High purity substances – prototypes of elements of Periodic Table

Introduction

In 2019, the United Nations marked the 150^th anniversary of D.I. Mendeleev's creation of the Periodic Table of Elements. A great many scientific studies are devoted to this table, new elements are discovered. The Periodic Table has become a basic and integral component of scientific and technological progress in chemistry and related sciences.

The Periodic Table contains not only all the known chemical elements that make up the world, but also reflects the possibility of replenishing it with new elements. In principle, the chemical elements of Periodic Table are virtual, idealized substances until they are obtained in their material embodiment. Obtaining high purity simple substances, which can be called prototypes of the elements of Periodic Table, has been achieved for many centuries by developing methods of chemical separation, synthesis and analysis [1], [2]. D.I. Mendeleev managed to form his Periodic Table without the benefit of established definitions of the many chemical elements we know today were discovered and investigated. Table 1 contains information about the elements known to mankind in various historical eras [3].

The creation of Periodic Table delivered a powerful impetus, and it became an essential tool in the development of fundamental and applied chemistry, chemical technology, analytical chemistry, and material sciences. It drove the desire and need to obtain the elements of the Periodic Table in their material form in order to establish their empirical underlying properties. It was clear that real-world chemical elements contained impurities and surface contaminants veiling and distorting these properties, and in order to get closer to the idealized elements of the Periodic Table, it was necessary to obtain extremely pure samples. This immediately raised the question of the possibility of controlling such purity. In the 1960s, P.L. Kapitsa and a member of the Academy of Sciences of the USSR, N.E. Alekseevsky, suggested using the value of residual resistance at helium temperatures approaching absolute zero in degrees Kelvin as a general criterion for the purity of a substance [4], [5]. This parameter has been used for a long time for an integral assessment of the purity of a substance. But such an approach eventually reaches limitations, as a significant contribution to the value of residual resistance is made not only by the chemical composition, but also by the structure, which complicates accurate measurement.

In the early 1970s, academician G.G. Devyatykh thought to create a collection of high purity substances, inclusive of all elements of the Periodic Table. He proposed to bring together record-breaking pure substances – prototypes of elements of the Periodic Table, to characterize them in detail by the content of impurities and to study their properties [6]. He understood that the achieved purity at any given time is a temporary characteristic and will constantly increase with the development of scientific and technological progress, and with it, the level of knowledge of the surrounding world will increase. In 1974, in Nizhny Novgorod, at the Institute of Chemistry of the USSR Academy of Sciences, now known as the G.G. Devyatykh Institute of Chemistry of High Purity Substances of the Russian Academy of Sciences (RAS), a unique Exhibition-collection of high purity substances was created, the only one of its kind in the world. To control the chemical purity of the samples in the Exhibition-collection, special analytical chemistry methods were used to determine the full impurity composition.

This article discusses the current state of the Exhibition-collection of especially pure substances as a material embodiment of D.I. Mendeleev's Periodic Table. In addition, three main components of the science of high purity substances are discussed – chemistry, technology and analytics. Particular attention is paid to the problems of modern analytical chemistry of high purity substances and the use of the latter in metrology of chemical analysis as the basic standards of comparison – “individual moles”.

Exhibition-collection of high purity substances: current state and main areas of work

The Exhibition-collection of high purity substances was created by a decision of the Presidium of the USSR Academy of Sciences on April 4, 1974. The leading organizations of the country working in the field of chemistry of high purity substances took part in the creation of samples for the Exhibition. To date, the Exhibition presents samples of 76 chemical elements in the form of pure substances, obtained from 145 research and industrial enterprises in Russia and the Commonwealth of Independent States (CIS) countries. The Exhibition reflects the current state of the art of purity of substances in Russia and its development trajectory over time. A detailed description of the samples in the Exhibition is given in the manuscript by G.G. Devyatykh, Yu.A. Karpov and L.I. Osipova “Exhibition-collection of high purity substances” [6].

After 2000, the Exhibition-collection received more than 100 samples of simple solid and volatile substances, molecular solids and materials from more than 30 institutes and research and production organizations. Samples of high purity isotope-enriched substances are presented for the first time; a number of samples of simple substances arrived in the form of nanopowders and nanostructured materials; the molecular compounds section was replenished with samples of previously absent oxides and halides as well as new types of glasses.

The relevance of the information presented in the databases of the Exhibition is maintained by regular monitoring of the purity level of high purity substances and materials, the acquisition and metrological certification of new samples, the development of methods for describing the total impurity composition and the study of the laws underlying the Periodic Table and the derived properties of various classes of substances. To adequately reflect the latest purity achievement capabilities, it is necessary to continuously update and replenish the Exhibition with new samples with the highest possible purity levels and to formulate and populate new sections (isotope-enriched substances, oxides, glasses, optical materials and precursors).

The Exhibition-collection and its ongoing work allows us to observe the situation in the world of the latest nomenclature and quality of high purity industrial products and high purity substances obtained in research organizations. Information on the impurity composition, the purity level of classes of substances with similar properties and temporal dynamics is necessary for manufacturers and developers to formulate an objective assessment of development trends for a number of materials science problems. The Exhibition enables researchers and engineers to make informed forecasts and support the development of new functional materials and devices.

Figure 1 shows the achieved world level of purity of simple substances and samples of the Exhibition, which demonstrates that the level of purity is currently comparable with global purity gold standards.

Fig. 1:

The achieved world level of purity of simple substances and samples of the Exhibition at the end of the 20^th century [6]. Notes. In cells: first line: element second line: achieved world level of purity of simple substances third line (marked in red): purity level of samples of the Exhibition-collection.

Values ​​are given in N – i.e., the number of nines indicating the content of the main substance, which reflects the level of purity for metals. For example, a sample containing 99.99% of the basic substance has a purity of four nines or 4 N. All simple substances (except fluorine) in the catalogs of companies and institutions outside of Russia have a purity level in the range of 5 N–8 N. In the purest state, elements of groups 12–16, and 18 were obtained. This picture has not changed much in the past decade. Thus, the pace of increasing purity of simple substances in recent years has been quite slow. We can consider the possibility of having reached a certain plateau: apparently, the costs of increasing the level of purity, maintaining this purity and increasing the sensitivity of the analysis are large and economically inexpedient.

From Fig. 1, it can also be seen that the purity level of 11 elements (Y, Nb, W, Fe, Co, Ni, Ag, Ge, Sn, Bi, and S) – the best examples of the Exhibition-collection, still surpasses the modern level abroad, even though some Exhibition designs arrived in the 1970–80s [6]. In order to correctly compare industrial brands and samples in the Exhibition, for the latter, the level of purity is indicated only for metal impurities. Moreover, the list of determined impurities for Exhibition samples can include a significantly larger number of them than the limited set of 10–25 impurities from catalogs. This enables a more reliable assessment of the purity level of the Exhibition's samples.

In general, 41 elements out of 76 presented at the Exhibition now correspond to the achieved world level or are close to it.

Though we can say that over the past few years, we have not noticed a major increase in the world level of purity, there have been great changes since the end of the 20^th century. Comparison against the state of the art at the end of the 20^th century shows that the level of purity achieved in the next 15–20 years has increased in most elements (Fig. 2) [1], [6], [7]. At the end of the 20^th century, most samples of simple substances in the Exhibition-collection matched the world-record level of purity or held it, and now only about half achieve that level. To clarify the situation, it is necessary to examine the purest samples recently produced in Russia and abroad. This is especially true for REE and elements of groups 12–16 obtained in the purest state.

Fig. 2:

A histogram of the distribution of elements by the maximum achieved level of purity: blue bars, at the end of the 20^th century; red bars, currently [1], [6], [7]. Note: column 5N contains values from 4N5 to 5N4, etc.

The main directions of the Exhibition, as a scientific project, are:

• Collection formation: collection and metrological certification of samples of high purity substances of various classes (simple substances, including isotope-enriched, volatile and molecular compounds) as well as high purity materials;

• Monitoring data on scientific developments, manufacturing organizations, the range and quality of high purity substances in our country and abroad;

• Development of methods for predicting the total impurity composition from incomplete analysis data for various classes of high purity substances;

• Identification of patterns of formation of the impurity composition of high purity substances and its effect on the properties of the substance.

These directions naturally follow from the fact that the Exhibition-collection is a material manifestation of the Periodic Table, and the samples of elements collected in it are material table artifacts. The implementation of these areas, especially the latter two, is largely determined by the currently existing capabilities of the applied technology and analytical control methods.

Consideration of the impurity composition of the samples of the Exhibition-collection indicates the following patterns. The impurities contained in them can be divided into four groups.

The first usually includes interstitial impurities or the so-called “gas-forming impurities” – oxygen, hydrogen, nitrogen and carbon. These elements are widespread in the environment, have a strong chemical affinity for most elements of Periodic Table, are difficult to remove from the substance to be purified, and it is easy to contaminate the pure material already obtained. Therefore, gas-forming impurities are contained in high purity substances in abnormally high concentrations and most often limit the purity of the substance as a whole.

The second group is the impurities of the most common elements, such as silicon, aluminum, iron, calcium, magnesium, sodium, potassium, etc. The impurities of these elements are sometimes called “household” or “vulgar”. They enter the pure substance from as raw materials, reagents, from the walls of the apparatus and from the air.

The third group consists of chemical analogs of the basis (“matrix”) of a substance. Examples include alkaline elements for sodium, alkaline Earth for calcium and rare earth for lanthanum.

The fourth group includes the remaining impurity elements, the content of which depends on the specific object of study.

The achieved purity of substances increases over the years, but progress in this area depends not only on improving technology and analysis methods, but to a large extent on pollution from the environment, equipment, reagents, storage methods and economics. The costs of deep cleaning and analysis of substances increase exponentially with increasing purity. As a compromise, cleaning and analysis processes often aim to increase not the overall purity of the total composition (this purity is called “academic”), but only target purity as defined by individual technologically important impurities.

The main directions in developing chemistry, technology and analytics of high purity substances

The current state of chemistry and technology of high purity substances is described in detail in a collective monograph [1].

The main components of the chemistry of high purity substances include:

• Elementary separation processes;

• Theory of processes of deep purification of substances;

• Production of substances with extremely low impurity content;

• Testing of the properties of high purity substances;

• Determination of impurities in high purity substances and their effect on properties.

Materials based on high purity substances have various functional purposes. Their intended use is possible if the content of limited impurities is below a certain, often threshold level. Material science based on high purity substances includes the following main sections:

• Identification of unique properties, the use of which enables the creation of new functional materials;

• Formulating requirements for the composition, structure and purity of the new material;

• Scientific basis and methods for producing materials based on high purity substances;

• Examination of the technical and operational characteristics of materials.

In turn, the technology of high purity substances and materials is developing in the following areas:

• Theory of separation apparatuses;

• Apparatus design of the processes of separation of mixtures and deep purification of substances;

• Rational schemes for producing high pure substances and materials based on them;

• Combination of technologies in a chain: substance-material-product.

Problems of modern analytical chemistry of high purity substances

Usually distinguish between chemical, isotopic and phase purity [1], [8]. The chemical purity of an element is characterized by the magnitude of presence of atoms of other elements – impurities in the sample under study.

Similarly, isotopic purity is determined by the isotopic composition of isotopic enriched elements.

The preparation and study of the properties of monoisotopic substances represent a new step in the development of the chemistry of high pure substances. At the same time, the concept of an individual substance, a key one in chemistry, is deepened: not only atoms of other chemical elements, but also atoms of the base element with a mass different from the mass of the main isotope are considered as impurities. An example of modern advances in the field of monoisotopic substances is 99.999% enriched 28Si material, recently developed by Russian scientists for a revised estimate of the Avogadro constant [9].

The intention of this project is also to show the availability of 28Si single crystals as a guarantee for the future realization of the redefined kilogram.

Phase purity characterizes a continuous medium by its structural homogeneity. Similarly, impurities here are micro- and nano-sized particles having the same (or very close) chemical composition, but differing in the spatial arrangement of atoms (packages) in the particle. The degree of phase purity is determined by the fraction of the volume of the sample occupied by phase inclusions.

In the study of the properties of high purity substances, the most important components are:

• Study of the properties of substances with a record degree of purity;

• Determination of the properties of substances with a precisely characterized composition of impurities;

• Establishment of the mechanism and boundaries of the influence of impurities on the properties of substances;

• Comparison of the contributions of impurity, structural, isotopic and dimensional factors to the properties of high purity substances.

In the global problem of obtaining, researching and using high purity substances, chemical analysis techniques and treatment of purity data play a central role. The very term “high purity” implies that this purity is established and evaluated using analytical chemistry methods. This is an extreme task, constantly requiring the creation of new highly sensitive methods of analysis and methodological approaches to their implementation. As noted earlier, the main criterion for the chemical purity of a substance is its impurity composition. Attempts to evaluate the purity of a substance by direct high-precision determination of its matrix components did not bring success because the highest accuracy in determining large contents is hundredths of a percent, which is clearly not enough to control the achieved level of purity. The requirements for substance purity are constantly growing. For reference, if in the 1940s, the content of regulated impurities was 10⁻²–10⁻⁴%, in the 1950s – 10⁻⁴–10⁻⁵%, and in the 1970s 10⁻⁶–10⁻⁷%, then at present, for a number of semiconductor materials, the content of impurities is allowed to be no more than 10⁻⁷–10⁻⁸% (and in some cases significantly less, up to 10⁻¹¹%). The fulfillment of these requirements necessitated solving a number of major fundamental problems. The task at hand is the detection of ultra-small impurity contents against the background of the main substance and environmental components. Such a task resembles the purpose of radar – the detection of a weak signal against a background of strong noise. Under these conditions, it is easy to mistake a false signal for a useful one or not to see a useful signal at all.

In analytical chemistry, two main approaches were proposed to solve the problem – the use of physical methods of analysis, potentially suitable for measuring small signals, and special metrological approaches. The basis of physical methods of analysis is the theory of an analytical signal that is strictly specific to each chemical element. The essence of the analysis method is that the sample under study is externally exposed, which causes an appropriate response called the analytical signal. This signal is separated from possible interference and then subjected to identification and measurement. The methods of exposure can be high temperatures, various types of radiation in a wide range of frequencies (optical, X-ray, etc.), bombardment with elementary particles, magnetic field and many others.

Different methods of exposure affect various parts of the atomic structure – outer electron shells (atomic spectral analysis), middle electron shells (X-ray analysis), lower electron shells (mass spectrometry), and finally, the nucleus (nuclear physical methods of analysis). Academician I.P. Alimarin described this trend as a path “from the periphery to the center of the atom” [10].

Simultaneously with the choice of the analysis method, a metrological problem arises of guaranteeing a real, not false detection of the analytical signal against interference from the matrix, reagents, laboratory glassware, reagents and noise of the equipment itself.

The most common solution to this problem is to estimate the detection limit proposed by Kaiser in 1955 [11]. The detection limit is a general criterion for detecting a signal in the presence of interference, based on statistical concepts of the probability of a “false alarm” (that is, it is decided that the signal is present when it is not really there) and the probability of “skipping the signal” (it is decided that there is no signal when in reality it is present). Signs of the actual presence of impurities in the sample are only results that exceed the fluctuations in the correction of the blank experiment by 3S (triple standard deviation, the Kaiser criterion). This methodology is extended not only to pure substances, but also to other areas of science, where ultra-low concentrations are priority characteristics (toxicology, ecology, pharmacy, geology, forensics, electronics, optics, nuclear technology, nanomaterials).

Multi-element sensitivity is also necessary in metrology high-purity substances, since purity is characterized only by information on the total impurity composition.

Potentially suitable for the analysis of high purity substances were: mass spectrometric methods with various ion sources; atomic spectral methods; nuclear-physical (primarily activation) methods; methods combining those listed with the concentration of defined impurities. Information about these methods, their capabilities and limitations is summarized in a collective monograph [12].

The analytical chemistry of high purity substances and chemical metrology are connected by one fundamental problem – in the International System of Units, there are seven basic physical units and only one of them does not have its own material standard – this is the amount of the “mole” substance. Namely, this unit is fundamental in analytical measurements. And here, as a compromise, high purity substances come to the rescue, each of them we can consider not only as a prototype of the element of Periodic System, but also as an individual standard against which the measured impurity concentration is compared. Thus, the most important metrological requirement is satisfied – mandatory reference to a standard or metrological traceability [13], [14], [15], [16], [17], [18]. The metrological basis of this binding is the confirmed purity of a high purity substance or material, which is most effectively realized through an inter-method comparative experiment using analytical methods with high sensitivity [14], [19], [20], [21]. Tables 2 and 3 show examples of the inter-method and inter-laboratory experiment, the result of which were standard samples of high purity substances, certified both in total chemical purity and in impurity composition.

Cognition of the material nature of matter is a dialectical process. Therefore, despite significant advances in analytical chemistry of high purity substances, new, even more complex problems have emerged in this area. There are many of them, but we will note two. It is known that most elements of Periodic Table have several isotopes. Despite their apparent identity, the properties of isotopes differ and, therefore, the need for isotope analysis arises, the main role in the implementation of which is played by mass spectrometry.

A relatively new analytical problem is surface analysis, layer-by-layer analysis, substantial analysis, analysis of micro- and nano-inclusions and also analysis of nano-objects. These types of analysis are a complex fundamental and applied problem. To date, more than 70 methods have been proposed for these purposes, and complex and unique devices have been created [13].

Conclusion

Thus, a material analog of D.I. Mendeleev's Periodic Table was created in the form of a collection of high purity substances – prototypes of chemical elements, which are the result of a complex of studies in the field of chemistry, technology and analytics of high purity substances.

Further large-scale studies are underway to establish new criteria for the purity of matter and their achievement, which represent a continuous process of striving for the ideal of the chemical elements of the Periodic Table.