Toward the Thirtieth Anniversary of Soviet Physics

X-ray Structural Analysis and X-ray Spectroscopy over 30 Years

S. T. Konobeevsky

The study of the structure of matter by means of X-rays is a comparatively young branch of physics, one that has developed widely during the last 35 years, since the discovery of the phenomenon of diffraction of X-rays in crystals. The merits of this method and the enormous tasks confronting it justify the appearance of a special article devoted to the work of Soviet scientists in the field of X-ray structural analysis, all the more since the latter was one of the principal directions of Soviet post-October physics.

In the field of atomic physics and, especially, the physics of the solid state, X-rays play a fundamental role arising from the commensurability of the dimensions of the atom and the wavelength of X-rays. Therefore at the present time an understanding of the phenomena occurring in solids, as well as the development of the theory of the condensed state, is unthinkable without information about structure obtained by means of the X-ray method.

X-ray structural analysis acquires special significance also because its results are used in the broadest way in a number of sciences adjacent to physics: in chemistry, in geology, in metallurgy, and in many other branches of the technical sciences.

It is natural that in our country, where science has been assigned the honorable task of serving the growing socialist industry, X-ray science and X-ray structural analysis had to find their proper place. Soviet X-ray science developed in close connection with the growth of Soviet industry and technology.

Among the works on the diffraction of X-rays carried out in the pre-revolutionary period, first of all one should mention the excellent study by the Moscow University professor Yu. V. Wulff (1913), in which he succeeded in showing very elegantly that the phenomenon of X-ray diffraction, discovered by Laue and his students, can be interpreted as reflection from the planar atomic networks of a crystal.

Following this, Yu. V. Wulff and N. E. Uspensky (1913) carried out two more experimental works that marked the beginning of the experimental study of crystal structure. One of them is especially interesting because it represents the first attempt at monochromatizing X-ray radiation by reflection in a crystal. One may also recall the work of A. P. Minakov and A. Z. Tal (1915) on the form of interference maxima of X-ray beams.

In 1922 there appeared the work of N. E. Uspensky and S. T. Konobeevsky, in which the method of X-ray analysis was applied to determine the structure of metal foils obtained by cold rolling. In this work, an important fact for the practice of metalworking was established for the first time: the orientation of crystallites in cold-deformed metals. This peculiar arrangement is now known under the name of texture. In practice it is important as a means of characterizing deformations; theoretically, because a change in the orientation of crystallites is an indication of the peculiar character of the flow of crystalline matter through the formation of slip planes.

In the study of the textures of metals, Soviet roentgenology occupies by no means the last place. By the author of this article and his students, a number of works on the study of metal textures were carried out: 1) a difference was established between the texture of surface and internal layers (Konobeevsky, 1926); 2) G. S. Zhdanov developed a method for constructing and analyzing pole figures and constructed a special texture camera; 3) he also studied the textures of duralumin (1934); 4) Zhdanov, with collaborators, studied the textures of a number of metals and alloys; 5) Zhdanov and V. I. Iveronova studied recrystallization textures of brass and other alloys; Konobeevsky and Umansky studied the textures of thin films obtained by condensation.

A number of other Soviet researchers also devoted their work to the study of textures (Bakhmetev, 1931; Kurdyumov, 1930). A. E. Bryukhanov studied textures by the acoustic method, while Titov, N. L. Bryukhatov, and others did so by the magnetic method developed by N. S. Akulov. Numerous works on the crystalline anisotropy of cold-deformed metals were carried out in factories, chiefly by former students of the Institute of Physics of Moscow University.

The second direction (chronologically) to have taken shape should be considered the study of plastic deformation and of the structure of the real crystal. The pioneering works in this direction belong to A. F. Ioffe and his school.

In 1924, while observing the roentgenogram of a crystal of rock salt, A. F. Ioffe, with his collaborators (M. V. Kirpicheva and M. A. Levitskaya), succeeded in showing that plastic deformation, without disrupting the crystal lattice, leads to the rotation of blocks (slip packets) into which the crystal breaks up during deformation. At the same time, the remarkable phenomenon of the hardening of a rock-salt crystal when it is stretched under water was discovered.

It should be noted, in connection with this, that the discovery by Klassen-Neklyudova of the phenomenon of abrupt deformation in crystals, which was subsequently studied by N. N. Davidenkov, M. V. Yakutovich, E. Yakovleva and others. I. V. Obreimov and L. V. Shubnikov are responsible for a new and very successful method of obtaining metallic single crystals, later widely used by Soviet physicists for studying the properties and structure of metallic single crystals. Later, working in Kharkov, I. V. Obreimov initiated a careful study of the phenomenon of twinning in crystals, using both optical and X-ray methods (Obreimov and Brilliantov, Garber, and others). Garber discovered the remarkable phenomenon of “elastic,” or reversible, twinning. Twinning was also studied by M. V. Yakutovich, D. B. Gogoberidze, G. Kolesnikov, and others.

Theoretical studies on plastic deformation and the strength of crystals were carried out by many physicists of A. F. Ioffe’s school, above all Ya. I. Frenkel, T. A. Kontorova, N. Ya. Selyakov, and A. V. Stepanov. The latter is responsible for highly original ideas in this field, in particular the broadly developed parallel he drew between plastic deformation and local melting on slip planes.

Speaking of this circle of works, one cannot fail to recall the remarkable results obtained by P. A. Rebinder’s school on the influence of the medium on the plastic properties of crystals (the lowering of the yield point under the influence of the penetration of surface-active molecules into the crystal), as well as the numerous works on the mechanical properties of crystals by V. D. Kuznetsov and his collaborators, summarized in his detailed work Physics of the Solid State, awarded the Stalin Prize. Among V. D. Kuznetsov’s works, the studies on metal cutting had especially great practical significance. However, a more detailed exposition of the results of these works would go beyond the scope of the present article, devoted chiefly to the radiographic method of investigating matter.

Among other works on single crystals, one may note the author’s work with I. I. Mirer, which gave radiographic proof of the existence of elastically bent slip packets, and especially the work of A. I. El’nikov, who carefully analyzed radiograms of an aluminum single crystal deformed at different temperatures. A. P. Komar also took part in the discussion of the internal structure of deformed single crystals.

In 1939 the author, together with M. P. Shaskolskaya, crystallized rock salt on a plastically deformed crystal; it was shown that the new layers grow in an elastically deformed state. Interesting works on the study of the structure of deformed metal by the method of magnetic suspensions were carried out by N. S. Akulov, M. V. Dekhtyar, S. Raevskii, and others.

Interest in X-ray science on the part of metallurgy, for which the introduction of X-ray methods of investigation meant decisive progress in understanding the nature and properties of alloys, led to a considerable number of works by Soviet physicists in the field of the structure of metals and phase transformations.

One of the main lines of this group of investigations likewise takes its origin from the Leningrad Physico-Technical Institute. In 1926 N. Ya. Selyakov, G. V. Kurdyumov, and N. T. Gudtsov for the first time determined the structure of martensite. In later works by G. V. Kurdyumov and his numerous pupils, the most important facts relating to the hardening and tempering of steel were established. The dependence of the tetragonal parameter of martensite on the carbon content was found; the mutual orientations of austenite and the martensite (ferrite) arising from it were studied on single crystals; the mechanism and kinetics of transformations during tempering of hardened steel were elucidated.

Later, G. V. Kurdyumov, who by that time was already working at the Dnepropetrovsk Physico-Technical Institute, which had separated from the LPTI, began a systematic cycle of investigations of transformations during the quenching of eutectoid alloys of the copper group. The investigation was carried out by various methods: X-ray, metallographic, thermal, and others. G. V. Kurdyumov and his immediate pupils and collaborators succeeded in discovering an entirely new field, previously unknown to metallurgists, of the most complex metastable phases passing one into another by means of diffusionless transformations. The idea of G. V. Kurdyumov underlying his investigations is that the transformations take place not by the growth of grains of a new phase from centers, as is commonly assumed for most structural transformations in metals, but by a crystallographic transformation of large volumes of the lattice, carried out in a manner analogous to the formation of twins in calcite.

The basis for such a view is provided by the regularities of the structure and orientation of the newly forming phase. One of the brilliant results of this circle of investigations is the discovery of a peculiar structural reversibility of phase transitions. Systematic experimental material, highly convincing, has been obtained on copper—aluminum alloys (Kaminskii, Miretskii, Gridnev, Stelletskaya), copper—tin (Isaychev), copper—zinc, and others. Recently, by an ingenious method of investigation on single crystals, Kurdyumov (together with Lysak) succeeded in proving the tetragonality of the lattice of tempered martensite and in establishing the dependence of the parameters on the tempering temperature, and also in finding a new low-carbon carbide during the tempering of steel.

A pupil of G. V. Kurdyumov, V. Nevolodov, applied the homological-pair method, known in spectroscopy, to the development of a convenient method for the quantitative determination of residual austenite in steel.

This entire circle of works is of the utmost importance for theoretical physical metallurgy and serves as a solid basis for the development of many technological processes for the treatment of steel and non-ferrous alloys.

Another group of works (connected chiefly with the Moscow school of X-ray specialists), of theoretical significance and yielding considerable practical results, is a series of investigations into transformations of the type of decomposition of a solid solution and into the effect of residual stresses on this process. For the first time, in the work of S. T. Konobeevsky and Ya. P. Selissky (1932), the remarkable fact was discovered that a solid solution of aluminum in magnesium decomposes under the action of plastic deformation, and the idea was put forward—subsequently receiving theoretical development in the works of N. S. Gorsky and the author of the present article—of the peculiar phenomenon of uphill diffusion caused by elastic stresses. Thereafter V. P. Tarasova, applying the same method of annealing a cold-deformed metal, established the true equilibrium diagram of the copper–tin alloy, radically different from earlier conceptions. The solubility of tin in copper at low temperatures proved to be only 1% instead of the previously accepted 14.5%. This circumstance compelled a revision of other analogous phase diagrams as well (Cu—Zn, Cu—Sb, Cu—Si, etc.) and the introduction into them of equally decisive changes. The new method proposed by Soviet investigators for establishing equilibrium—annealing of a strongly cold-worked metal—fully justified itself and is now generally recognized. The influence of stresses on the $\beta \to \alpha$ transformation in tin (tin pest) was demonstrated with exceptional clarity in the works of M. M. Chertok. It was shown by him that the so-called acute tin pest is a consequence of autocatalytic acceleration of the process owing to stresses arising during transformations. In subsequent works by the author of the present article it was shown that the special quasi-eutectoid type of decomposition of a solid solution is also the result of autocatalytic acceleration of the process owing to developing stresses. Along with this, a theory was advanced of the metastable equilibrium of a disperse phase with a supersaturated solution, which has recently made it possible to provide a quantitative theory of the aging phenomena in alloys of the duralumin type.

From the theory indicated here there follows, of necessity, the existence of the phenomenon of reversion during aging, i.e. the ability of an alloy that is in the state of a highly dispersed precipitate to pass into a soft (quenched) state upon a slight increase in temperature. This phenomenon was studied in detail by D. A. Petrov (IONKh) and especially by G. Ya. Sergeev and others. Recently B. G. Livshits and L. I. Tsyprun observed the phenomenon of reversion during aging of technical iron. S. T. Kishkin successfully applied the idea of accelerating transformation processes by deformation to the explanation of the high hardness of martensite. A major role of the phenomenon of dispersion precipi-

... play in magnetic materials. Important X-ray investigations on Fe—Ni—Al alloys belong to G. V. Kurdyumov and A. P. Komar. Many works by Soviet metallurgists and magnetologists have been devoted to the study of the physical properties of alloys of this type; especially extensive investigations were carried out by B. G. Livshits and A. S. Zaimovskii with collaborators.

Closely connected with the preceding direction are investigations of residual stresses arising during deformation of a metal, and of the phenomena of recovery and recrystallization. In work on work-hardening and recovery of metals, X-ray investigations are fundamental. Making broad use of X-ray methods, a group of investigators (E. F. Bakhmetev, S. I. Gubkin, G. F. Kosolapov, and B. M. Rovinskii) over the course of several years (1929–1933) carefully studied the process of deformation in the hot and cold states and the structural changes in alloys. One of the important results of these works was the establishment of a connection between the rate of deformation and the temperature interval in which deformation may be regarded as hot.

Many works were devoted to the quantitative measurement of stresses of the second and third kind (Ya. P. Selisskii, Ya. S. Umanskii, B. M. Rovinskii, G. M. Rovenskii, V. I. Iveronova, and others). Ya. S. Umanskii established the existence of two types of return (recovery): in one (Al) the stresses of the 2nd and 3rd kind are relieved simultaneously; in the other (Cu) the stresses of the 3rd kind are more stable than the stresses of the 2nd kind. In the works of V. I. Iveronova with collaborators, devoted to the behavior of deformed alloys during annealing, it was established that alloys behave in a manner opposite to pure metals, retaining broadening of the Debye lines while there is considerable relief of stresses of the 3rd kind (change in the intensity of the lines). These latter facts are well explained on the basis of the theory of ascending diffusion, developed by the author of this article. The same theory makes it possible to find an explanation of the remarkable anomaly in the mechanical properties of Al-bronze, discovered by G. M. Rovenskii, where low-temperature annealing of a stable solid solution leads to an increase in the hardness and strength of the alloy, whereas secondary deformation after annealing, on the contrary, leads to a decrease in hardness.

Let us also note V. I. Iveronova’s X-ray investigations on the fatigue of metals, in which it was possible to observe and study structural changes long before the destruction of the metal, as well as the phenomenon she discovered of anisotropy of stresses of the 2nd kind in rails. An interesting fact concerning the appearance of texture in balls of operating bearings was reported by L. V. Altschuller and V. A. Tsukerman.

The establishment of quantitative regularities in the kinetics of recovery and recrystallization we owe chiefly to the works of M. O. Kornfeld, who conducted his investigations mainly on single crystals. The rule that the asymptote of the recovery curves depends on the degree...

deformation to which the crystal had been subjected, could quite justly have been called Kornfeld’s rule.

A great deal could be said about the works on recrystallization associated with X-ray methods, since these works were of essential importance for establishing technological regimes of heat treatment and were widely carried out at many plants possessing X-ray equipment. Among the more fundamental and systematically conducted works one should mention V. I. Iveronova’s study, as well as the study by G. S. Zhdanov and V. I. Iveronova, of the recrystallization temperatures of a number of binary alloys, which also received theoretical illumination in the light of the ideas already indicated above.

Let us turn to the group of works connected with the activity of an outstanding scientific school, deservedly enjoying worldwide recognition—the school of the late Academician N. S. Kurnakov. The doctrine created by him—physicochemical analysis of complex multicomponent mixtures of variable composition—belongs among the greatest achievements of modern chemical science. It lies at the foundation of all metallurgy, making it possible, through the study of properties, to establish the chemical nature and state of an alloy of any composition and in any temperature interval. It would be entirely fair to say that the circle of these ideas developed from the source created by the activity of the brilliant Russian metallurgist D. K. Chernov, who first established the principal features of the most important state diagram of the iron–carbon alloy.

Without having the possibility of giving a full characterization of the inexhaustible wealth of ideas and practical results of the scientific activity of N. S. Kurnakov’s school, we shall note here only two discoveries that had exceptional significance for the most recent physical metallurgy. These are, first, N. S. Kurnakov’s discovery of such intermetallic phases whose region lies outside the composition that is for them the “ideal” stoichiometric composition. These compounds, as we would now say, cannot be observed in a state of complete ordering. The second is the phenomenon, discovered by N. S. Kurnakov together with Zhemchuzhnyi and Zasedatelev (1916), of ordering of a quenched solid solution of gold–copper in the temperature interval below 400°. As is known, this discovery created an entirely new and most important field of investigation in metallophysics.

Beginning in 1930, at the Institute of Physicochemical Analysis in Leningrad, a series of investigations began on the X-ray study of alloys. One of N. V. Ageev’s first works on the structure of the Ag—Cu alloy (1930) acquaints us with a new phenomenon: the existence of two essentially different types of decomposition of a solid solution—homogeneous, in which the concentration of the solid solution in the process of precipitation decreases continuously, and heterogeneous, where the transformation occurs completely within a limited volume, and the propagation of the transformation has a “frontal” character. Considerably later (in 1943) the author of the present article gave an explana-

such a dual character of decomposition—an explanation that takes into account the magnitude of the volume changes during the transformation and the autocatalytic acceleration of the process that arises thereby.

The use of X-ray analysis for establishing and refining phase diagrams was widely practiced at the Institute of Physicochemical Analysis (subsequently the Institute of General and Inorganic Chemistry named after Academician Kurnakov). Mention should be made of the work of N. V. Ageev and D. Schoichet, who, by X-ray analysis, established a special and somewhat paradoxical course of transformations in a silver–aluminum alloy in the region of 5–10% Al. The solid solution \(\beta\) below \(610^\circ\) decomposes into a mixture of phases \(\alpha+\gamma\), which at about \(390^\circ\) again passes, by a peritectoid transformation, into the homogeneous phase \(\beta'\). Let us note, incidentally, that the first work on establishing equilibrium in the Ag—Al system also belongs to our compatriot G. I. Petrenko.

A great many works by the collaborators of the IONKh have been devoted to the X-ray study of solid solutions and to the study of the course of lattice parameters as a function of composition. Especially extensive investigations of both binary and ternary solutions have recently been carried out by V. G. Kuznetsov. These investigations apparently lead to the conclusion that solid solutions should be treated as true chemical compounds of variable composition—an idea put forward in his time by D. I. Mendeleev.

A large group of works was devoted to the X-ray study of ordering phenomena in alloys. Worthy of attention is the fact, established by N. V. Ageev and D. Schoichet (1935), that the degree of ordering in a gold–copper alloy near the 50% composition changes asymmetrically on both sides of 50% and sharply departs from the theoretically possible one. I. I. Kornilov studied the rate of transition to the ordered state as a function of composition in a solid solution. In this connection one should also recall the work of N. S. Gorsky, a pupil of I. V. Obreimov, carried out by him in Kharkov in 1930, in which the course of the tetragonal parameter of the Au—Cu compound as a function of temperature was established for the first time. Finally, special note should be made of the works of E. S. Makarov, who (partly together with N. V. Ageev) succeeded in tracing continuous structural changes in an extensive group of nickel-arsenide type phases, in establishing the remarkable transition from defective lattices to interstitial solid solutions, and, finally, in creating very general points of view on the crystallographic kinship of a number of crystal structures, which are of great value in theoretical crystal chemistry.

Despite the variety of tasks of X-ray structural analysis in problems of solid-state physics, in various fields of chemistry, metallurgy, and apparently also in biophysics, where the application of radiography, unfortunately, is still in an embryonic state, the most direct object of this method nevertheless remains determination.

atomic structure of crystals. We may say with justifiable pride that the foundations of this method were to a considerable extent laid in our country in the works of the most talented Russian scientist-crystallographer E. S. Fedorov. The greatest merit of E. S. Fedorov is the doctrine he developed of the internal symmetry of crystals—the creation of the theory of space groups. It is impossible to overestimate the significance of this mathematical theory, which made it possible to create a powerful computational apparatus that, like a kind of magic wand, reproduces the most complex crystalline structures using only a few films with spots or lines of X-ray interferences photographed on them. E. F. Bakhmetev may be regarded as a pioneer in the field of X-ray study of the structure of crystals; in the period 1933–1935, together with a number of collaborators, he carried out several studies on determining the structure of intermetallic compounds \((\mathrm{FeAl}_3, \mathrm{Mg}_3\mathrm{Ni}\), etc.). True, Bakhmetev did not succeed in bringing these works to a complete determination of the structure, but he developed highly important apparatus, which later formed the basis of many more advanced instruments for X-ray structural analysis now used by our laboratories.

More systematic work on the structure of crystals began in 1933–1939 at the Karpov Physico-Chemical Institute (G. S. Zhdanov), at the Institute of Organic Chemistry of the Academy of Sciences (A. I. Kitaigorodskii), and at the Institute of Crystallography of the Academy of Sciences (N. V. Belov).

At the Institute of Crystallography, N. V. Belov developed convenient methods for using a card file of the so-called strips for summing Patterson and Bragg series, making it possible to synthesize the periodic lattice of a crystal on the basis of X-ray-metric data. N. V. Belov’s great contribution lies in the theory of close packing of atoms that he developed. Substantially supplementing the data obtained through the application of the doctrine of symmetry, Belov’s method makes it possible to orient oneself reliably in the complex geometry of atomic arrangement and to arrive at the solution of structural problems, examples of which have been demonstrated in a number of works of the Institute of Crystallography. The objects of investigation here are various minerals.

The merit of G. S. Zhdanov, who in 1938 organized an X-ray structural laboratory at the Karpov Institute, is his extensive and systematic work on applying the X-ray method to theoretical chemistry. The lines of work here are: 1) investigation of the structures of solid compounds (complete determination of the structure of boron carbide, titanium nitride, etc.), 2) investigation of the structures of complex compounds (sodium fluoroberyllate, Will’s salt, simple and complex cyanides), 3) study of the structures of certain organic compounds. It is necessary to note the work on the complete determination of the structure of boron carbide, which has a complex and somewhat unexpected arrangement of atoms—three carbon atoms in the form of a linear chain—and especially the complete study of the structure of carborundums, where

a whole series of modifications was found, some of which differ by gigantic periods along the principal axis. This constitutes an as-yet unsolved puzzle of crystal chemistry. Also important are the works on the deformation of the valence angles of the C—N—O bond in certain organic compounds.

At the Institute of Organic Chemistry of the Academy of Sciences, A. I. Kitaigorodskii is working successfully. On the basis of the fruitful idea he advanced—the dense packing of organic molecules and the concept of the intermolecular radius—it has become easy to decipher complex organic structures, many of which were first found by Kitaigorodskii. Much is promised by Kitaigorodskii’s attempt at a theoretical development of the doctrine of symmetry for molecules of complex form.

A very large and useful body of work on the study of insertion structures was carried out and is being carried out by the group of Ya. S. Umanskii, who had been working in contact with the Hard-Alloys Combine since 1937. The structure of intermetallic compounds in W—Co and Mo—Co alloys was determined. It was established that the phase Ni$_4$W is a highly distinctive superstructure based on the face-centered lattice of nickel. Structures were also studied in a number of other alloys of variable composition, containing carbon, hydrides, nitrides, and carbides of niobium, zirconium, titanium, etc. All these works are of very substantial importance for the technology of hard alloys.

Closely adjoining this circle of works are investigations of the structure of thin films, both natural (the structure of oxides, the structure of a nitrided or cyanided surface) and artificial, obtained by sputtering or evaporation in vacuum. In all these cases, X-ray, as well as electron-diffraction, determination of structure plays a large role. Among electron-diffraction investigations one should note the first works in our Union by P. S. Tartakovskii, as well as by V. E. Lashkarev. The latter, using the peculiarity of the electron-diffraction method that a beam of electrons is scattered not by charges but by the potential field of the lattice, succeeded in localizing the position of ionized hydrogen atoms in the molecule NH$_4$Cl. S. G. Kalashnikov, working by the method of slow electrons, obtained a very clear picture of the thermal motions of surface atoms. To him belongs the ingenious idea of explaining anomalous (half-order) reflection orders as a consequence of the “echeloned” character of the surface structure. One may also note the electron-diffraction investigation of G. A. Effendiev (1939), who developed a subtle technique for obtaining alloys by the successive evaporation of two metals and subsequent diffusion. In this way various intermetallic phases were obtained and investigated. As is known, almost simultaneously S. A. Vekshinskii proposed another method for obtaining thin alloy films by simultaneous evaporation of them from two or several sources. Natural layers of oxide films were studied by V. I. Arkharov and

especially thoroughly by P. D. Dankov. The electron-diffraction study of the structure of crystals deposited from their suspension in liquids is being carried out by Z. I. Pinsker, who has considerably improved the technical design of the electron-diffraction camera. Here it is not even possible to enumerate the numerous works of Soviet electron-diffraction researchers devoted to the structures of surface films obtained by the method of electrolytic coating or by chemical-thermal treatment. They have, however, great technological significance. Of fundamental significance in the question of the possibly amorphous character of films at very small thicknesses are the investigations of A. I. Shal’nikov. Questions of the very mechanism of crystallization during the condensation of a metal on a substrate are apparently receiving a distinctive solution (condensation in the molecular beam itself near the substrate) in a recent work by M. M. Umanskii and the author of this article.

Our enumeration of experimental works on X-ray structural analysis must be supplemented by a reference to the excellent investigations of V. I. Danilov, carried out by him at the Dnepropetrovsk Physico-Technical Institute and devoted to the study of the structure of liquids and of crystallization processes. Among the facts he discovered, the most interesting is the special structure of metallic melts of eutectic composition, where it proved possible to note, as it were, the independent existence of two liquid structures. His works on the study of supercooling and crystallization of liquids are also very valuable. By careful purification of liquid metals (bismuth) from impurities, V. I. Danilov showed that there exists a quite definite magnitude of supercooling necessary for the beginning of the spontaneous nucleation of crystalline centers. In the same institute G. I. Aksenov developed methods for X-ray measurement of elastic stresses—a problem of great practical importance.

All that has been said above relates to the application of X-rays to the investigation of the structure of matter. Significant works by Soviet scientists have also been devoted to the study of the X-rays themselves, i.e. to questions of X-ray science and X-ray spectroscopy, although the volume of these works may perhaps be somewhat inferior to that of the works of the first group.

The investigation of the processes of scattering, emission, and absorption of X-rays played an enormous role in the development of modern ideas about the structure of the atom, as well as about the corpuscular-wave nature of radiation. The study of the fine structure of X-ray emission and absorption spectra of atoms in solids served as important material in the development of modern ideas about their electronic structure. All these investigations fall within the field of X-ray spectroscopy.

The earliest works of Soviet scientists in this field are associated with the names of Academician A. F. Ioffe, P. I. Lukirskii, N. I. Dobronravov, N. Ya. Selyakov, A. I. Alikhanov, M. I. Korsunskii,

V. E. Lashkarev and V. P. Linnik. They concern the study of the laws of photoelectric absorption (Ioffe, Lukirsky, Dobronravov), the phenomenon of total internal reflection by a plane surface of solids and the determination of the refractive index by this method (Alikhanov, Lashkarev, Linnik), the study of the structure of surface organic films on solid substrates (Alikhanov), and the spatial distribution and velocities of recoil electrons (Lukirsky). The result of these works was the development of experimental methods, some of which should at present be regarded as classical (Lukirsky, Alikhanov). Also among the pioneering works on the study of the so-called influence of chemical bonding on x-ray emission spectra are the works of Korsunsky and Selyakov.

The most consistent and systematic development of work on x-ray spectroscopy in our Union over the last 10–12 years has been carried out by the group of Moscow x-ray specialists headed by I. B. Borovsky. The works of Prof. Borovsky and his students posed and, to a considerable extent, solved the following two tasks of modern x-ray-spectral investigations:

1) The development and application of the method of x-ray-spectral analysis as a new method of analytical chemistry.

2) The development of methods and theoretical premises for studying the fine structure of x-ray absorption and emission spectra and for determining the energy spectrum of electrons in solid and liquid bodies.

Among the works in the first direction one may note a fundamental improvement in the methodology of x-ray-spectral chemical quantitative and qualitative analyses, connected with the use of spectrographs with bent crystals, photographic and ionization recording, and also with new techniques of analysis (semiquantitative analysis, the standard method, the method of the width of the standard line). The methodological investigations carried out led to the fact that at present x-ray-spectral chemical analysis is the leading and controlling method in quantitative determinations of a number of rare elements. Thus, quantitative analysis can be performed at element contents from 0.1–0.5% to approximately 8%, with an accuracy of 10–11%. Qualitative analysis, with a sensitivity of 0.05–0.06% for 90 elements of the periodic system, takes 1 hour. As a result of this development of the method, with its aid it has been possible to solve a number of practical problems of great significance. At present the apparatus and methods developed by the above-mentioned workers are used in all operating laboratories and are being introduced into a number of factory laboratories. It should also be noted that the use of x-ray-spectral chemical analysis was also successfully carried out in Leningrad by Protopopov, and in Kharkov by Borisov and Fogel.

Studies by Soviet scientists of the fine structure of X-ray absorption and emission spectra—i.e., measurements of the distribution of intensity by wavelengths, determinations of line width, the position of the maximum and the short-wave boundary of a line, and of fluctuations of the absorption coefficient in the principal edge and on its short-wave side—also led to a number of interesting results of general theoretical significance. Thus, it was predicted for the first time and experimentally shown that the principal \(K\)-edge of the elements of the iron group has a complex multiplet structure not only for atoms in pure metals, but also in compounds with different cation valencies, as well as in binary alloys (Borovskii, Dekhtiar, Sorokin). In these works it was shown that the number of electrons in the conduction band, while the structures of the alloys are preserved, changes depending on concentration. M. A. Blokhin studied and gave a rational explanation for the numerous satellites of the \(\beta\)-spectra (Cr).

The study of the “far” or Kronig fine structure was the subject of the work of B. M. Levitskii (GIFTI), who set himself the task of investigating the absorption spectra of Zn in single crystals. According to Kronig’s theory, anisotropy of the fine structure was to be expected depending on the direction of the absorbed beam. The theory of the Soviet physicist A. I. Kostarev, on the contrary, gave no grounds for expecting such anisotropy. Levitskii obtained results confirming Kostarev’s theory.

A. I. Krasnikov was an innovator in the field of sources for obtaining X-ray spectra. By constructing a tube in which a large surface serves as the anode, Krasnikov achieved a high total radiation power. Irradiation of an object with this tube produces strong X-ray fluorescence. By increasing the distances in reflection from a flat crystal it is possible to obtain greater dispersion and to observe the displacement of the emission lines of elements in alloys as compared with the spectra of pure metals.

With this necessarily brief and incomplete enumeration of experimental works in the field of structural X-rayography and X-ray spectroscopy we must limit ourselves.

From the survey presented it can be seen that, in solving the problems posed by life itself and by the development of industry at gigantic rates in the Soviet country, Soviet X-ray specialists based their work on integral theoretical conceptions; theoretical thought developed together with the development of experimental research. Lacking the possibility of setting forth in any detail even the principal works on the theory of metals and alloys, I shall have to indicate only some of them. The concepts of the nature of the metallic state created by Ya. I. Frenkel exerted an enormous stimulating influence on experimental work in the physics of metals. Of no less importance were also the ideas on diffusion, the nature of plastic deformation, and other matters of L. D. Lan-

Dau (partly together with E. M. Lifshitz) also deserves credit for developing many ideas fundamental to the science of the solid state (the theory of phase transitions, questions of variable structure); far from all of them have yet been used by experimental physics.

The problem of the structure of intermetallic phases, both of singular and of variable composition, has figured repeatedly in the works of Soviet physicists and chemists. Apparently, the works explaining, on the basis of the electronic theory of alloys, the course of solubility-limit boundaries in copper alloys (Konobeevskii, 1936–1943) and the regularities in the structure of defective lattices (1938–1944) may be acknowledged as at least a modest but definite success of the crystal-chemical theory of alloys. Apparently, attempts (Konobeevskii, 1946) to interpret X-ray patterns as the energy “spectrum” of a crystal, or, in other words, to obtain information about the internal energy of a crystal using exclusively experimental X-ray-diffraction data, promise the possibility of advancing still further in the crystal-chemical theory. Mention should be made of the works on the theory of the solid state by A. G. Samoilovich (Gorky). A new variant of the statistical theory of the solid state, developed by N. N. Bogolyubov and his pupils, deserves great attention and study. These ideas (the application of a self-consistent molecular field) were first expressed by A. A. Vlasov. Considerable clarity has also been introduced into questions of metastable equilibrium in solid heterogeneous systems thanks to the works of Soviet scientists (Konobeevskii, Kurdiumov, Mirkin, Pines, Dankov, and others). The theoretical explanation of the phenomena of reversion during aging of Al alloys should be regarded as an evident success of the theory of disperse equilibrium developed in our Union. The connection, proposed on the basis of purely theoretical considerations, between internal stresses and the phenomenon of directed diffusion in alloys (Konobeevskii, Gorsky, Iveronova) was confirmed in the works of G. M. Rovenskii and V. I. Iveronova. This entirely new idea promises to lead to important practical results. Mathematical analysis is beginning to be introduced into the practice of metal treatment; rigorous theory is gradually taking the place of the vague and unclearly formulated notions of metallurgists that were widespread only recently.

It remains now only briefly to touch upon the question of the “material base” of our X-rayography, and the work of Soviet scientists in designing new types of X-ray apparatus. The principal apparatus is the X-ray tube. As early as 1920, when soft X-ray tubes with beam emission through transparent glass had not yet become common, K. V. Vasiliev, an assistant of Yu. V. Vul’f, designed and built a glass tube with beam emission through aluminum foil. Many still remember this “Vasiliev tube,” which in its time rendered no small service to X-rayography. K. V. Vasiliev also built the first chambers in our Union for investigations by the Laue and Debye methods. Thanks to the works

first of all the Kiev group of physicists headed by S. D. Gertsri­ksen, who invented “getan” glass, and then the laboratory of the “Svetlana” plant, our industry obtained glass transparent to soft X-rays, and around 1935 the “Svetlana” plant began to produce sealed X-ray tubes with anodes of copper, iron, cobalt, etc. However, alongside this, the need for demountable tubes did not disappear. The Leningrad Physico-Technical Institute developed several types of a controllable tube of the “Hadding” type, which, despite its shortcomings, for a long time supplied our structural laboratories.

Of special importance is the increase in the power of X-ray tubes. The pioneer in the construction of powerful X-ray tubes with a rotating anode was P. I. Strelnikov, who first proposed and realized this idea long before anode rotation began to be used abroad.

Several years before the war S. V. Sergeev (VIAM) realized another design, based on an ingenious idea: the rotating anode shield constitutes the nozzle of a diffusion mercury pump, whereby the problem of sealing between high vacuum and fore-vacuum is solved by itself. In the X-ray Laboratory of the Scientific Research Institute of Physics of Moscow State University, a tube with a rotating anode is at present being designed, based on an entirely new principle of hydrodynamic sealing, eliminating altogether any sort of sealing bearings.

Great successes in the construction of units (a tube with cameras) have been achieved by Tsukerman and Altschuller. The tube of the latter, improved by the Institute of Physics of Moscow State University (M. M. Umanskii), is an example of express apparatus. Making broad use of the method of focusing radiation, in this apparatus it is possible to reduce the exposure process for obtaining Debye and Bohlini photographs to 1–5 minutes. A very interesting idea for obtaining radiation with a minimum of continuous spectrum by emitting rays in a direction coinciding with the path of the cathode beam has been realized by Blokhin. A number of original designs of cameras for structural analysis have been proposed. The texture camera of Zhdanov, and the Heller camera, have already been mentioned above; many types of cameras have been proposed and realized at the Institute of Physics of Moscow State University; Bakhmetev has created convenient and precise rotation and oscillation cameras, improved at the Scientific Research Institute of Physics and now being manufactured there in small series. Finally, recently in the same place Yu. A. Bagaryatskii designed monochromators with a bent crystal for X-ray structural analysis.

The ionization method has not yet found wide application in our country. Nevertheless, one may point to substantial successes in the use of counters and of a proportional amplifier in X-ray spectroscopy (Borovskii), as well as for X-ray radiography (Tsukerman—Altschuller). A very advanced ionization spectrograph with counters was constructed by B. M. Levitskii in the X-ray la-

laboratory of the All-Union Institute of Aviation Materials. Especially noteworthy is the method of pulsed X-ray cinematography, developed to a very high degree by the work of Zuckerman, which is finding broad application for shadow X-ray photography of rapidly occurring processes. I shall not describe it in detail, since this lies beyond the scope of the present article. A somewhat different variant of the same technique has been developed in the laboratory of electrical discharges in gases of the Institute of Physics of Moscow State University (Spivak, Reikhrudel).

Everything said above characterizes the great development of work in X-ray science and X-ray structural analysis, and the considerable successes achieved by our Soviet physicists in this field over the thirty years of the existence of the Soviet state. Yet immeasurably greater tasks still lie ahead. One may be confident that our scientists’ awareness of their responsibility to the country and to Soviet science will multiply and deepen these achievements in the very near future.