Physicists ‘undiscovered’ technetium carbide
An international team of scientists led by Artem Oganov, Head of Computational Materials Discovery Lab at MIPT, has proven that technetium carbide does not exist — it was pure technetium that was mistakenly considered as such. This is important from the view point of chemistry of transition metal carbides which are in many ways considered as promising substances. The article was published in RSC Advances.
Transition metals are elements whose electrons responsible for forming chemical bonds are located in a specific position within the atom: they sit on d-orbitals. The list of such metals also includes well-defined iron and copper, as well as radioactive technetium, which the scientists managed to synthesize only in the mid-20th century using accelerators, and later extracted from radioactive waste. Compounds of transition metals with carbon are called carbides, they are usually hard, heat-resistant substances, with a varying ratio of carbon and metal content: there are, for example, a chromium carbide CrC2 and a chromium carbide Cr23C6. An important question is what carbides can be synthesized in principle, which intrigues not just the theorists, but also engineering professionals and chemical technology specialists. While engineers strive for strong and heat resistant coatings for cutting tools, the chemists are attracted by carbides of transition metals for their ability to act as chemical catalysts similar to expensive platinum plates.
There is no universal and simple way to predict the existence of certain chemical compounds to date, this is why the substance thought to be technetium carbide turned out to be a controversial one: some researchers claimed that they managed to synthesize it, others doubted the correctness of the published data. Using an available USPEX algorithm, a group of scientists headed by Artem Oganov (Professor of Skoltech and the State University of New York, and Head of Lab at MIPT, Professor of the Russian Academy of Sciences) including Dr.Qinggao Wang (MIPT and Anyang Normal University, Anyang, People's Republic of China) have modeled a number of transition metal carbides and convincingly demonstrated that carbide technetium cannot be obtained.
What was done and how
To find out whether low-carbon carbides (containing much fewer carbon atoms then metal atoms) are stable, the authors succeeded in calculating the two key parameters: the energy of metal atoms' mutual interconnection (ECoh) and the energy for introducing carbon into a transition metal (EC-dis) — that is, the energy required to insert carbon in the crystal lattice. Whenever the EC-dis value is negative (which means that carbon insertion is favorable), carbon atoms occupy the octahedral voids (internodal space) in the metal lattice. In such metals as ruthenium or osmium both values are too great, and these metals are too inert to form more or less stable compounds: they cannot form carbides in principle.
To assess the stability of high-carbon compounds, the authors have calculated the energy required to form monocarbide (ETMC). Some of ETMC values were negative, meaning that the formation of such monocarbides was energetically favorable, and they must be stable when actually synthesized. Among the metals that can be successfully fused with carbon are titanium, vanadium, zirconium, niobium, hafnium and tantalum. For them, the monocarbide formation energy and the carbon insertion energy are both negative, i.e., these processes are energetically favorable, which means monocarbides of these metals exist and are stable.
Iron, chromium, magnesium and technetium belong to the Green Group, having positive formation energy for FeC, CrC, MnC and TcC, therefore, these monocarbides are unstable. In addition, the energy of carbon insertion is also greater than zero, therefore having it in the lattice is energy-inefficient: it turns out that the previously "discovered" technetium monocarbide stumbles at the fundamental laws of nature, therefore one can only synthesize Tc10C , Tc8C and Tc6C. This is the outcome of USPEX simulation algorithm, and it is perfectly consistent with the findings of researchers who have actually obtained these compounds.
Physicists have also been able to explain the data that was previously interpreted in favor of technetium monocarbide. Previously, the key evidence was represented by a radiograph showing two indicative peaks.
The X-ray Phase Identification Method is based on the fact that different substances have different interplanar spacing (atom balls of various substances have different diameters, and hence the thickness of layers that they form). Therefore, every substance produces a unique line pattern on a radiograph. By analyzing the location and intensity of the lines it is possible to draw a conclusion about how much of a certain substance is contained in the sample.
However, when they modeled the x-ray scattering process in pure technetium, scientists saw a very similar picture: therefore, the previous group might have mistakenly assumed the pure element's trace for that of technetium carbide. Not only "undiscovery" of the disputed compound allows to answer the question about an exotic substance, but it also systematizes our knowledge about transition metal carbide prospects in general.
"Chemistry of transition metal carbides is controversial — there may be different articles on the same material arguing — some 'for', and others 'against' — the possibility of its existence. In this paper we added a modicum of clarity as to the causes of the formation of these compounds, and created a foundation for future research and quest for new carbides useful in practical applications. Besides, sometimes an "undiscovery" of a substance such as ??? at the right moment can help save time and efforts of contemporary and future researchers in the field," Oleg Feya, the study co-author and a Computational Materials Discovery Lab fellow, commented.