High temperature equilibrium of 3D and 2D chalcogenide perovskites

TL;DR

This study investigates the high-temperature thermodynamics and synthesis challenges of 3D and 2D chalcogenide perovskites, revealing their close Gibbs free energies and the difficulty in obtaining phase-pure materials.

Contribution

It provides a comprehensive thermodynamic analysis and experimental characterization of 3D and 2D chalcogenide perovskites, highlighting their formation limitations.

Findings

Gibbs free energies of 3D and 2D phases are very close at high temperatures

Solid-state synthesis often results in mixed phases rather than pure compounds

Multiple characterization techniques are essential for accurate phase identification

Abstract

Chalcogenide perovskites have been recently under the researchers spotlight 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 environmental and thermal stability. In addition to the 3D perovskite BaZrS$_3$, the Ba-Zr-S compositional space contains various 2-D Ruddlesden-Popper phases Ba$_{x+1}$Zr$_x$S$_{3x+1}$ (with $x=$ 1, 2, 3) which have recently been reported. In this work it will be 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 analysis of the product of the BaS and ZrS$_2$ solid-state reaction, in different stoichiometric conditions, present a mixture of BaZrS$_3$ and Ba$_4$Zr$_3$S$_{10}$. To carefully resolve the composition, XRD, SEM and EDS analysis were complemented with Raman spectroscopy. For this purpose, the phonon modes, and the consequent Raman spectra, were 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 chalcogenide materials.