First-Principles Calculations of Spectroscopic Signatures III

Methods

The density functional theory (DFT) permits the computation of the electronic ground-state energy and the ground-state charge density of a given atomic configuration (solids and molecules). These quantities result from the Kohn-Sham (KS) eigenvalue equations, which are solved within a self-consistent scheme using efficient and established numerical algorithms. The periodic unit cell approach discretises the differential operators into matrices acting on the Fourier coefficients of the wavefunctions (plane waves). The infinite sums are truncated according to the convergence of macroscopic quantities (usually the total energy). The continuous Brillouin zone is integrated on a grid of k-point samples. The number of k-points is reduced by symmetry relations and typically amounts to∼100 for bulk systems, and∼10 for surfaces. Similarly to the number of plane waves, the optimal number of k-points is determined by the convergence of macroscopic quantities. The matrices are diagonalised within efficient iteration schemes, most importantly the well established Davidson block diagonalisation and a residual minimization scheme (direct inversion in the iterative subspace, RMM-DIIS).

Results

Subproject A: Subproject A is devoted to the investigation of LiNbO₃, LiTaO₃, and their LiNbO₃-LiTaO₃ solid solutions as investigated in the research group FOR5044. The latter are mostly simulated by DFT as implemented in the VASP and QuantumEspresso codes within the special quasirandom structures (SQS) model, which allows to simulate disordered alloys within the supercell approach. The approach to the calculation of the nonlinear optical response based on the calculation of the polarization in the time-domain, which we implemented during the previous project runtime, has been applied to strained LiNbO₃ and LiTaO₃ materials, revealing how the optical response can be 2manipulated by external stimuli. Ground and exited state properties of the solid soultions have been calculated from first principles. Ferroelectric domain walls in LiNbO₃ and LiTaO₃ have been modeled atomistically, the domain form has been modeled by an Ising approach on the basis of the coupling coefficients calculated within DFT.

Subproject 2: Subproject 2 is dedicated to the investigation of the adamantane-based molecular clusters and their unique nonlinear optical properties as investigated in the research group FOR2824. Prototypical model systems are the adamantane shaped [(RT)₄E₆] clusters (with R = organic group; T = C, Si, Ge, Sn; E = O, S, Se, CH₂). After calculating the ground state atomic and electronic structure of different molecules, we have furthed explored the effect of chemical modifications (ligand field) and habitus (single molecular cluster, molecular crystals) on the optical response. This was the continuation of the investigation started in the previous compute time grant.

Discussion

Subproject 1: With the compute time provided by the NHR4CES at the Lichtenberg 2 cluster a row of novel and relevant results could be produced that led to the publication of not less than 12 peer reviewed publication related to subproject 1. The investigation of ferroelectric solid solutions in cooperation with our experimental partners yield insight concerning atomic and electronic structure, optical spectroscopy signatures, lattice dynamics, heat capacity, and defect structures. The calculated properties have been compared with corresponding experimental results from the groups of Profs. H. Fritze and H. Schmidt (TU Clausthal), S. Ganschow (IKZ Berlin) M. Imlau (TU Osnabrück), and C. Sil- berhorn (Uni Paderborn) showing an excellent agreement and providing further insight towards the materials properties. Most remarkable is the investigation of ferrolectric domain walls (DWs) performed in the framework of a cooperation with Prof. Lukas Eng (TU Dresden), in which we were able to explain the origin of the conductivity of DWs in otherwise insulating materials.

Subproject 2: Within that last year we could elevate the insight achieved by the atomistic models by developing analytical tools that allow to relate optical nonlinearties with particularly relevant electronic transitions. We could demonstrate that the optical nonlinearities have their origin in the ligand field, in particular in the delocalized π-orbitals of the phenyl-substituents. The molecular core is not directly involved in the transitions leading to the generation of the optical nonlinerities, however it influences the intensity of the optical response by symmetry breaking. Our calculations explain the results of the experimental investigation of this class of molecules, synthetized by our experimental colleagues (Prof. S. Dehnen at the KIT and Prof. P. W. Schreiner at the Justus-Liebig- Universität Gießen) and characterized by optical measurements within the group of Prof. K. Volz at the Philipps Universität Marburg. Moreover, they provide a path toward the design of molecules with optimized optical response.

Additional Project Information

DFG classification: 307-02 Theoretical Condensed Matter Physics
Software: VASP, QuantumEspresso, Yambo
Cluster: Lichtenberg

Publications

Ziese, F.; Sanna, S. Nonlinear optical response of tetrel modified tetraphenyl-adamantane clusters, ACS Omega, 9, 49816 (2024).  https://pubs.acs.org/doi/10.1021/acsomega.4c08541?goto=supporting-info

Bernhardt, F.; Gharat, S.; Kapp, A.; Pfeiffer, F.; Buchbeck, R.; Hempel, F.; Pashkin, O.; Kehr, S. C.; Rüsing, M.; Sanna, S.; Eng, L. M. Lattice Dynamics of LiNb₁₋ₓTaₓO₃ Solid Solutions: Theory and Experiment, Phys. Stat. Sol. (a), 202300968 (2024). https://doi.org/10.1002/pssa.202300968

Verhoff, L. M.; Pionteck, M. N.; Rüsing, M.; Fritze, H.; Eng, L. M.; Sanna, S. 2-dimensional electronic conductivity in insulating ferroelectrics: Peculiar properties of domain walls, Phys. Rev. Res. Lett., 6, L042015 (2024). https://doi.org/10.1103/PhysRevResearch.6.L042015

Ziese, F.; Wang, J.; Rojas-León, I.; Dehnen, S.; Sanna, S. Origin of the non-linear optical response in organotetrel molecules, (hetero)adamantane-type clusters with organic substituents, and related species, J. Phys. Chem. A, 128, 8360 (2024). https://doi.org/10.1021/acs.jpca.4c03990

Bernhardt, F.; Pfeiffer, F. A.; Schug, F.; Pfannstiel, A.; Hehemann, T.; Ganschow, S.; Imlau, M.; Sanna, S. Ground- and excited-state properties of LiNb₁₋ₓTaₓO₃ solid solutions, Phys. Rev. Mat., 8, 054403 (2024). https://doi.org/10.1103/PhysRevMaterials.8.054403

Wang, J.; Rojas-León, I.; Rinn, N.; Guggolz, L.; Ziese, F.; Sanna, S.; Rosemann, N. W.; Dehnen, S. Strain-Induced Structural Rearrangement Towards a White-Light-Emitting Adamantane-Type Cluster Dimer, Angew. Chem. Int. Ed., e202411752 (2024). https://doi.org/10.1002/anie.202411752

Yakhnevych, U.; Sargsyan, V.; El Azzouzi, F.; Kapp, A.; Bernhardt, F.; Suhak, Y.; Ganschow, S.; Schmidt, H.; Sanna, S.; Fritze, H. Acoustic Loss in LiNb₁₋ₓTaₓO₃ at Temperatures up to 900°C, Phys. Stat. Sol. (a), 2400106 (2024) https://doi.org/10.21268/20240311-1

El Azzouzi, F.; Klimm, D.; Kapp, A.; Verhoff, L. M.; Schäfer, N. A.; Ganschow, S.; Becker, K.-D.; Sanna, S.; Fritze, H. Evolution of the electrical conductivity of LiNb₁₋ₓTaₓO₃ solid solutions across the ferroelectric phase transformation, Phys. Stat. Sol. (a), 2300966 (2024). http://dx.doi.org/10.1002/pssa.202300966

Bernhardt, F.; Verhoff, L. M.; Schäfer, N. A.; Kapp, A.; Fink, C.; Al Nachwati, W.; Bashir, U.; Klimm, D.; El Azzouzi, F.; Yakhnevych, U.; Suhak, Y.; Schmidt, H.; Becker, K.-D.; Ganschow, S.; Fritze, H.; Sanna, S. Ferroelectric to paraelectric structural transition in LiTaO₃ and LiNbO₃, Phys. Rev. Mat., 8, 054406 (2024). https://doi.org/10.1103/PhysRevMaterials.8.054406

Rinn, N.; Rojas-León, I.; Peerless, B.; Gowrisankar, S.; Ziese, F.; Rosemann, N. W.; Pilgrim, W.-C.; Sanna, S.; Schreiner, P. R.; Dehnen, S. Adamantane-Type Clusters: Compounds with a Ubiquitous Architecture but a Wide Variety of Compositions and Unexpected Materials Properties, Chem. Sci., 15, 9438 (2024). https://doi.org/10.1039/D4SC01136H

Pfannstiel, A.; Hehemann, T.; Schäfer, N. A.; Sanna, S.; Suhak, Y.; Vittadello, L.; Sauerwein, F.; Dömer, N.; Koelmann, J.; Fritze, H.; Imlau, M. Small electron polarons bound to interstitial tantalum defects in lithium tantalate, J. Phys.: Cond. Matt., 36, 355701 (2024). https://doi.org/10.1088/1361-648x/ad4d47

Gaczynski, P.; Suhak, Y.; Ganschow, S.; Sanna, S.; Fritze, H.; Becker, K.-D. A High-Temperature Optical Spectroscopy Study of the Fundamental Absorption Edge in the LiNbO₃-LiTaO₃ Solid Solution, Phys. Stat. Sol. (a), 2300972 (2024). https://doi.org/10.1002/pssa.202300972

Kofahl, C.; Ganschow, S.; Bernhardt, F.; El Azzouzi, F.; Sanna, S.; Fritze, H.; Schmidt, H. Li Diffusion in lithium niobate-tantalate solid solutions, Solid State Ionics, 409, 116514 (2024). https://doi.org/10.1016/j.ssi.2024.116514

Pionteck, M. N.; Sanna, S. Photoelastic properties of stoichiometric lithium niobate from first-principles calculations, Phys. Stat. Sol. (a), 2300970 (2024). https://doi.org/10.1002/pssa.202300970

Yakhnevych, U.; El Azzouzi, F.; Bernhardt, F.; Kofahl, C.; Suhak, Y.; Sanna, S.; Becker, K.-D.; Schmidt, H.; Ganschow, S.; Fritze, H. Oxygen partial pressure and temperature dependent electrical conductivity of lithium-niobate-tantalate solid solutions, Solid State Ionics, 407, 116487 (2024). https://doi.org/10.1016/j.ssi.2024.116487