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CONCLUDING REMARKS
C. Johnson
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplkrnent au no 12, Tome 37, Dbcembre 1976, page C6-941
CONCLUDING REMARKS
C. E. JOHNSONOliver Lodge Laboratory University of Liverpool U. K.
This has been the third International Conference on the Applications of the Mossbauer Effect to be held on the shore of, or close to, the Mediterranean Sea. The previous ones were at Ayelet Hashahar [I J
in 1972 and at Bendor [2] in 1974, although it looks as though we shall not meet in this part of the world again for several years. I have been to every paper of every session of this meeting and I shall try to give a summary of the work we have heard about. I suspect that I would have learnt more if I had missed a few talks, as one's concentration is apt to flag after listening to so many varied topics as those which have been discussed during the week. The area in which Mossbauer spectroscopy has made important contri- butions is now so wide that no-one could hope to follow it all. Nevertheless, it is a valuable feature of a conference like this that it brings together experts in many differing fields who have a common interest. I shall start with some statistics on the attendance at the Conference, broken down [3] by countries. The total of 220 people who came was made up as follows : Germany 52 U. S. A. 29 France 23 Belgium Denmark Greece
1
Nederlands1
10 3 each Israel Italy Sweden U. K. Eastern countries 15 14 other countries - 23 220The overall number and distribution is close to those for the previous meetings, although there has been an increase in the number of countries with one or two participants.
To come to the papers, there were 13 invited talks and 57 contributed talks, the latter having been selected from 210 contributed papers. Those that I
shall mention are very much a matter of my personal taste of course. Three of the review papers were of very general interest to almost all Mossbauer spectro-
scopists, whether they are directly concerned with these topics or not. These were by Blume [4] on spin relaxation, Mossbauer [5] on fundamental aspects of y-ray emission and absorption, and Wagner [6]
on the use of scattering measurements. I think that these papers alone would make the Conference Proceedings a valuable publication.
Now that the technique of Mossbauer spectroscopy has become mature, there are not the spectacular advances every year that there used to be in its early days. Progress is slower and has to be worked for harder. There were a few exciting new discoveries presented during the conference, but in the main the work we heard about was directed towards getting a more complete understanding of the physics and chemistry of solid materials. This is necessarily a
slow (but rewarding) process as new measurements are related to previous data and assimilated into our knowledge. I shall try to indicate which aspects of the traditional topics studied through hyperfine interactions - chemical bonding, metals and alloys, magnetism and so on - are currently being pursued. 1. Chemical bonding.
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The most numerous group of papers submitted was on the use of chemical (isomer) shift data to study chemical bonding. The fundamental problem here is to establish the relative contributions of charge transfer and overlap distortion to the electron distribution in the chemical bond. As often happens with basic questions there is no definite general answer : in some cases one effect predominates while in others there is evidence that both are present.2. Metals and alloys.
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Our understanding of electric quadrupole splittings and magnetic hyperfine fields in the metallic state is still imperfect, despite the large quantity of data which has been built up. Here the uncertainty seems to be in our knowledge of the distribution of the conduction electrons. We believe we have a good knowledge of the charge density distribution of the core electrons from x-ray diffrac- tion measurements, but we are largely ignorant of the details of the outer electrons and their influence on the hyperfine couplings. We just do not know how to describe the size and shape of the charge and spin distributions of these electrons in a metallic lattice. The mixing of core electron wave-functions into the conduction band by the non-cubic potential in anC6-942 C. E. JOHNSON alloy has been considered [7], and the effect on the
efg. has been examined. There will also be subtle exchange effects on the conduction electron wave- functions which will effect the interaction between atoms, transferred hyperfine interactions, etc., in magnetic alloys.
3. Magnetism.
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I was especially interested in the progress that is being made in the study of crystal fields in paramagnetic ions in solids. It is becoming apparent that Mossbauer spectroscopy is now a very sensitive tool for obtaining data on crystal fields and in many cases rivals EPR for the richness of the information obtainable. Spectra where the electron spin relaxation rates are intermediate (neither very fast nor very slow compared with the magnetic hyper- fine frequency) are no longer a nuisance ; they are full of information which may be extracted by using a computer program to fit the data. Varret [8] des- cribed the observation of a crystallographic phase change in ferrous fluosilicate at low temperatures, and showed that by observing the Mossbauer spectrum in magnetic fields as a function of temperature it is possible to deduce the crystal field splittings and states in great detail. Future work looks very promising. Of the other established topics of the conference it seems that in radiation damage the Mossbauer Effect has given results of particular value.Among new topics there was a very good review of the applications of Mossbauer spectroscopy to catalysis by Dumesic [9] which explained for the non-specialist how work on small particles and surfaces could be designed to give data of practical importance. In this inter-disciplinary subject we learnt new mean- ings for the terms dispersion and shell model. In the contributed papers that followed there were several studies which had proved relevant to real industrial problems. Work on Fe,O, catalysts [lo] used in the manufacture of ammonia has shown how the particle size may be stabilized by the addition of alumina, thus increasing the useful life of the catalyst. In another study [ l l ] the effect of Pd-Sn as a colloidal catalyst in the electroless plating of copper in making printed wire boards has actually led to the modification of an industrial process in order to improve its efficiency. In both these applications it was evident that applied science can be as exciting, creative and intellectually demanding as pure science.
A number of new and interesting measurements was reported. A new Mossbauer isotope was reported [12] for the first time at this conference. It is Hglg9 which has a y-ray energy of 158 keV. The spectra were measured with a 4 Ci source and the
Refer [I] Perspectives in Mossbauer Spectroscopy ed. S . G . Cohen
and M. Pasternak (Plenum Press, New York) 1973. [2] J. Physique Colloq. 35 (1974) C6.
measured absorption was less than 0.000 5
%,
i. e.$ of a percent of a percent ! Detection was possible using the current integration technique pioneered by Kankeleit. There was a description of the observa- tion of a Bragg reflection of resonant y-rays by Moss- bauer scattering from a myoglobin crystal, thus unabling the phase of the scattered radiation to be determined [13]. Perlow [14] described the study of some strange effects found when rf radiation was applied to very pure copper.
In less serious vein I should like to award the prize of one-upmanship to Blasius and Gonser [15] for having a reference in Japanese and the prize for irony for a question to Uli Gonser (Mr. Texture) asking whether he had thought of the effects of texture in his precision phase analysis !
It is usual for the speaker giving the conference summary to pay tribute to the Organizing Committee for their spendid work in making the conference such a success. In this case I cannot do this as I am a member of that committee. (Normally the Committee gangs up on some unfortunate candidate who is not around to defend himself, and the honour is bestowed upon him in his absence). However, I can mention a few points we have come across in arranging the conference which might be useful to future organizers. Perhaps the biggest problem of a conference such as this is how to get all the contributed papers discus- sed. For some subjects (metallurgy, biology, environ- mental applications) spontaneous extra sessions have been arranged. In the Bendor conference the rappor- teur system was used with great success in some sub- jects, although the overall success was not enough to encourage us to repeat it here. It may be that poster sessions would be more appropriate. Or maybe a mixture of presentations would be best. Another point that is difficult to get right is the balance bet- ween invited talks and presented contributions. The choice of subjects for the main sessions requires great care if we are to ensure a lively conference. To those of you submitting abstracts for future con- ferences I would urge you to be as complete and informative as you can be ; many papers turned out to be far better than could ever have been guessed from their abstracts, and their inclusion for presenta- tion might well have improved this conference.
The fact that all the sessions were extremely well attended, despite the rival attractions of the sea and sun and this beautiful island, is a handsome tribute to the success of the conference. Our thanks go to Thanos Kostikas and Thanassis Simopoulos and their team for the careful planning which has made our week here so enjoyable.
[3] Statistical jargon : e. g. in population statistics one often reads about people ((broken down by age and sex B.
CONCLUDING REMARKS C6-943 [5] MOSSBAUER, R., J. Physique Colloq. 37 (1976) C6-5, C6-11. [I21 WURTINGER, W., J. Physique Colloq. 37 (1976) C6-697 ; [6] WAGNER, F., J. Physique Colloq. 37 (1976) C6-673. KOCH, W., WAGNER, F. E., FLACH, D. and KALVIUS, G. M.,
[7] KOLK, B., J. Physique Colloq. 37 (1976) C6-355. J. Physique Colloq. 37 (1976) C6-693. [8] VARRET, F., J. Physique Colloq. 37 (1976) C6-437.
[9] DUMESIC, J. A,, J. Physique Colloq. 37 (1976) C6-233. [13] PARAK, F., MOSSBAUER, R. L., HOPPE, W., THOMANEK, U. F. and BADE, D., J. Physique Colloq. 37 (1976) C6-703. [lo] CLAUSEN, B. A., TOPSBE, H., M ~ R U P , S., CANDIA, R.,
JENSEN, E. J., SBRENSEN, O., BARANSKI, A. and PAT- PERLOW, G. I., POTZEL, W. and KOCH,
w.,
J. physique TEK, A., J. Physique Colloq. 37 (1976) C6-245. Colloq. 37 (1976) C6-427.[ l l ] COHEN, R. L., MEEK, R. L. and WEST, K. W., J. Physique [15] BLASIUS, A. and GONSER, U., J. Physique Colloq. 37 (1976)
