An international team of scientists led by Artem Oganov, Head of Computational Materials Discovery at MIPT, has proven that technetium carbide does not exist — what previous researchers had obtained was pure technetium that was mistakenly considered as carbide. This is important from the view point of chemistry of transition metal carbides which for many applications are considered as promising substances. The article was published in RSC Advances.
Transition metals are elements with chemically active electrons on d-orbitals. The list of such metals includes heavily used iron and copper, as well as radioactive technetium, which scientists managed to synthesize only in the mid-20th century using particle accelerators, and later extracted from radioactive waste. Compounds of transition metals with carbon (carbides) are usually hard refractory 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 which carbides can be synthesized in principle, and this intrigues not just the theorists, but also engineers and chemical technologists. 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 catalysts similar to expensive platinum plates.
Technetium carbide proved to be an elusive one: some researchers claimed that they managed to synthesize it, others doubted the correctness of the published data. Using their USPEX algorithm for compound prediction, a group of scientists led 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 (Anyang Normal University, 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 than metal atoms) are stable, the authors calculated two key parameters: the energy of metal atoms’ bonding (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. When EC-dis value is negative, carbon insertion into the octahedral voids of the metal lattice is favorable. In such metals as ruthenium or osmium both values are very large, reflecting that these metals are quite inert: 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 carbides is energetically favorable. Among the metals that form such carbides 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 have positive formation energies for FeC, CrC, MnC and TcC, and therefore, these monocarbides are unstable. Direct calculations using evolutionary algorithm USPEX show that only low-carbon technetium carbides can exist (Tc10C , Tc8C and Tc6C), and purported TcC is impossible.
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 powder X-ray diffraction patterns with two characteristic 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 diffraction pattern. 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, and even better matching experimental data. This proves that previous researchers have mistaken the high-temperature cubic phase of the pure element for its 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 ÒñÑ 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.