High temperature equilibrium of 3D and 2D chalcogenide perovskites Prakriti Kayastha 1 , Devendra Tiwari 1 , Adam Holland 2 , Oliver S Hutter 1 , Ken Durose 3 , Lucy D Whalley 1 , Giulia Longo 1 1 Northumbria University, Newcastle upon Tyne, UK, 2 Horiba UK, Northampton, UK, 3 University of Liverpool, UK Chalcogenide perovskites have recently gained interest as a non-toxic, stable, and earth-abundant candidate for photovoltaic absorber materials. 1 BaZrS 3 is one of the most widely studied materials in this family as it has a high absorption coefficient, and a large bandgap ideal for application in tandem cells. However material synthesis remains a challenge and these materials typically need very high temperatures (800-1100 0 C) to be crystallized in the desired phase, hindering thin film processing and device integration. 1 In addition, materials from the 2D Ruddlesden-Popper (RP) series Ba n+1 Zr n S 3n+1 have been reported to form alongside the BaZrS 3 target phase. 2 In contrast to recent experimental interest, the quantum chemical properties of this system are relatively underexplored, in particular,a systematic study of phase formation in the Ba-Zr-S system is lacking. In this talk, we will presenta first-principles thermodynamic model for the Ba-Zr-S system implemented using our open-source code ThermoPot. 3 We will use this to demonstrate that at high temperatures the Gibbs free energy of the 3D perovskite and 2D RP materials (n=1,2,3) are only a few kJ/mol apart, indicating that both can be formed during high-temperature synthesis. In addition, we will present phase diagrams as a function of temperature and pressure to identify favourable conditions for the formation of BaZrS 3 . We will then present results from our database of Raman spectra for all the competing elemental, binary and ternary phases of the Ba-Zr-S system. 4 Ourcalculations and subsequent analysis, combined with results from experimental Raman studies and X-ray diffraction, 5 confirm the formation of an unusual RP phase: Ba 4 Zr 3 S 10 . This is suppressed through the addition of an above-stoichiometric amount of ZrS 2 , resulting in a more phase-pure BaZrS 3 material. Finally, we will use our results to discuss the impact of exchange-correlation functional (GGA vs hybrid) on calculated phonon frequencies, and highlight the importance of multimodal analysis when differentiating between materials that share structural similarity. References 1. Tiwari et al, 2021 J Phys. Energy 3 034010. 2. Niu et al, 2022 J Mater. Res. 34 3819.
3. Kayastha et al, 2022, Zenodo, doi:10.5281/zenodo.7351356. 4. github.com/NU-CEM/2022_BaZrS3_vibrational_spectroscopy. 5. Kayatha et al, 2023 Solar RRL, under review; arXiv:2212.01429
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