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PLENARY SESSION.Fifty years of colour centre physics
H. Pick
To cite this version:
H. Pick. PLENARY SESSION.Fifty years of colour centre physics. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-1-C6-6. �10.1051/jphyscol:1980601�. �jpa-00219996�
PLENARY SESSION,
Fifty years of colour centre physics
H. Pick
Physics Department University of Stuttgart, Germany
1. Introduction. - In natural science the 17th century brought forth the development of terrestrial and celestial mechanics. The 18th and 19th centuries were characterized by the establishment of elec- trostatics, electrodynamics and early atomic phy- sics. The 20th century has been called the century of modern physics. This includes relativity, quantum mechanics of atoms, nuclear and solid state physics.
From a very modest beginning in fields like crystallo- graphy, phosphorescence and photoconductivity, solid state physics particularly grew up to become an extended branch of o w physico-technical world.
Colour centre research may be considered historically as the first chapter of modern solid state physics.
The Organizing Committee of the 3rd Europhysical Topical Conference, and in particular my old friend A. B. Lidiard, encouraged me to summarize the historical development of colour centre research.
Personally connected with the field since 1934, I had the great privilege to meet and to know nearly all those scientists who stimulated and promoted this exciting new branch of the physics of solids. This report does not claim to be historically reliable : it is far more a personal recollection based upon knowledge of the participants and their work.
The initiation of what we now call Colour Centre Physics was taken more than 50 years ago by R. W. Pohl and his coworkers at the university of Gottingen. This new topic and activity remained con- nected to this place for more than one decade before it spread out to England, the United States ofAmerica, Italy, the Netherlands, Japan and other countries.
Since 1949 international meetings, symposia and con- ferences brought together the members of a rapidly growing family of colour centre specialists.
Between 1956 and 1974 a series of seven International Colour Centre Conferences was held. The organizers of the subsequent meeting held in Gatlinburg in 1977 were anxious to cover a broader field. Therefore they changed the name to International Conference on Defects in Insulating Crystals. In fact this is essentially a synonym of the title of the three Europhysical Topical Conferences, held in Marseille-Luminy 1973, Berlin 1976, and Canterbury 1979 : Lattice Defects in Ionic Crystals. Incidentally, the topics of all these conferences from the first in Gottingen in 1949 to the
present in Canterbury in 1979 have not changed much.
Besides problems of colour centres in alkali halides there were always presentations concerning other materials, general properties of ionic crystals, pho- nons, transport phenomena, photochemistry, band structure, crystal growth, radiation damage, etc.
Strongly convinced that colour centre research is the most important and effective root of modern solid state science and technology, I think it may be wise to recall the situation in physics 60 years ago, so as to understand the exceptional position and implication of the early activity in this field.
2. The situation in physics before 1920. - As a consequence of exciting experimental studies on the spectroscopy of gases, thermal radiation, radio- activity, and X-ray scattering at the end of the 19th and the beginning of the 20th century new branches of physics developed rapidly. Important discoveries and events characterize the time between 1895 and 1915 :
1895 W. C. Rontgen discovers X-rays.
1896 H. Becquerel observes radioactivity.
1900 M. Planck introduces h as an important uni- versal constant.
1902 E. Rutherford publishes a theory on radio- active decay.
1905107 A. Einstein presents the theory of relativity, the photoelectric equation and the first quan- tum theory of the specific heat of solids.
19 1 1 H. Kammerling Onnes discovers supercon- ductivity.
1912 M. v. Laue demonstrates the periodic struc- ture of crystals and the wave nature of X-rays.
1913 W. H. Bragg develops methods to analyse crystal structures by X-rays.
19 13 G. J. Moseley introduces X-ray spectroscopy.
1913 N. Bohr presents an atomic model and the first quantum theory of atomic spectra.
19 15 A. Sommerfeld explains the fine structure and generalizes the analysis of atomic spectra.
These are a few highlights of a 20 year period.
Scientists believed in a new beginning of basic under- standing of physics and natural sciences. Famous
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980601
C6-2 H. PICK
research centres and academic schools came into being.
N. Bohr since 1916 in Copenhagen,
J. J. Thomson and E. Rutherford since 1915 in Cambridge,
M. Planck, M. v. Laue, and A. Einstein since 1914 in Berlin,
A. Sommerfeld since 1915 in Munich,
may be mentioned as examples. They attracted talented scientists and students.
But the flourishing international activity and exchange was suddenly interrupted by world-war one.
The optimistic belief in rapid progress was replaced by disappointment and a wide-spread pessimistic men- tality. The period of high prosperity was followed by economic difficulties and austerity. F. Reiche, Berlin, who published the first German book on quantum theory, wrote in 1920 : (( We all have to realize that we have not made the slightest progress conceiving the essence of things. What is the profound reason for the erratic processes in nature ? Many of the problems are in the depth of the dark. ))
3. The early beginning at Gottingen. -This was the situation when R. W. Pohl agreed to take a chair at the university of Gottingen. He changed from Berlin where he had received his Ph. D. degree in 1905 and where he had worked on the ionization of gases, on X-ray diffraction, and on photoelectric surface emis- sion. He had published a great number of papers, many of them together with B. Walter, P. Pringsheim and J. Franck. U p to that time Pohl's work followed the general line. It was also interrupted by the war until 1920 when he resumed his work in Gottingen.
Contrary to the prewar situation in Berlin he found no modern equipment, no vacuum pumps, very little money and personnel in the new place. He had to concentrate on cheap or small science. Accordingly he decided to jump from surface- and vacuum physics to physics of the bulk, which he speculated would not need expensive vacuum conditions. A few years later he had to change his mind.
There was some tradition in crystal physics in Gottingen : in 1910 W. Voigt had published a textbook on Crystal Physics treating macroscopic tensor pro- perties and interactions. In 1915 M. Born had written his first edition of Lattice Dynamics.
M. Born and J. Franck, also changing from Berlin, joined the Physics Institute in 1921. These three young scientists worked independently in different fields : M. Born on lattice dynamics, J. Franck on atomic spectroscopy, R. W. Pohl on luminescence, photo- conductivity and optical properties of crystals.
There was strong competition as well as friendly disregard between the three groups. Playing on words they called themselves and each other the Bornierten, the Franckierten and the Pohlierten which means translated into English : the narrow minded, the
prepaid and the polished people. Refering to Pohl the most important coworkers who joined his group during the following decade are :
B. Gudden (1920), Z. Gyulai (1922), S. Kyropou- 10s (1923), R. Hilsch (1924), A. Smakula (1926), E. Mollwo (1928), K. Korth, W. Lehfeldt and 0. Sta- siw (1930), G. Glaser and H. Pick (1934). Nevertheless most of the publications of that period obviously indi- cate Pohl's writing hand, but all the results emerged from the work of small teams with strong staying power and long-term planning. It was experimental work mainly, with few theoretical speculations, based on sensitive feeling for physical connections.
Before going into details it might be informative to survey previous activities in this field.
4. The prehistory of colour centre research. -
The prehistory of colour centre research goes back to the year 1863 when H. Rose [I] was able to colour sodium chloride by heating in sodium. This result was confirmed in 1905 by H. Siedentopf [2]. Ten years before, E. Goldstein [3] had connected the yellow and blue coloration of natural rocksalt with the nearby presence of radioactive minerals.
W. C. Rontgen [4] published an extended paper in 1921 on the photoconductivity of X-rayed rocksalt.
He found, among other things, a much higher conduc- tivity than in untreated material. To summarize, in the early twenties it was known that :
Sodium chloride may be coloured by X-rays and also by radioactive radiation.
Sodium chloride may be coloured also by heat treatment in sodium vapour.
Sodium chloride becomes strongly photocon- ductive as a result of coloration.
Besides these very special observations on coloured NaCl there was some specific knowledge on general solid state problems :
Very important investigations had been carried out around 1908 by K. Baedeker [5]. Investigating the conductivity of non-metallic CuI he found the wrong sign for the Hall voltage, as we now connect !with the presence of positive electronic carriers, and - what is still more important - he realized a strong dependence of carrier density on the stoichiometric excess of Iodine. Ten years earlier E. Riecke [6] had speculated in a paper on the theory of galvanism and heat.
..
)) in the space between molecules there may be accepted mobile positive and negative electrical particles with different mobilities and numbers ... )).Both these indications of positive electronic carriers were forgotten, and rediscovered 30 years later as a result of V centre and semiconductor studies.
The Dulong-Petit law on the specific heat of solids had been extended by A. Einstein [7] and P. Debye [8] to the quantum theoretical treatment of its temperature dependence.
The electron theory of metals had been developed by E. Riecke [6] (1898), P. Drude [9] (1900), and H. A. Lorentz [lo] (1909). But there was little agree- ment with experimental results on metals. It was found much later by A. H. Wilson [ll] (1931) that these ideas describe reasonably well the behaviour of semiconductors.
Ph. Lenard [12] (19 10) had investigated phospho- rescence of fluorides with heavy-metal activators.
He attempted to apply the quantum physics of atomic systems, but he failed.
In 1914 R. W. Pohl and P. Pringsheim had published a book on photoelectric phenomena [13].
They considered the influence of thin surface layers upon the photoelectric emission from metals.
Indeed there was no general picture of non-metallic solids, very little activity, and much less interest in solid state problems than in atomic and nuclear phy- sics.
5. The local period 1921-1936. - At the beginning, the main activity of R. W. Pohl and his group in Gottingen was not at all connected with colour centres in alkali halides, rather with photoconductivity of crystals with high dielectric constants like diamond, zinc sulphide, alkaline-earth phosphors and others.
The publication of W. C . Rontgen [4] on the properties of X-rayed natural minerals like rocksalt, especially the yellow and blue species, however, found great interest in Gottingen and stimulated detailed work on alkali halide crystals. There was no doubt that coloured
NaCl showed a much higher than
uncoloured crystals, and this was not at all connected with a high dielectric constant. To understand the underlying elementary processes in the bulk of the crystal one had to answer an important question : what is the origin of the yellow and blue colour in NaCl crystals ?This question was raised and answered.
However, Pohl and his coworkers were not satisfied with the obvious interpretation of atomically dispersed metal. They rather initiated broad and systematic research programmes to unravel a bundle of problems.
Research areas and approximate starting dates were : Photoconductivity of coloured alkali halide crystals [14].
Optical absorption spectra of coloured natural single crystals [15].
Connection between photoconductivity and phosphorescence [16].
Growth of synthetic alkali halide crystals [17].
Development of methods of additive colo- ration [18].
Spectroscopic analysis of alkali halide phos- phors doped with heavy metals [19].
Fundamental optical absorption in the ultra- violet [20].
Photochemical reactions in alkali halides [21].
Photochemical reactions in silver halides [22].
1927 Fundamental optical absorption of lead and thallium halides [23].
1929 Optical absorption in the vacuum ultraviolet region [24].
1930 Fundamental optical absorption and disper- sion in the infrared [25].
1930 Motion of colour centres in an electric field at elevated temperatures [26].
1933 Ionic conductivity of single crystals [27].
The result of this research activity, published in more than 150 papers, is a broad body of empirical knowledge covering the properties of pure, doped, irradiated, additively coloured and photochemically treated alkali halide single crystals and crystalline layers. A first summary of this dedicated work was presented to an international audience by R. W. Pohl [28] and R. Hilsch 1291 in 1936 at a meeting of the Physical Society in London. At this meeting J. H. de Boer [30] proposed a more refined F-centre model which considers the atomically dispersed excess alkali as an unpaired electron in an anion vacancy.
By the way, the name colour centre (Farbzentrum) emerged around 1930. Until 1928 it was common to speak of coloured crystals or crystals with strange coloration [31]. In connection with A. Smakula's formula [32] for the number of dispersion electrons, one began to speak of centres of coloration and briefly of colour centres [33].
The international reaction to the work of Pohl and his co-workers was formulated by F. Seitz [55] :
(( It is a rather remarkable characteristic of this work that, although the investigators have never had a very deep interest in the fundamental interpretation..
.
the experiments they have carried out have been exactly those that are needed to furnish the basis of an inter- pretation. This fact indicates that they possess a very deep intuitive sense concerning the physically impor- tant quantities...
D.What was known and well established at the end of the local period of colour centre research ? A few of the most important findings and well established results are :
The fundamental electronic absorption in the ultraviolet [35, 361 and the fundamental vibrational absorption in the infrared [37, 38, 39, 401 of all alkali halides, later on identified as band to band, exciton and phonon excitation.
The temperature dependence of intrinsic and extrinsic ionic conductivity interpreted on the basis of thermodynamic Schottky-disorder [27].
The temperature dependence of the absorption spectra of colour centres, halogen excess centres (now called V centres) hydrogen centres (originally named U centres) and other chemical defect centres (heavy metals mainly) (summarized in [28, 29, 341).
The connection between integral absorption and density of centres is well established by Smakula's formula [32].
C6-4 H. PICK
The photo- and dark conductivity released by optical excitation at low temperatures [41, 431, or thermal activation of colour centres at high tempe- ratures [44]. Primary and secondary electronic currents at lower temperatures [41, 421. Their mean free paths and efficiencies as well as the mobilities of colour centres at higher temperatures had been studied carefully [44,45].
The photochemical decomposition of H- cen- tres [46] and the F + F' conversion [47] were analysed and compared with studies of the elementary photo- graphic process in silver halides.
The properties of alkali-halide phosphors con- taining heavy metals [29].
Confining ourselves to the special problems of the F centre, with the general accepted model of an unpaired electron trapped in an anion vacancy, there still remain important questions :
What is the precise site of the unpaired electron ? Does it stay in the centre of the vacancy ? Does it belong to one of the neighbouring cations, or to all of them simultaneously? What happens after the optical excitation : is there a fluorescence emission ? Are there many-vacancy centres ? Do corresponding cation-vacancy centres exist ? What are V centres ? These and many other questions were raised by scientists in many countries who became fascinated by the new and promising field.
6. The early international period 1936-1956. -
The international period of colour research began around 1936. It was characterized by animated inter- national competition and cooperation, by intensive theoretical activity,' by fruitful interaction between experimentalists and theorists and by a number of newly developed experimental methods and tech- niques.
This was possible because of a rapid development of measuring techniques and devices in the fields of microwave instrumentation, infrared detectors, spec- troscopy, electronics and low-temperature equipment, mainly after world-war two. Among these methods one of the most effective for centre research is Electron Spin Resonance (ESR) combined with the Electron Nuclear Double ~ e s o n a n c e (ENDOR) technique.
With their help comprehensive information on local atomic structures, on electron nuclear interaction, on symmetry elements and wave functions have been obtained. Two final results may be mentioned :
With respect to the F centre, it was found that in the highly symmetric groundstate the unpaired elec- tron spreads - with continously decreasing density -
over more than 100 neighbouring ions of the anion vacancy [48, 491. Accordingly the F centre and all the other F-type centres have to be considered as centres typical of the crystal matrix. Their structures, energy levels and electron wave functions are primarily
given by the respective lattice and its lattice defects (vacancies).
This does not hold for the corresponding V centres. They all have molecular character, they are molecules or molecular ions imbedded in defects of the lattice, the wave functions of the unpaired holes essentially spreading over only a small number of anions, as for instance, over two halogen ions in C1;
in KC1
Ol,
centre) [50].The methods of near and far infrared spectroscopy have brought about extensive knowledge of dynamical properties of crystals with defects and centres as may be seen from two examples :
The in-phase vibrational motion of two cation neighbours of the F centre is observed as a disturbed lattice phonons in the far infrared region (near 100 pm in KBr) below the optical phonon band of the undis- turbed lattice [51].
The vibration of the substitutional H- ion is found in the near infrared as a local phonon mode above the optical phonon band [52, 531 near 22 pm.
In addition, a disturbed lattice phonon may be excited in the far infrared (near 113 pm in KBr).
The near infrared range of the spectrum holds special interest because of the fluorescence of the F and F-type centres. These emission properties have proved to be of great importance for the understanding of excitation, de-excitation and relaxation processes.
They were found rather late. F. Seitz [55] quoted S. Pekar's [56] theoretical conclusion that lumines- cence of F centres should exist, but he had to report that outstanding experimental evidence does not seem to favour this point of view. One year later it was found [54].
Besides normal absorption spectroscopy, methods of excitation, fluorescence and magnetooptical spec- troscopy were skillfully developed [57, 58, 591 and used for the detailed study of energy states and transitions. They proved to be especially helpful for the analysis of more complicated centres like F, [60].
Additional information for the study of dipole centres (OH-, off centre Li') was obtained by the investi- gation of paraelectric, paraelastic, electrocaloric and dipole-resonance properties [6 1-65].
These few examples have been selected, looking ahead of 30 years development, to demonstrate the capacity of such new experimental and theoretical achievements. Going back to the early international period we still are at the beginning of world-wide cooperation and methodical refinement. This scien- tific exchange was stimulated extraordinarily by two review articles of F. Seitz [55]. The first from the year 1946 preferentially covers the work of Pohl's group and reports on 30 original publications. The second from the year 1954, however, refers to 160 papers from many countries : Belgium, Canada, England, Ger- many, Italy, Japan and the United States of America.
7. The period of international colour centre confe- rences 1956-1977. - The following period of colour centre research is characterized by a series of seven conferences held every 3 years.
The conference in Argonne (Ill) 1956 (with 40 contributed papers) came about as a result of an invitation from the very active group around P. Pringsheim, C. J. Delbecq and P. Yuster. Extended discussions on ESR spectra of F and V centres, on the theory of states and wave functions, on luminescence and on colour centres in other materials (MgO, CaF, ...) are the highlights of the first meeting of the family of colour centre researchers.
Three years later A. 8. Scott invited the commu- nity to come to Corvallis (Or) (60 papers). Radiation dammage, thermoluminescence and photochemical (F + 2 ) reactions inspired somk interest besides the most informative experiments on ESR and lumines- cence. For the first time H - vibrational modes were presented.
In 1962 I had the privilege to host a conference in my home university of Stuttgart (Germany) (75 papers). Besides the well known topics the magnetic properties of F and F-aggregate centres and the phonon properties of hydrogen doped crystals found great interest.
The famous conference in Urbana (Ill) in 1965 organized by R. J. Maurer, W. D. Compton, F. C . Brown and their collegues with more than 200 contributions was governed by theoretical discussions on optical transitions, on concepts for the construc- tion of wave functions, on band structures and on phonon modes. For the first time the F-centre excited state was seriously considered.
This well established pattern of topics which included the theory of transitions and states, defect formation, the structure of F and V centres, transport and phonon properties, was kept during the confe- rences in :
Rome (Italy) 1968 with G. Chiarotti as chairman.
Reading ( U K ) 1971 organized by C . D. Clark and his collegues and
Serzdai (Japan) 1974 with M. Ueta as chairman.
The conference programmes indicated a certain degree of saturation.
Only the last conference under the chairmanship of R. F. Wood in Gatlirzburg (Terziz) in 1977 on Defects in Insulating Crystals with nearly 50
%
out of 270 papers on other than alkali halide crystals is characte- rized by the attempt to shift from colour centres to more general solid state problems.Confereizces came together by personal initiative based on unanimous agreement, with few official accessories. They never agreed to publish Coizference Proceedings, thus leaving it free to all contributors to publish their results in adequate journals. In Europe members of the European Physical Society raised the idea of Topical Conferences. Accordingly three Europhysical Topical Corzferences on Lattice Defects irz Ionic Crystals have been organized so far in Marseille-Luminy (France) 1973, Berlin-West (Ger- many) 1976 and Canterbury (England) 1979. They all cover the field of colour centres with approximately 50
%
of all contributions.8. Summary and retrospect. - Considering inter- national journals on solid state research one still sees a remarkable activity in the special field of colour centre physics, with an output of at least 30 papers per month.
They complete our knowledge of the structure, states and transitions of centres, mainly those of more complicated nature. They bring proposals for appli- cation of colour centre physics for lasers, for infor- mation storage, for paraelectric cooling, etc. They present remarkable refinement concerning short-time effects and computer-supported theoretical treatments and they introduce other and new materials.
But a great deal of fundamental research in solid state certainly has meanwhile shifted to semiconduc- tors, organic materials, polymers, new materials and magnetic crystals.
Nevertheless, the broad basic knowledge of the properties of pure and doped, irradiated and coloured alkali halide crystals was and is still extremely helpful for any study of the solid state. Accordingly a number of authors found good reasons to summarize and to survey the literature and to present monographic publications.
A retrospective view of 50 years of colour centre work indicates a fertilizing period with a great number of empirical findings. The results of this period (192 1-36) have been summarized in review articles by R. W. Pohl [28] and R. Hilsch [29] and discussed by N. F. Mott and R. W. Gurney [66] and F. Seitz [55].
The results of the second or early international period (1936-56) were summarized by H. Pick [67, 691, J. H. Schulman and W. D. Compton [68] and J. J. Markham [70].
In 1968, around the culmination of international cooperation, experimental subtlety and strong theore- tical penetration, a number of monographic articles were written by specialists.
In 1964 B. S. Gourary and F. S. Adrian [71] wrote an article on colour centre wave functions.
I think it is not unfair to say that the years between In 1968 Physics of Color Centers edited by 1956 (Argonne) and 1974 (Sendai) are the ripening and W- 33. Fowler [721 was published. It comprises survey harvesting period of colour centre research. Not On :
organized and represented by any international Theory of Centres, W. B. Fowler.
society, the attendees of the Irzter~zatio~al Colour Centre FA Centres, F. Liity.
C6-6 H. PICK
Laser Spectroscopy, H. Mahr.
Zero Phonon Transitions, D. B. Fitchen.
Moments and Degeneracy in Optical Spectra, C. H. Henry and C. P. Slichter.
Localized Modes and Resonance States, M.
V. Klein.
ESR and ENDOR Spectroscopy, H. Seidel and H. C. Wolf.
A special article on the structure of F and V centres reviewed the situation around 1970 r 3 ] .
Finally several textbooks by F. C. Brown [74], B. Henderson [75], P. D. Townsend and J. C . Kelly [76], are to be mentioned. In these colour centre physics is an integral part of the general treatment of structure and properties of solids. They appeared approximately 50 years after the first publications in the field. These and others [77] demonstrate the basic role Colour Centre Physics still plays in the general understanding of the solid state.
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[68] SCHULMAN, J. H. and COMPTON, W. D., Color Centers in Solids, Pergamon Press, Oxford, 1962.
[69] PICK, H., Struktur von Sltoerstellen in Alkalihalogenid-Kristallen, Springer Tracts 38, Heidelberg, 1965.
[70] MARKHAM, J. J., F-centers in Alkali Halides, Academic Press, New York, 1966.
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[72] FOWLER, W. B., Physics of Color Centers, Academ~c Press, New York, 1968.
[73] PICK, H., Structure of Trapped Electron and Trapped Hole Centers in : Optical Properties of Solids edited b y F. Abeles, North Holland Pub. Cp., Amsterdam, 1972.
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1771 PICK, H., Einfuehrung in die Festkoerperphysik, Wiss. Buchges., Darmstadt, 1978.
