‘side-drifting’: filling the powder into the opening of the specimen holder from the side.

Among these techniques a) is the simplest and most commonly used, although it carries a higher risk of inducing
preferred orientation than with other preparation techniques and is therefore sometimes regarded as ‘bad practice’.
Techniques b) and c) may yield specimens with reduced preferred orientation as they allow preparation of powder
10


EN 13925-2:2003 (E)


specimens with less densely packed surface layers and reduce the shear applied to crystals in the surface layer
during the packing process.
For diffractometers working in transmission geometry configurations, standard specimen mount techniques are:


capillary (cylindrical specimens): filling the powder into a capillary made from thin glass or dusting it onto the
surface of a thin wire of metal or glass;



flat specimen: dusting the powder onto, for example, a thin polymeric transparency or a Pt wire gauze, or by
preparing a self-supporting specimen, e.g. by compacting the powder between two plates.

Capillary mounting techniques can be very efficient in avoiding preferred orientation as, among the available
techniques, it yields the least densely packed specimen mounts.
Samples that are subject to severe preferred orientation are sometimes diluted with another powder consisting of
particles of approximately spherical shape or dispersed in a viscous material to reduce the directional effects.
Powders used for dilution can be crystalline or amorphous (powdered glass, acacia, starch, gelatine, amorphous
boron, etc). Commonly used viscous materials are grease, Vaseline, collodion, etc. Spherical agglomerates of
powder particles can also be produced by techniques such as aerosol spray-drying of a suspension of the powder
in a liquid containing a small amount of a binder. However, the presence of an additive will produce additional
diffraction lines or diffuse halos.

4.4

Analysis of small quantities of sample

Small amounts of a powder can be investigated with diffractometers working in reflection geometry using either a
specimen holder with a small and possibly shallow cavity or simply a flat plate specimen holder. Both types of
specimen holders can be made from common materials or, preferably, they can be so-called "low background

specimen holders". They are cut in a non-diffracting orientation from single-crystalline silicon, quartz or other
wafers, for instance a plate of single-crystalline silicon cut parallel to one of the {510} lattice planes. If necessary, a
small quantity of grease, oil, amorphous glue or even double-sided adhesive tape may be used.
Diffraction lines from such thin specimens may differ from those using an ‘infinitely’ thick specimen made from the
same material. The dependence of the irradiated volume on diffraction angle can differ between thick and thin
specimens.
A thin layer specimen comprises a limited number of crystallites and may therefore cause irreproducible line
profiles.
Alternatively, small quantities of a powder sample can often conveniently be investigated by transmission
techniques such as Debije-Scherrer, Guinier and others, mounting the sample into a capillary or using a thin flat
mount.
Very small specimens, block or powder, can be analysed using special XRPD instruments having a narrow
(typically 100 µm x 100 µm) intense beam to obtain high spatial resolution and preferably equipped with a Position
Sensitive Detector (PSD).

4.5

Reactive samples and non-ambient conditions

Samples that react with the surrounding atmosphere (oxygen, moisture, etc.) can be investigated with a
diffractometer working in reflection or transmission geometry after mounting it in a glove-box under inert gas in an
environmental cell. The specimen surface can be protected with a suitable thin film transparent to X-rays. However,
scattering from this film can hinder pattern analysis unless it is mounted at an adequate distance from the
specimen surface to be excluded by the diffracted beam collimation [8]. Highly reactive samples can also be
mounted in fused glass capillaries under inert gas or vacuum, the X-ray diffraction analysis being carried out in
transmission geometry.
Reactive samples can also be investigated under non-ambient conditions (see clause 8 of EN 13925-1:2003).
Specimen mounting specific to diffraction experiments under non-ambient conditions (e.g. high and low
temperature, pressure ...) shall be used.
11



EN 13925-2:2003 (E)

5
5.1

Data collection
General considerations
)

Data collection3 shall follow the strategy best suited to the aims of the work and the kind of specimen under
examination. For that purpose a particular diffractometer configuration and appropriate settings of adjustable optical
parameters shall be carefully chosen, adjusted and monitored according to well-defined procedures. The
monitoring of instrument performance is described in prEN 13925-3.
The background of a diffraction pattern arises from inelastic and elastic scattering. Inelastic scattering (e.g.
fluorescence) can be reduced by a careful choice of the wavelength of the X-ray source and/or by using a
monochromator in the diffracted beam. Elastic scattering (e.g. air scattering) can be considerably reduced by
optimising the settings of anti-scatter slits, available with many diffractometer configurations [8]. If a Kβ filter is
used, the profile of the continuous background is severely disturbed due to the absorption edges appearing on the
low angle side of each diffraction line. This can result in problems for profile fitting, background subtraction and
determination of line intensities and profiles.
To increase the number of crystallites contributing to the intensity profile of the diffraction lines, large areas of
specimen illuminated by the incident X-ray beam or rotating specimen holders can be used. In the case of a
diffractometer working in Bragg-Brentano geometry the specimen rotation improves the crystallite statistics (see
4.3.2) only by improving the representativeness of the specimen without reducing preferred orientation within the
diffracting plane. The effects of limited crystallite statistics (and to a lesser extent preferred orientation) on the
powder diffraction pattern can be reduced using diffractometers with scanning one- and two-dimensional Position
)
Sensitive Detectors (PSD). They can simultaneously collect data from several Debije4 -Scherrer cones and

integrate them over large regions. Techniques such as specimen rotation and use of a scanning PSD detector can
be particularly helpful when making measurements from solid block specimens where the intrinsic crystallite
characteristics cannot be changed. When the sample is a loose powder it is often possible to reduce preferred
orientation by applying special treatment to the powder or using an appropriate specimen holder as described in
4.3.4.

5.2

Angular range and mode of data collection

For qualitative analysis of a “completely” unknown sample the angular range for data collection and the
corresponding range of d-spacings should start from the largest accessible d-spacing and continue to a d-spacing
–10
of 1.5 Å (1 Å = 10
m) or less. In special cases (e.g. well known sample types in a series, materials with large unit
cells and low symmetry, special types of analysis other than phase analysis) a reduced 2θ range may be used. The
2θ range and the ∆2θ sampling interval (step size) shall be chosen, along with the counting time per step, to
optimise resolution and counting statistics according to the type of analysis and the sample. It has been
recommended that the step size be chosen to give a minimum of 5 to 10 data points (depending on the data
reduction method to be used) above the half maximum of a diffraction line for phase identification and various other
types of analysis.
Data collection to cover the d-spacing range of interest may be accomplished by sequential collection at points
covering the equivalent 2θ range or by parallel acquisition of data using either a position sensitive detector (one- or
two-dimensional) or an energy dispersive diffractometer.
5.3

Parameters relevant to the quality of collected data

This paragraph refers to parameters useful in classifying the quality of collected data. Some of them are closely
related to the instrument quality and are also referred to in prEN 13925-3. Data collection quality is influenced by:



ratio of step size to Full Width at Half Maximum (FWHM);

3)The collected data (raw data) are often referred to as “observed data” and should not be confused with “measured data”. The result of a
diffraction measurement is determined via a qualified data evaluation (see WI00138078 - Terminology).
4)Debije is often spelled as "Debye"

12


EN 13925-2:2003 (E)



total counts observed in relation to the field of application;



the FWHM of a well resolved diffraction line;



angular range;



signal to background ratio.

For many types of analysis the parameters have to relate at least to the two most intense lines. The value of the

parameters is improved if they are determined for several regions of the diffractogram (e.g. at low, medium and
high 2θ). For whole powder pattern analysis, additional quality assessment parameters can be used. For
instrument characterisation a form shall be used that is designed for that purpose. The parameters used for data
collection shall be summarised and documented by using a result report form similar to that presented in annex B.
5)
Additional information on the quality of collected data can be obtained through measurements on control samples
as described in prEN 13925-3.

6

Data processing and analysis

The degree of sophistication in diffraction data analysis depends on the a priori available information about the
sample, the type of analysis to be carried out and the effort that can be applied.
The graphical representation of the results of any data evaluation procedure together with the visualisation of
portions of the diffraction pattern by computer graphics should be used when available.
The relationships between the various steps of a diffraction data collection process are shown in annex C.
Dedicated software may be used for data collection and processing according to the field of application and level of
sophistication required. Software functionality shall be checked before use in XRPD measurements e.g. by crosschecking the results or outputs against those produced by an independent method. Such checks shall be recorded.

6.1

Background

Background is regarded here as information in the data that is to be excluded from the analysis. The background
signal (see 5.1) can be fitted using appropriate mathematical functions (e.g. a polynomial). This can be done by
using the minima between the diffraction lines. Alternatively background subtraction is included in pattern
)
decomposition programs6 .
In case of heavily overlapping diffraction lines, care should be taken not to remove intensity belonging to the

diffraction lines.

6.2

Peak searching

Peak searching on diffraction data provides a list of approximate parameters (line position 2θ, maximum intensity
Imax, Full Width at Half Maximum (FWHM), etc.) for both isolated and overlapping diffraction lines in the pattern.
Peak searching accuracy is reduced when overlaps increase. For digital data, an automatic or interactive peak
search program can be used. Such programs usually involve a combination of mathematical operations (e.g.
subtraction of background, stripping the Kα2 component, signal differentiation, local curve fitting) and use of
analytical functions (see annex D) to determine line profile parameters.
If required, integrated intensities are evaluated after background subtraction by:

5) Control samples are samples used in a laboratory in order to check and monitor the instrument performance.
6) Calculated R-values (see section 6.3.) are highly dependent on the background.

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EN 13925-2:2003 (E)

∑ (I obs ) j ∆2θ

I int =

(7)

j


where
(Iobs)j

is the observed intensity at the current position 2θ;

∆(2θ)

is the sampling interval.

Figures of Merit (FOM) for checking the internal consistency of observed intensities are described in annex E.
Most current X-ray diffractometers can be supplied with data processing software for these purposes. The line
positions in degrees 2θ can be converted into d-spacings using the Bragg equation (see equation (1) of EN 139251:2003). The "d-I" list produced is used for data analysis in several fields of application.

6.3

Pattern decomposition into individual line profiles including background subtraction

Pattern decomposition is a term that traditionally refers to a least-squares processing method to unravel
overlapping diffraction line profiles. This is performed by fitting the entire diffraction pattern (Rietveld method) or
segments of the diffraction pattern. Pattern decomposition is performed by fitting profile shape functions to each of
the line profiles using computer programs based on least-squares or maximum likelihood methods. It yields a list of
refined parameters (line position, integral intensity, integral breath, line shape, etc.) for each diffraction line in the
angular range examined. Small sections containing only a small number of lines can be analysed separately. This
segmentation is performed by choosing background regions between the peaks, to begin and end each fitted range
taking care to retain the useful information contained in the tails of the diffraction lines. Typically the diffraction lines
are described by the analytical functions shown in annex D or by the fundamental parameters approach, which can
give a better match to the data than analytical functions.
Among the most common criteria (Figures of Merit) adopted to characterise the goodness of fit the following Rvalue can be used:

Rwp = [


∑ wi ((Iobs )i − (Icalc )i )2 / ∑ wi (Icalc )i2 ]1/ 2

(8)

where
Iobs is the observed intensity at the current point i;
Icalc

is the calculated intensity at each point i of the fitted function;
2

wi = 1/σ
σ

2

is an appropriate weight factor per step associated with each Iobs;

is the variance of Iobs

Parameters defining the model are refined until the following quantity is a minimum:

χ2 =

∑ wi2 [(I obs )i − (I calc )i ]2

(9)

where the symbols of the equation are the same as referred for equation (8).

The efficiency of pattern decomposition decreases rapidly as the number of line overlaps increases and no
structural model is used. This difficulty can be partially minimised by reducing the number of free parameters (e.g.
by applying certain mathematical constraints to the line profile parameters), and/or their range of values
(mathematical restraints). Parameters for checking the internal consistency of observed FWHMs are described in
E.2.

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EN 13925-2:2003 (E)

After subtraction of the background from the observed intensities, the evaluation of the maximum intensities is
performed by simple observation of the peak heights (Imax). The evaluation of the integrated intensities is given by
equation (7).
If all crystalline phases present in the specimen have been identified and their crystal structures (or at least their
crystal systems and approximate lattice parameters) are known, the decomposition of the powder pattern can use
this information to constrain a whole pattern fitting procedure, e.g. by a Rietveld-type method if the full crystal
structure is known.

6.4

Phase identification

The identification of the crystalline phases of an unknown sample by XRPD is usually based on the visual or
computer assisted comparison of its X-ray powder pattern with the experimental or calculated patterns of single
phase powder samples. The comparison can be based on a more or less extended angular range of the full
diffraction pattern or on reduced data sets such as d-I lists of the unknown sample and single-phase samples. In
the latter case line positions and intensities have to be determined manually or by automatic peak searching as
described in 6.2. Highly automated “search/match“ routines generate a list of matches that may serve as first
approximation for a detailed interpretation. In many cases the speed, reliability and completeness of the results of

phase identification strongly depend on the availability of additional information about the sample, most importantly
about the chemical elements present as main and minor constituents. Information provided by optical microscopy
and by electron microscopy is also helpful. The detection limit for phase identification by XRPD depends strongly
on the sample. In favourable cases the detection limit can be as low as 0,1 % to 1 % in mass. In less favourable
cases the detection limit can easily exceed 10 % in mass. When lattice parameters are available, a check of the
phase identification can be made by indexing the complete powder pattern using the procedure described in 6.5.
The results of a phase identification made by XRPD can be validated by comparing the experimentally measured
and preliminarily interpreted powder pattern of the unknown sample with the calculated powder patterns of the
individual phases detected in this sample if the corresponding 3-dimensional structure data are available (e.g. in
structure databases such as the Inorganic Crystal Structure Database (ICSD [9]), the Metals Crystallographic Data
File (MCDF or CRYSTMET) [10] or the Cambridge Structural Database (CSD) [11]). This comparison of
experimental to calculated powder patterns can be performed using the Rietveld method or a powder pattern
simulation program.

6.5

Indexing

Indexing of an XRPD pattern consists of attributing indices {hkl} to each diffraction line. This can be done by
comparison with reference patterns or by an automatic indexing program. Whether the mathematical solution found
by an automatic indexing program is the true crystallographic unit cell or only a “pseudo-cell“ depends on:
1)

the completeness (especially in the low 2θ angle region) and the uncertainty of the experimental data;

2)

the size and symmetry of the unit cell.

It is good practice to check the meaningfulness of the mathematical result of an indexing procedure by additional

crystallographic considerations (structure refinement or structure determination) and by independent methods (e.g.
optical, and/or density measurement).
Indexing of an X-ray powder pattern requires precise determination of diffraction line positions (typically ∆|2θ| better
than 0,03°). Correction for instrumental contributions is advisable. Along with indexing, the lattice parameters and
sometimes the possible space group(s) are obtained. The quality of indexing is assessed using the Figures of Merit
reported in E.3.

6.6

Lattice parameter refinement

Lattice parameter refinement requires knowledge of the crystal systems of the phases present, estimated values of
the corresponding lattice parameters and experimental line positions with low uncertainties. The refinement is
usually performed by a least-squares method. The dependence of the lattice parameters on environmental
conditions (such as temperature and pressure) and instrumental contributions have to be taken into consideration.
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EN 13925-2:2003 (E)

The accuracy of lattice parameter refinements shall be assessed using a reference material with certified lattice
parameters.

6.7

Other types of analysis

Other types of analysis outlined in EN 13925-1 and not mentioned here will be the subject of future standards.

16



EN 13925-2:2003 (E)

Annex A
(informative)
Relationship between the XRPD standards
(Linking of the topics within the Standards)

Outlines of Diffraction Physics
See EN 13925-1
(General Principles)

Type of Analysis

Nature and Format of
Specimens
Choice of Geometry

See prEN 13925-3 (Instruments)

Equipment
components and their
characterisation

(Physical Model)
Data Collection and
Processing
See prEN 13925-4
(Reference Materials)

See EN 13925-2
(Procedures)

Working Standard
Alignment &
Verification

Specimen Preparation

Data collection strategies
Calibration

Performance Testing
Performance Monitoring
Data Processing and Analysis

Equipment Characterisation
Form

Analysis Results
Form

Characterisation Form
"Reference Material"

17


EN 13925-2:2003 (E)


Annex B (informative)
Example of Report Form
Diffractometer settings* :
*NOTE
Tube

For instrument characterisation a form should be used that is designed for that purpose.
Type

Anode

Focus

Power

kV Gen

mA Gen

Take-off

Age on
live

Type of analysis and field of application :

Sample preparation: characterisation and holder
Milling

Milling techniques


Crystallite size

Sieving

Sieving techniques

Particle size

Surface treatment
Specimen holder

Type

NOTE

Calibration
Calibration specimen

Monitoring procedure

Frequency of calibration

Date of last calibration

Data collection
Continuous
scan

Step/Scan


Angular range

Degrees/min

Time per step

Total acquisition
time

NOTE

Data processing and analysis
Evaluation criteria

Method

Results (with precision and accuracy when required)

18

Date

Signature
of
responsible

XRPD

person


Date

Signature of QA person responsible

____________________________________

______________________________ _______