Based on the main scientific goals of the project, as described in the DoA, the main objectives and milestones have been achieved, with relatively minor deviations:
1. A number of calculations have been performed on different materials to probe the structural, phonon and electronic properties using Density Functional Theory (DFT) and lattice dynamics. Collaboration with the theoretical group of Prof. Alfonso Muñoz at the University of La Laguna (Spain) within the MALTA Consolider Team network has been important in this regard. The range of studied materials were: SbPO4, Ga2S3,As2S3, As2Se3, As2Te3, Sb2S3, Sb2Se3, Bi2S3, and SnSb2Te4. In particular, GW calculations have been employed to better describe the electronic properties of some of the mentioned systems, such as for the low bandgap β-As2Te3 compound.
2. The Quasi-harmonic Approximation (QHA) has been employed to study the dynamical stability of the two lower energy phases at close to room pressure of Sb2Se3. From DFT calculations, it was found that the tetradymite R-3m phase of this compound was more stable than the experimentally known orthorhombic Pnma phase. From lattice dynamics we observed that both phases were dynamically stable at 0 GPa. The QHA was employed in order to compute the Gibbs free energies of both phases of Sb2Se3 and present their energy differences at different pressure values. It was observed that the R-3m phase persists as being the most energetically stable phase at any temperature up to 1000 K, for 3 and 4 GPa. Regarding MD calculations, these have been performed for the high-pressure disordered Im-3m phase of Sb2Se3 in order to search for a lower energetic structure, since the disordered phase was found to be energetically and dynamically more stable at high pressure (~50 GPa). It was found that a lower minimum configuration with C2 symmetry could coexist at low pressure values, however further probing would be required in order to confirm the dynamical stability of this new phase.In addition to what was initially proposed, and as an alternative to MD calculations, we have initiated calculations with the Stochastic Self-Consistent Harmonic Approximation (SSCHA) code to compute the anharmonic properties of different materials thanks to an external collaboration with one of the developers of the code, Dr. Ion Errea at the University of the Basque Country (Spain). Initial efforts were devoted to SnSe in order to learn the code and further explore anharmonic properties of the materials object of the COMEX project in the near future. Moreover, calculations by employing 3rd order force constants (phonon-phonon interactions) have also been performed in order to compute the lattice thermal-conductivity as a function of pressure of prospective thermoelectrics, i.e. β-As2Te3 and Ga2S3.
3. A number of studies were addressed by employing the quantum theory of atoms in molecules (QTAIM), mainly to probe the charge density topologies as a function of pressure. For respective calculations, further external collaboration has been established with the main developer of the Critic2 code, Dr. Alberto Otero at the University of Oviedo (Spain) within the MALTA Consolider Team network. Respective studies were applied for instance on SbPO4, As2S3, SnSb2Te4 (already published) as well as on β-As2Te3 and Ga2S3 (manuscripts under preparation).
