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Notes about the origins of the BCA 'named' lectures can be found on page 20 'Crystallography News' no. 62 Sept 97).
The 1999 BCA Lonsdale Lecture was delivered as one of the Plenary sessions at the XVIII IUCr Congress in Glasgow on 7 August 1999. The Lecturer, Dr Richard Welberry of the Australian National University,(ANU), Canberra, was introduced by Howard Flack, a highly appropriate choice as both had been students in Dame Kathleen's Laboratory in University College London. Although Richard was not himself one of Professor Lonsdale's students, arriving to do his Ph.D. in 1967 just a year before her retirement, his project on disorder in crystals reflected one of her long term interests.
The lecture began with a reference to Kathleen Lonsdale's 1948 book,
"Crystals and X-rays", which included pictures of diffuse scattering, and to
the work of Flack and Glazer in 1970 based around measurements of diffuse
scattering using Laue photographs and beginning to model this scattering.
The measurements now being made by Richard Welberry at ANU use flat-cone
Weissenberg geometry with a Position Sensitive Detector. Images are
typically collected in 100 crystal w steps which
are then combined and undistorted to produce reciprocal space images.
Modern images such as these provide a great deal of detail in the X-ray
diffuse scattering patterns, giving the opportunity to develop detailed
methods of interpretation.
The basic framework of the methods used is a Taylor expansion of the diffraction equation in terms of displacements of atoms from their mean positions. In schematic terms this expansion can be written as Idiff = I0 + I1 + I2 + I3 + ...
The first three of these terms, representing correlations between pairs of atoms, can be described most simply in the following way:
One of the difficulties in trying to interpret a diffuse scattering pattern
in terms of even a relatively straightforward expansion such as this is the
large number of terms involved. This leads to the use of computer
simulations to aid interpretation, and building physical models to help
explain the data.
Monte Carlo simulations are the main approach now used to model diffuse scattering. Atoms are moved around "randomly" subject to some simple physical model; the usual one represents "molecular" units as rigid bodies and intermolecular forces by simple Hooke's law relationships. Large numbers of repeat units are required for such simulations to reproduce the observed diffraction pattern in sufficient detail to match the high quality observations available. Early use of arrays of 10 x 10 x 10 unit cells led to seriously degraded patterns, now simulations usually have 32 x 32 x 32 unit cells.
The applications of this method were illustrated by examples from Richard's own work:
Recent work aims to move from the current need for some initial insight into the problem before construction of plausible models, to a method adjusting parameters automatically using a least squares procedure, which would, in principle, allow for the interpretation of diffuse scattering ab initio. However, in this method, the calculations of the necessary differentials are not straightforward and the differentials have to be evaluated numerically.
The first system tested with this procedure was Fe3(CO)12, where the basic disorder is a 180° flip of the Fe3 unit; that in the carbonyl groups can be ignored to first order. The parameters in the diffuse scattering interpretation procedure included 8 occupancy (pair-wise) correlations, 4 centre-of-mass relaxation size-effects and 4 orientational relaxation size-effects. The differentials for each can be evaluated numerically; the refinement progressed towards a reasonable fit in around 8 cycles. One important aspect of this process was that initial attempts at interpreting the diffuse scattering pattern based on just two sections through the diffraction pattern showed that these were not sufficient; the inclusion of a third section was required for consistency. Apart from that, considerable detail can be reproduced by the modelling procedure, lending strength to the argument that the method will soon allow a quantitative interpretation of diffuse scattering.
Having constructed the model automatically, the configurations produced can now be inspected. For example, comparisons can be made with Bragg scattering results by averaging the generated configurations. Local structural detail and correlations between molecules can also be examined. Librational modes are not accessible by examining the Bragg scattering alone but can be distinguished by diffuse scattering . In this case of Fe3(CO)12 there is clear evidence of concerted motion. The work is now being extended to more complex systems; the continuing huge increases in computing power (a factor of 1000 increase every 10 years over the last 30 years) are making these difficult problems tenable.
This clear, educational and informative Lonsdale lecture concluded with an illuminating demonstration of optical transforms. This illustrated how the effects discussed above lead to characteristic patterns of diffuse scattering, and delightfully illustrated how these are built up from simple distortions from a regular structure. Diffuse scattering remains non-routine and challenging in its analysis but this lecture demonstrated the enormous strides that are being made towards its interpretation.
During IUCr XVIII an evening symposium celebrated the great personal
contribution to X-ray crystallography of J. Monteath Robertson, F.R.S.,
Gardiner Professor of Chemistry at the University of Glasgow from 1942 to
1970.
The symposium was organised and chaired by one of JMR's ex-students, Jim Trotter (Ph.D. 1957). An overview of Robertson's career was presented by Robert Bryan (Ph.D. 1957), a pupil of another internationally distinguished Glasgow crystallographer, Jim Speakman. Besides touching on some of the highlights of Robertson's career - his solution of the phthalocyanine structure by isomorphous replacement and the development and application of the heavy atom method - Bryan gave a graphic account of JMR's lectures to first year undergraduates - an old and honorable tradition for holders of chairs at ancient Scottish universities.
Jack Dunitz (Ph.D. 1946), Sydney Abrahams (Ph.D. 1949), and Michael Rossmann (Ph.D. 1956), all ex-students of JMR, described their memories of him during the heroic days of the 1940s and 1950s when crystallographic apparatus was primitive and a mechanical calculating machine represented leading-edge technology. Their contributions painted a rounded portrait of JMR, both as a research scientist and as a teacher. JMR's training as an organic chemist allowed him to see more clearly than most that X-ray analysis could revolutionise organic chemistry, rendering obsolete purely chemical methods of structure determination. Today, he is hailed as founding father of both chemical crystallography and crystal engineering. As a teacher JMR trained over fifty graduate students, many of whom have gone on to make their own distinguished contributions to the subject, both here and abroad. Dunitz, in a moving conclusion to his talk, emphasized the importance of the Robertson diaspora of young and enthusiastic crystallographers to the development of chemical crystallography in many countries, notably the U.S.A., Canada, Australia, and Switzerland. Today, Robertson's department has a large crystallography group with active programs in protein structure analysis, experimental charge density determination, organometallic and cluster chemistry, direct methods and crystallographic programming. Neil Isaacs, who leads the protein crystallography group, described current activities.
The large audience included about 70 crystallographers with Glasgow connections, including Durward Cruickshank who was persuaded by JMR to accept a chair in Glasgow in the 1960s, and George Sim (Ph.D. 1955) who succeeded JMR in the Gardiner Chair. It is pleasant to note that JMR's daughter, Mrs Patricia Escott, her two sons, and Miss Ann Boyd, JMR's secretary for many years, were also present.
Kenneth W. Muir (Ph.D. 1967)
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