Bandgap Engineering in Pressurized Calcium Sulfide

Calcium sulfide (CaS) is a semiconductor with excellent optoelectronic properties and its electronic structures are readily modulated by applying pressure. Here, we conduct in situ X-ray diffraction and ultraviolet–visible light spectroscopy experiments to investigate the evolution of band gap across the phase transition from B1 to B2 type structures. Upon pressurizing to 50 GPa, we observe the band gap decreased from the initial 3.51 to 1.18 eV. Through first-principles calculations, we reveal that the band gap narrowing is driven by a pressure-induced direct-to-indirect transition, accompanied by enhanced interactions between S 3p and Ca 3d states. Releasing pressure to ambient conditions recover the band gap, implying a fully reversible phase transition. Our results suggest that pressure-induced polymorphism and bandgap engineering tune the electronic properties of CaS, making it a promising optoelectronic material in the visible to deep-ultraviolet spectral regions.