Periodic Reporting for period 1 - COMEX (COmputational Modelling for EXtreme conditions)
Reporting period: 2018-05-01 to 2020-04-30
Pressure is an important thermodynamic variable in order to understand the properties of materials because it allows for a precise control over the interatomic distances and hence the atomic interactions. Moreover, the application of high pressure (HP), many times in combination with high temperature (HT), allows the synthesis of new phases of materials with completely different properties to those from stable materials at room conditions (RC). Sesquichalcogenides (SCs) are compounds with A2X3 stoichiometry (A being a trivalent cation and X=S,Se,Te) which have been studied because of their many interesting applications. In particular, Bi2Te3 is considered the best thermoelectric material (TM) at RC. A great activity in the study of SCs and respective behaviour at HP has occurred in the last years since the discovery of the topological insulating (TI) and superconducting behaviour of tetradymite-like (R-3m phase) Bi2Se3, Bi2Te3 and Sb2Te3. In fact, a strong interest has aroused regarding the bridge between the TIs and high-performance TM materials. Several studies have contributed to the understanding of the pressure-induced electronic topological transition and the TI behaviour of SCs at HP. More recently, a HP study on Sb2S3 has discussed pressure-induced second-order isostructural phase transitions and electronic topological transitions at HP in several SCs, similar to what occurs for Sb2Se3 and Bi2S3. These interesting properties have motivated great interest to explore the properties of SCs. Moreover, studies of the properties of SCs are still required since their behaviours at HP and HT are not fully understood and these have been scarcely explored, especially regarding some the crystalline phases of SCs with A=As,Sb. In particular, the R-3m phase of Sb2Se3 (β-Sb2Se3) has been predicted to be TI, but such a phase has not been reported to date. On the other hand, the R-3m phase of As2Te3 (β-As2Te3) has been reported and predicted to become a TI under compression, but this prediction has yet to be experimentally confirmed. Finally, almost nothing is known about the properties of the different crystalline phases of As2S3 (orpiment and anorpiment) and As2Se3 (α, β and γ) even at RC. It is possible that γ-As2Se3 could crystallize in the R-3m phase and show TI properties.
Thus, the objective of this project is to study, from a theoretical perspective, the characterization of the structural, vibrational and electronic properties of SCs, which are potential candidates for TI and/or TM when induced under extreme conditions. This work will complement experimental studies of SCs and provide basic understanding of respective physical-chemical properties, which will be important for respective implementation in technological devices.
Thus, the objective of this project is to study, from a theoretical perspective, the characterization of the structural, vibrational and electronic properties of SCs, which are potential candidates for TI and/or TM when induced under extreme conditions. This work will complement experimental studies of SCs and provide basic understanding of respective physical-chemical properties, which will be important for respective implementation in technological devices.
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).
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).
The work carried out does not create new market opportunities, strengthen competitiveness nor growth of companies. However respective work has enhanced innovation capacity of the researchers involved in the project, since they had to address new and challenging concepts, such as a newly defined type of chemical bonding named metavalent bonding. Moreover, the project has allowed the researcher to apply new and successful anharmonic strategies (SSCHA code) to explain the extraordinary properties of materials that evidence such a bonding character. Finally, it is worth mentioning that the group-14 and -15 chalcogenides which evidence metavalent bonding are technologically important since they possess excellent properties for phase change materials to apply for computing memories (GeTe, Sb2Te3, GeSb2Te4). Thse systems are among the best TMs known to date (SnSe, PbTe, Sb2Te3, Bi2Te3) and show TI properties (β-As2Te3, Bi2 Se3, Sb2Te3, and Bi2Te3) likely to be applied in future spintronics and quantum computation. Therefore, this project addresses important issues related to information technologies (phase change memories, spintronics, quantum computation) and to climate change or the environment (highly-efficient TM to harvest energy from waste heat) that could bring enormous benefits for the society.