First Principles Thermodynamic Model of BaZrS₃ Synthesis

The BaZrS₃ chalcogenide perovskite shows strong light absorption, high chemical stability, is nontoxic, and is made from earth-abundant elements. These properties make it a promising candidate material for application in optoelectronic technologies, including next-generation photovoltaic absorber materials [1]. It has been proposed as a more chemically stable alternative to the widely studied lead-based metal halide perovskite family [2]. Although the chemical and physical properties of this perovskite are favorable, a scalable synthesis technique remains an open challenge. Standard solid-state synthesis requires temperatures that are not suitable for device integration (&gt;900°C).<br/><br/>Several studies have now established that the perovskite forms at moderate temperatures (500-600°C) through liquid-flux assisted synthesis [3]. This method is based on the formation of BaS₃ in a sulfur-rich environment. Other methods based on molecular precursors or nanoparticles have also been proposed, but no good quality crystalline thin films have been formed to date [4,5]. Progress is hindered by our limited understanding of the underlying reaction thermodynamics.<br/><br/>In our work, we use density functional theory and lattice dynamics to calculate the thermodynamic and vibrational properties of BaZrS₃and its competing ternary, binary, and elemental phases. We consider the experimentally reported BaS_x, ZrS_x(x = 1, 2, 3) and BaZrS₃ phases. Using our open-source code ThemoPot [6] we calculate the temperature and pressure-dependent Gibbs free energy of formation with reference to competing ternary, binary, and elemental phases.<br/><br/>We find that to promote the formation of BaZrS₃through liquid-flux there is a “goldilocks” zone for temperature and sulfur partial pressure. This is driven by the high sensitivity of Gibbs formation energy to the sulfur gas allotrope. At intermediate temperatures (500°C) and higher pressures (&gt;10³Pa) the S₈allotrope dominates [7] and suppresses the formation of BaS₃. At lower pressures (&lt;10²Pa) the S₂ allotrope dominates and BaS₂ forms. At intermediate pressures, the S₂ allotrope dominates and forms BaS₃(10²-10³Pa). We find good agreement between our results and those reported in the experimental literature [5]. Our work provides insights into the reaction thermodynamics of this promising material and suggests the experimental regimes to target for future synthesis.<br/><br/>References:<br/>[1] Sopiha et al, Adv. Opt. Mater. 2022 10 2101704.<br/>[2] Comparotto et al, ACS Appl. Energy Mater. 2022 5 6335.<br/>[3] Yang et al, Chem. Mater. 2023 35 4743.<br/>[4] Pradhan et al, Angew. Chem. 2023 62 202301049.<br/>[5] Yang et al, J Am Chem. Soc. 2022 44 15928.<br/>[6] https://github.com/NU-CEM/ThermoPot<br/>[7] Jackson and Walsh, J Mater. Chem. A 2014 2 7829.