High‐Temperature Equilibrium of 3D and 2D Chalcogenide Perovskites

Chalcogenide perovskites have been recently proposed as novel absorber materials for photovoltaic applications. BaZrS 3 , the most investigated compound of this family, shows a high absorption coefficient, a bandgap of around 1.8 eV, and excellent stability. In addition to the 3D perovskite BaZrS 3 , the Ba–Zr–S compositional space contains various 2D Ruddlesden–Popper phases Ba n + 1 Zr n S 3 n + 1 (with n = 1, 2, 3) which have recently been reported. Herein, it is shown that at high temperature the Gibbs free energies of 3D and 2D perovskites are very close, suggesting that 2D phases can be easily formed at high temperatures. The product of the BaS and ZrS 2 solid‐state reaction, in different stoichiometric conditions, presents a mixture of BaZrS 3 and Ba 4 Zr 3 S 10 . To carefully resolve the composition, X‐ray diffraction, scanning electron microscopy, and energy‐dispersive X‐ray spectroscopy analysis are complemented with Raman spectroscopy. For this purpose, the phonon dispersions, and the consequent Raman spectra, are calculated for the 3D and 2D chalcogenide perovskites, as well as for the binary precursors. This thorough characterization demonstrates the thermodynamic limitations and experimental difficulties in forming phase‐pure chalcogenide perovskites through solid‐state synthesis and the importance of using multiple techniques to soundly resolve the composition of these materials.