Reduced temperature solid-state synthesis of barium sulfide: a greener alternative

Barium sulfide (BaS) serves as a commonly used precursor for advanced barium-based materials, including the emerging perovskite photovoltaic absorber BaZrS₃. However, conventional BaS production methods are highly energy-intensive, requiring temperatures well exceeding 1000 °C and emitting large quantities of CO₂ and SO₂, and raising valid environmental concerns. This work presents a novel solid-state synthesis route for BaS that drastically reduces the environmental and energy demands. By employing a finely milled mixture of barium hydroxide [Ba(OH)₂] and elemental sulfur, we achieve an efficient conversion (90%) to BaS at a remarkably low annealing temperature of 500 °C. The process is enabled by a low-pressure annealing environment, which facilitates the rapid vaporisation of H₂O byproducts while maintaining a controlled sulfur partial pressure. This prevents unwanted side reactions and enhances conversion efficiency by continuously removing waste gases, including water vapour and trace amounts of SO₂, while preserving optimal reaction conditions. The success of the process was confirmed through X-ray diffraction, and aided by Fourier transform infrared spectroscopy, and Raman spectroscopy. The extent of conversion was quantitatively determined using Rietveld refinement of the diffraction patterns. This method offers a more sustainable and economically viable pathway for BaS production. Residual gas analysis has shown a significant reduction in CO₂ and SO₂ production compared to earlier processes.