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Honouring the two Braggs: the first Xray crystal structure and the first X-ray
spectrometer
John R. Helliwell

School of Chemistry , University of Manchester , Manchester ,

M13 9PL , UK
Published online: 14 May 2013.

To cite this article: John R. Helliwell (2013) Honouring the two Braggs: the first X-ray crystal
structure and the first X-ray spectrometer, Crystallography Reviews, 19:3, 108-116, DOI:
10.1080/0889311X.2013.797410
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Crystallography Reviews, 2013

Vol. 19, No. 3, 108116, http://dx.doi.org/10.1080/0889311X.2013.797410

Honouring the two Braggs: the rst X-ray crystal structure and the rst
X-ray spectrometer
John R. Helliwell*
School of Chemistry, University of Manchester, Manchester M13 9PL, UK

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(Received 15 March 2013; nal version received 16 April 2013)

In the Centennial celebrations of the birth of X-ray crystal structure analysis, a key feature is to
mark the articles which are the rst crystal structure analysis studies. This mini review
describes the historical development and quotes key statements of W.L. Bragg (WLB) as
well as W.H. Bragg (WHB) and the perspectives offered by key players of the time period.
The rst crystal layout, as stated by WLB, is the face-centred cubic arrangement evident in
the Laue Laboratory diffraction photographs recorded from a crystal (of zinc blende) and
provided to WHB. The rst crystal structure, as stated by WLB, and explicitly remarked
upon by P.P. Ewald, as well as WLBs ofcial biographer, D.C. Phillips, is sodium chloride
and which was published in June 1913. The use of the X-ray spectrometer of WHB, and the
measurements by WHB, at Leeds University, with this device are acknowledged by WLB in
his article. This 1913 article also contains numerous raw diffraction data in the form of
Laue photographs measured by WLB of NaCl, and most importantly of KCl, in
Cambridge. WLB seemed to anticipate the use of these two isomorphous and closely related
alkali halide crystal structures in his article of 1912. The X-ray spectrometer as the
forerunner of all X-ray diffractometer designs is also a remarkable initiative of WHB.
Keywords: rst X-ray crystal layout; face-centred cubic layout; rst X-ray crystal structure;
sodium chloride; Laue diffraction photographs; the X-ray spectrometer

Contents

PAGE

1.

Introduction

109

2.

The words of WLB

109

3.

The words of WHB

111

4.

The words of P.P. Ewald

111

5.

The words of D.C. Phillips

113

6.

Other perspectives

113

7.

Conclusions

114

Notes on contributor

115

References

115

*Email: john.helliwell@manchester.ac.uk
2013 Taylor & Francis

Crystallography Reviews

109

1. Introduction
In the Centennial celebrations of the birth of X-ray crystal structure analysis, a key feature is to
mark the article which is the rst crystal structure analysis. This mini review describes the historical development and quotes key statements of WLB and the perspectives offered by key players
of the time period.

2. The words of WLB

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An excerpt from WLBs rst article in 1912 [1]:

It is only the third point system, the element of whose pattern has a molecule at each corner and one at
the centre of each cube face, which will lend itself to the system of planes found to represent spots in
the photograph (recorded by Messrs Friedrich and Knipping). This last system, seeing that it forms an
arrangement of the closest possible packing, is according to the results of Pope and Barlow the most
probable one for the cubic form of zinc sulphide.
Which of these factors it is that decides the form of the interference pattern might be found by experiments with crystals in which the point system formed by the centres of all the atoms differs from that
formed by the centres of identical atoms.
In conclusion, I wish to thank Professor Pope for his kind help and advice on the subject of crystal
structure.

These concluding words, Which of these factors it is that decides the form of the interference
pattern, from WLB presciently explained the experiments that would lead to the rst crystal structure.
From WLBs words quoted by Ewald in [2]:
But let us hear in W.L. Braggs own words what the exciting sequence of events was after Laues
paper had reached W.H. Bragg in (the) form of an offprint. He tells the story in an address given
in 1942 in Cambridge at the rst conference on X-ray analysis in industry (held under the auspices
of the Institute of Physics), which was published in Science in Britain.
In order to examine the reected X-ray beam (from a crystal face) more thoroughly, my father
(William Henry Bragg (WHB)) built the X-ray spectrometer. The X-ray spectrometer opened up a
new world. By using measurements made with the X-ray spectrometer, many of them due to my
father, I was able to solve the structures of uorspar, cuprite, zinc blende, iron pyrites, sodium
nitrate and the calcite group of minerals. I had already solved KCl and NaCl, and my father had
analysed diamond. Between them, these crystals illustrated most of the fundamental principles of
the X-ray analysis of atomic patterns. These results were produced in a year of concentrated work,
for the war in 1914 put an end to research. I have gone into these early experiments in some detail
because it is a story which I alone can tell, and which I wish to put on record.

These reminiscences can be readily supplemented with the words of WLB in his book completed just two weeks before his death on 1 July 1971. Thus, from WLB (1975), The Development
of X-Ray Analysis, p. 25 [3]:
I found that, although the range of wavelengths represented by the spots did not make sense if one
assumed ZnS to be based on a simple cubic lattice everything fell into place if one assumed the
basic lattice to be face-centred cubic. These results showed, not only that Laues pictures were
made by a continuous range of X-ray wavelengths, a kind of white radiation, but also that X-ray
diffraction could be used to get information about the nature of the crystal pattern.

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110

J.R. Helliwell

Figure 1. The rst X-ray crystal structure, NaCl (see [4]).

Note: The WLB 1913 paper and his 1975 book have the identical gure, albeit with the left and right hand
sides switched around, but with the two different gure numbers; Figure 1 shows the gure and numbering,
from the 1913 article.

The next text page 27 [3] begins with the heading which is Section 5 in Chapter 2 of ref 3 in turn
headed THE START OF X-RAY ANALYSIS and then quotes the last paragraph of this section on
page 30.
Page 30 [3]:
The First Complete Analyses: The Alkali Halides
It was on this rather indirect and slender evidence that I assigned the structure of Fig 12 (see Figure 1) to
the alkaline halides in a paper read to The Royal Society in June 1913 [4]; fortunately further investigation established its correctness! These were the rst crystals to be analysed by X-rays. As the structure
was now established, it was possible to calculate dimensions from the crystal density and the mass of the
NaCl molecule. Half a molecule is associated with each small cube of side a = AB in Fig 12 (Figure 1) so

1
Mm = ra3 ,
2
where M is the molecular weight, m is the mass of the hydrogen atom, and is the density of the
crystal. This gave a value for a of 2.8 108 cm and so established a scale for the measurement of
all X-ray wavelengths and crystal spacings.

There is now a Chapter devoted to the rst crystal structure analysis:Page 53 [3]:
Chapter 5 THE FIRST ANALYSIS OF CRYSTAL STRUCTURE
The Method of Analysis
Although the NaCl structure was deduced from Laue photographs, the rst results with the X-ray
spectrometer showed at once how far more powerful it was as an analytical tool. When I started
work in the Leeds laboratory in the summer of 1913, my father was still mainly interested in exploring
the X-ray spectra. It fell to me to use the spectrometer [5] for determinations of crystalline arrangement and a number of inorganic structures were discovered. We wrote a joint paper on diamond
[6] and the other structures were described in a paper in The Royal Society Proceedings in 1913
which may be said to represent the start of X-ray crystallography.

Crystallography Reviews

111

It was very fortunate for me that I was able to work in my fathers laboratory. Young research students
nowadays can have little conception of the primitive conditions in a research laboratory some sixty
years ago. (However) In my fathers laboratory at Leeds there was a good workshop with an excellent mechanic in charge to carry out his ideas. It was the privilege of working with really effective
apparatus which made it possible for me to start my research career by working out a number of
crystal structures. The analysis depended on comparing the strength of the various orders of reection. When the planes are identical and evenly spaced the orders fell off regularly A marked departure from this regular diminution indicated that the planes were not simple. The (crystal structures
analysed) included uorspar, zinc blende, pyrites and calcite (in various forms).

3. The words of WHB

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WHBs own words, extracted from the article [7] of his sons role (see Figure 2):
From the work now described by W L Bragg it appears that the reection phenomena lead to a more
denite knowledge of crystal structure, and we may now complete various quantitative determinations. (namely) the (X-ray) wavelengths of various homogeneous rays as soon as their angles
of reection are known (from an NaCl or other single crystal).

[The unit cell parameter for the cubic NaCl having been established ingeniously from the
mass of a crystal, its volume and the atomic weights of sodium and chlorine, as described above.]
The X-ray spectrometer was also noted by WHB to be under similar development and use at
Liverpool University by Barkla and at Manchester University by Moseley and Darwin; for a
summary description see [8].
4. The words of P.P. Ewald
Page 65 of Fifty Years of X-ray Diffraction [2]:
Although this early paper (WLB 1912 [1]) does not yet contain a full structure determination, it comes
very close to one, in the case of such a simple compound as ZnS.

Page 69:
The great break-through to actual crystal structure determination and to the absolute measurement of
X-ray wavelengths occurred in W L Braggs (NaCl) paper [4].

Page 71:
In the series of fundamental papers published by both Braggs in 1913 and 1914 this paper by W L
Bragg unquestionably brings the greatest single advance it made all future structure determinations very much easier by providing an absolute wave-length scale It would, however, be an invidious undertaking to single out any one of the early papers as the most important one, so closely were
they all interlinked and so rapid was the progress at the time of their writing which formed a background for their formulation.

Page 72:
The joint paper The Structure of Diamond [6]:
was the rst example of a structure in which the effective scattering centres did not coincide with the
points of a simple (Bravais type) lattice. The determination of this structure was acclaimed as a great
triumph of the new methods. Whereas in the structures of rock salt, zinc blende and uorite the
absence of molecules in the accepted sense created an element of bewilderment, the beautiful conrmation of the tetravalency of carbon on purely optical principles made this structure and the method
by which it was obtained immediately acceptable to physicists and chemists alike.

J.R. Helliwell

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112

Figure 2. From [7]. Further text extracts of the various papers from WLB and/or WHB are in the centennial
celebration article [8].

Crystallography Reviews

113

Page 73:
The paper The Analysis of Crystals with the X-ray Spectrometer [9] shows remarkable progress in a
number of ways (this included the fact that) it is clearly recognized that for a complete structure
analysis the intensities of the reections have to be known and evaluated.

5. The words of D.C. Phillips

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David Phillips, who knew WLB directly, e.g. at The Royal Institution, wrote the Biographical
Memoir of WLB in The Royal Society series [10]. On pages 8889 referring to WLB 1912 he states
The critical test was to see whether these ideas (of WLB) explained the observations from (the) ZnS (Laue
diffraction photographs), including the absence of some spots predicted by Laues analysis. Here (W L)
Bragg inverted the argument and used the fact that the X-ray pulses can be regarded to be a white light
spectrum extending over a characteristic range of wavelengths and with maximum energy at certain wavelengths. The intensities of the Laue spots ought, therefore, to fall in a regular series depending upon which
part of the spectrum was responsible for each of them. Examinations showed that this did not work. (W
L) Bragg tried to explain the ZnS pattern (of diffraction spots) on the assumption that the structure is facecentred cubic and everything fell into place. Thus he showed that the Laue pictures were made by a continuous range of X-ray wavelengths and that X-ray diffraction could be used to get information about
the crystal structure. This was the start of the X-ray analysis of crystals The next papers were published
at about the same time (June 1913). In the rst of them WHB derived the wavelengths of various radiations
and correlated them with Barklas characteristic radiations, making use of the structure of rock salt which
had been worked out by his son, but not yet published. This paper was immediately followed by Braggs
detailed account [4] of NaCl and related structures described by Ewald [2] as the great breakthrough to
actual crystal structure determination and to the absolute measurement of X-ray wavelengths. The analysis depended mainly on Laue photographs taken in Cambridge, supported by some measurements with the
(WHB) spectrometer.

6. Other perspectives
The Royal Society Obituary Notice for WHB was written by Andrade and Lonsdale [11]. It contains a synoptic paragraph of their achievements (Figure 3). The emphasis given in [11] between
WLB and WHB, son and father, in that synopsis seems not to be correct not least with respect to
WHBs own words of his sons work (see above) from [7].

Figure 3. Text extract from [11] by Andrade and Lonsdale.

114

J.R. Helliwell
This perhaps explains the modern comment of the type:

11 November 2012 marks the centenary of the reading of the paper by William Lawrence Bragg (WLB)
to the Cambridge Philosophical Society outlining the foundations of X-ray crystallography. It included
the derivation of the rst correct atomic structure of a crystal, namely that of zinc blende, based on the
X-ray diffraction pattern recorded by Friedrich, Knipping and Laue in the spring of 1912.

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This comment can be read recently: in Acta Cryst Section A and in at least two newsletters of
national crystallographic societies or listened to on the radio by scientic commentators.
Perhaps most remarkably, the following appeared in the paperwork for the IUCr Madrid Congress of the Crystallography General Assembly papers opening the item on the International Year
of Crystallography (note now to be held in 2014 not 2013):
In 1912 Max Laue showed that X-rays were diffracted by crystals and W.L. (Lawrence) Bragg presented a
paper to the Cambridge Philosophical Society both presenting Braggs Law and the correct structure of
zinc blende, which he derived from the X-ray diffraction data obtained from Laue. W.H. and W.L.
Bragg rapidly carried out a number of key diffraction experiments of their own that led to the determination of crystal structures that were published in 1913. These ground breaking experiments mark the
birth of modern crystallography. The International Union of Crystallography (IUCr) is marking the centennial of these events by declaring 2013 the International Year of Crystallography (IYCr2013).

If the substance of the modern comments highlighted just above were to restrict themselves to
the layout of a crystal, it would be factually correct, namely face-centred cubic in the case of
WLBs 1912 analysis [1]. Those modern recent comments however, arguably much more important, also seem to basically ignore the importance of the X-ray spectrometer and the monochromatic X-ray measurements from the crystals that it empowers. The Laue method of white X-rays,
as WLB pointedly remarks [1], is very sensitive to the crystal orientation, whereby the intensities
of individual reections are altered. Of course the systematically absent reections have zero
intensity whatever the crystal orientation and which is why the Laue method was adequate for
WLB to use it to determine the face-centred cubic lattice layout from the diffraction photographs
[1] provided by Laue to WHB. Indeed the wavelength normalization of the Laue photographic
intensities has been a key step of the modern synchrotron Laue method of complete quantitative
crystal structure analyses [12]. Again, as WLB remarks, the move towards more complicated
crystal structures, like diamond, in 1913 into 1914 was rmly implanted within the use by
WLB, along with his father, of the X-ray spectrometer.
Further extensive documentation of the achievements of WLB is available in the volume by
Thomas and Phillips [13] and the Special Issue of Acta Crystallographica Section A: Foundations
of Crystallography January 2013 [14]. A tribute to the work in X-ray analysis of WHB is the
scientic summary written by North [15]. A further, recent, tribute to the work of WLB is
given by Thomas [16] and from which I quote
1913 marks the year when X-ray crystallography, through the determination of the structures of
sodium chloride, potassium chloride, potassium bromide and potassium iodide and of diamond
rst made its striking impact.

7. Conclusions
This Historical Note honours the rst crystal structure, that of WLBs NaCl, published in June
1913 [4]. The earlier, also truly remarkable, analysis by WLB of the face-centred cubic layout
of a crystal from the pattern of systematic absences in the Laue diffraction photographs of ZnS
[1], included WLBs anticipation in his concluding paragraph of what studies and experiments
he would have to make to determine the rst crystal structure, i.e. with the placement of the

Crystallography Reviews

115

Na and Cl atoms [4]. The prompt subsequent development of the X-ray spectrometer apparatus by
WHB in Leeds [5], and its use by his son and by himself, led to the more quantitative analysis of
the monochromatic X-ray measurements from single crystals. The rst X-ray crystal structure,
that of NaCl, described vividly in WLBs own words [2,3] is a truly absorbing, wondrous,
iconic story in the History of Crystallography and indeed of all of science. The critical nature
of the contribution of a new piece of apparatus, the X-ray spectrometer [5], and the measurements
it empowered in these studies is also a wondrous development.

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Notes on contributor
John R. Helliwell trained as a physicist (York
University, 1st class Hons degree) and then a
protein crystallographer (DPhil., Oxon). He
spent the rst 20 years of his research career,
from 1979, walking in the footsteps of
William Henry Bragg with the design and
build of the Daresbury Synchrotron Radiation
Source Station 7.2 tuneable synchrotron X-ray
spectrometer for protein crystallography. This
synchrotron radiation X-ray spectrometer was
followed by SRS wiggler Station 9.6, also tuneable, but extended to shorter X-ray wavelengths,
then SRS wiggler Station 9.5 which was rapidly tuneable, and nally the rapidly tuneable and
high X-ray intensity of the instrument ESRF BM14 (this latter project led by Andrew Thompson).
In the last 20 years, JRH switched emphasis and followed in the footsteps of William Lawrence
Bragg, based at the University of Manchester from 1989 as Professor of Structural Chemistry, and
where he has undertaken a wide variety of crystal structure analyses of proteins and nucleic acids,
using X-rays and more recently with neutrons. He is now an Emeritus Professor at the University
of Manchester.

References
[1] Bragg WL. The diffraction of short electromagnetic waves by a crystal. Proc Camb Phil Soc. 1912;XVII
(1):4357 [Communicated by Sir J.J. Thomson].
[2] Ewald PP. The immediate sequels to Laues discovery, Chapter 5. In: Ewald PP, editor. Fifty years of
X-ray diffraction. Utrecht: Published for the International Union of Crystallography by N V A
Oosthoek; 1962. p. 59, 6163.
[3] Bragg WL. The development of X-ray analysis. New York: Dover Publications; 1975.
[4] Bragg WL. The structure of some crystals as indicated by their diffraction of X-rays. Proc R Soc Lond
A. 1913;89:248277.
[5] Bragg WH. The X-ray spectrometer. Nature. 1914;94:199200.
[6] Bragg WH, Bragg WL. The structure of diamond. Proc R Soc Lond A. 1913;89:277291.
[7] Bragg WH. The reection of X-rays by crystals. (II.). Proc R Soc Lond A. 1913;89:246248.
[8] Helliwell JR. The centennial of the rst X-ray crystal structures. Crystallogr Rev. 2012;18(4):280297.
[9] Bragg WL. The analysis of crystals with the X-ray spectrometer. Proc R Soc A. 1914;89:468.
[10] Phillips DC. W L Bragg 18901971. Biograph Memoirs Fellows R Soc. 1979;25:75143.
[11] Andrade ENdaC, Lonsdale K. William Henry Bragg. 18621942. Obit Not Fell R Soc. 1943;4(12):
276300.
[12] Helliwell JR, Habash J, Cruickshank DWJ, Harding MM, Greenhough TJ, Campbell JW, Clifton IJ,
Elder M, Machin PA, Papiz MZ, Zurek S. The recording and analysis of Laue diffraction photographs.
J Appl Crystallogr. 1989;22:483497.

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J.R. Helliwell

[13] Thomas JM, Phillips D, editors. Selections and reections: the legacy of Sir Lawrence Bragg.
Middlesex: Science Reviews Ltd.; 1990.
[14] Wilkins SW. Editor Bragg centennial a special issue of. Acta Crystallogr Sect A. 2013;69:162.
[15] North ACT. William Henry Bragg: grandfather of X-ray analysis. Univ Leeds Rev. 1976;19:125148.
[16] Thomas JM. William Lawrence Bragg: the pioneer of X-ray crystallography and his pervasive inuence. Angew Chem Int Ed. 2012;51:1294612958.

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Subject Index
Crystal structures of
-diamond 2, 3, 7, 8
-NaCl 15
Face-centred cubic lattice 1, 2, 5, 7
Laue diffraction photographs 5, 7, 8
Systematically absent X-ray intensities 7
X-ray spectrometer 25, 7, 8
X-ray wavelengths 2, 3, 5, 8