Marck Lumeij

Ab initio study of the high‐temperature phase transition in crystalline GeO2

Germanium dioxide (GeO2) takes two forms at ambient pressure: a thermodynamically stable rutile‐type structure and a high‐temperature quartz‐type polymorph. Here, we investigate the phase stability at finite temperatures by ab initio phonon and thermochemical computations. We use gradient‐corrected density‐functional theory (PBE‐GGA) and pay particular attention to the modeling of the “semicore” germanium 3d orbitals (ascribing them either to the core or to the valence region). The phase transition is predicted correctly in both cases, and computed heat capacities and entropies are in excellent agreement with thermochemical database values. Nonetheless, the computed formation energies of α‐quartz‐type GeO2 (and, consequently, the predicted transition temperatures) differ significantly depending on theoretical method. Remarkably, the simpler and cheaper computational approach produces seemingly better results, not worse. In our opinion, GeO2 is a nice test case that illustrates both possibilities and limitations of modern ab initio thermochemistry.

[CoI(CN)2(CO)3]−, a new discovery from an 80-year-old reaction

A systematic study of the 80-year-old simple and fundamental reaction of cobalt(II) salt with cyanide under one atmosphere CO was carried out and two novel Co(I) complexes were isolated in the absence of hydroxide. These complexes are the first structurally characterized mixed cobalt CO-CN(-) compounds.