Periodic Trends in the Properties of Heavy Elements

Work with Peter Schwerdtfeger, Odile Smits, Paul Jerabek, and Sarah Löffelsender

Ever since the periodicity of the properties of the elements was recognized by Dimitrij Mendeleev and Lothar Meyer, inspiring them to craft version 1.0 of the Periodic Table of Elements (or rather an incomplete early alpha), the exploration of these trends defines chemistry as a scientific discipline. More recently, inspired by the successful synthesis and experimental investigations of super-heavy elements (SHE) 112-118, the continuation of periodic trends in Groups 12-18 has moved into the focus. These Main Group SHEs are of particular interest since the prevalence of strong relativistic effects gives rise to curiosities like the inert and thus noble-gas-like elements copernicium (Cn, Z=112) and flerovium (Fl, Z=114) as suggested by K. Pitzer in 1975. In contrast, the actual super-heavy noble-gas oganesson (Og, Z=118) has been speculated to be a solid semiconductor.

During my Postdoc with Peter Schwerdtfeger, I devised and implemented an efficient protocol for modeling bulk super-heavy elements to test the hypotheses mentioned above through first-principles calculations. This methodology consists of a set of (non-)relativistic projector augmented wave (PAW) pseudopotentials for the main-group SHEs and -- to account for the high polarizability and strong London-dispersion of the SHEs which are poorly described with canonical DFT -- atomic parameters for the DFT-D3 dispersion correction for these elements. Moreover, I composed an incremental free-energy-based approach to predict melting and boiling points from first-principles DFT simulations.

During the following years, I have used this methodology in several projects exploring

• the aggregate state and electronic nature of copernicium

• the aggregate state and electronic nature of oganesson

• the origin and continuation of periodic trends in the phase transitions of Group 12 (and Cn)

• the origin and continuation of periodic trends in the phase transitions of Group 11

Most notably, these projects have revealed the hypothetical melting and boiling points of oganesson (Og) with 330±11 K and 453±3 K, meaning this noble gas is a noble solid. See also the article “Oganesson: A Noble Gas Element That Is Neither Noble Nor a Gas”. Moreover, the large difference between the melting and boiling points suggests Og's rather “ignoble” nature, which has been confirmed by calculations of its electronic band gap with self-consistent GW theory.

Concerning Group 12 (Zn, Cd, Hg, Cn), we demonstrated that the decrease of the melting and boiling point of these elements is exclusively driven by relativistic effects. Accordingly, in the non-relativistic limit, the melting and boiling points of the Group 12 elements are virtually identical to that of Zn (all lie within 1230±30 K), as illustrated in the picture in the top left.

Surprisingly, things are totally different in the coinage metals of Group 11: Here, the influence of relativistic effects on the melting points is erratic, non-linear, and surprisingly small for Au, which turns out to be a result of phase-specific effect: liquid gold is strongly stabilized relative to the solid phase by relativistic effects, which eats up the complete increase of the melting point due to the relativistic increase of the cohesive energy. As a result, the melting point of Au is unimpressed by relativity, whereas its boiling point shows the typical linear correlation with the cohesive energy.

An outstanding task is to apply the same methodology to Group 14 and superheavy flerovium (Fl, Z=114), which is at least as interesting as Cn but also a bit more difficult: Here, spin-orbit effects cause a pseudo-closed-shell configuration, and a perturbative treatment which worked for Groups 11 and 12 is presumably no longer sufficient.