Transition metal silicides, a distinctive group of semiconducting materials that contain silicon, exhibit excellent oxidation resistance, low corrosion rates and high temperature stability, which make them a potential material for a range of future developments in electronic devices.
Despite their significance to modern technology, however, important aspects of the chemical bonding between their transition metal atoms and silicon are yet to be properly understood. One of the most significant, but poorly known, properties is the strength of these chemical bonds - the thermochemical bond dissociation energy.
Funded by the National Science Foundation, Researchers from the University of Utah have studied this property, and in this week’s The Journal of Chemical Physics, from AIP Publishing, they publish their crucial findings for several specific compounds. These include precise values of the bond dissociation energies of the class four and five transition metal silicide molecules: ZrSi, HfSi, TiSi, NbSi, VSi, and TaSi.
“The team measured the energy at which the diatomic silicides fall apart more quickly than they can be ionized by absorption of a second photon. This amount of energy is called the predissociation threshold. It provides an upper limit to the bond dissociation energy. However, the researchers have found that for molecules with certain electron configurations, if the molecule is cold, then the observation of a sharp predissociation threshold provides an accurate value of the thermochemical bond dissociation energy, and not simply an upper limit.”
“What I’m so pleased about with this new technique that we’ve developed is that it’s not just applicable to a small set of molecules,” said Michael Morse, one of the Authors. “It’s based on the fact that these small transition metal molecules have a density of electronic states that increases very rapidly as you get close to the dissociation limit, and that’s key in causing the molecule to fall apart as soon as you get above that limit […] The peculiarities of the transition metals make the method broadly applicable to that entire class of molecules, which are quite difficult to investigate by other means.”
This sharp threshold observation in a thick vibronic spectrum offers a new and very effective means of approximating the bond dissociation energy for transition metals bonded to other p-block elements. According to the team, the reservations regarding using this new technique are a lot smaller than those observed with former approaches.
Together with measuring the bond dissociation values for these molecules, the Researchers were also able to apply the predissociation thresholds to establish other essential values for specific molecules using thermochemical cycles, specifically enthalpies of formation and ionization energies.
The data obtained can be used by Chemists to create more accurate computational techniques regarding transition metal chemical bonding, together with improving researcher’s understanding of these bonds.
Quantum chemists are trying to develop new, efficient and accurate means of calculating these systems, and they’ve been quite successful with main group systems, and especially organic compounds. But, the transition metals are much more difficult because there are so many more ways the electrons can be arranged. Another problem is that in the past, there hasn’t been as much highly accurate data available that can be used to compare theory and experiment. Without accurate data, it’s hard to tell how good a computational method may be.
Michael Morse, one of the Authors
Going forward, the team will work with other diatomic molecules comprising transition metals. Actually, they already have results for the bond dissociation energies of ZrC, HfC, TiC, VC, TaC, WS, WC, WSi, NbC, WSe, and WCl that are getting readied for publication. By analyzing series of chemically related molecules, like these investigations of the metal-carbon and tungsten-halogen molecules, the team hopes to develop a wide picture of chemical bonding in the transition metal molecules.
There’s a big advantage that comes from this sort of wide-ranging, systematic study. It allows us to develop what I like to call ‘chemical intuition’ about chemical bonds.
Michael Morse, one of the Authors