Mechanism of Pressure-Driven Band Gap Evolutions in Lead-Free Halide Double Perovskites

Lead-free halide double perovskites Cs2BBiCl6 (B = Na, Ag) are potential alternatives in optoelectronic applications because of their nontoxicity and intrinsic stability. Intriguingly, the photoluminescence spectra of their cubic phases revealed that the band gap evolution under pressure strongly depends on B-site metals. Our first-principles calculations demonstrate that this distinct phenomenon is caused by orbital contributions of B-site cations (Na versus Ag) at the band edges. In contrast to Na in Cs2NaBiCl6 whose 3s valence orbitals contribute insignificantly to the band edge states, Ag in Cs2AgBiCl6 can cause large upward shifts of the valence band maximum energy under pressure because of the enlargement of the bonding–antibonding energy split of Ag–Cl bonds below the Fermi level. Other double perovskites with different B-site cations (K, Rb, Cu, and Au) in the +1 valence state exhibit band gap evolutions similar to Cs2NaBiCl6 and Cs2AgBiCl6, indicating that the B-site cation plays a critical role in regulating the electronic properties of lead-free halide double perovskites. Moreover, our calculations show that the optical absorbance coefficients of Cs2CuBiCl6 and Cs2AuBiCl6 can be as large as 105 cm–1 in the region of visible lights and could be further enhanced by external pressure. Our study reveals the mechanism of how s/d-block metals regulate the band gap of double perovskites and provides a guideline for the band engineering of optoelectronics under high pressure.