Rotational tunneling of the NH+4 ion in phase II of (NH4)2 PtBr6 by inelastic neutron scattering spectroscopy

Pressure-Induced Metallization of Lead-Free Halide Double Perovskite (NH4 )2 PtI6

Metallization has recently garnered significant interest due to its ability to greatly facilitate chemical reactions and dramatically change the properties of materials. Materials displaying metallization under low pressure are highly desired for understanding their potential properties. In this work, the effects of the pressure on the structural and electronic properties of lead-free halide double perovskite (NH₄ )₂ PtI₆ are investigated systematically. Remarkably, an unprecedented bandgap narrowing down to the Shockley-Queisser limit is observed at a very low pressure of 0.12 GPa, showing great promise in optoelectronic applications. More interestingly, the metallization of (NH₄ )₂ PtI₆ is initiated at 14.2 GPa, the lowest metallization pressure ever reported in halide perovskites, which is related to the continuous increase in the overlap between the valence and conduction band of I 5p orbital. Its structural evolution upon compression before the metallic transition is also tracked, from cubic Fm-3m to tetragonal P4/mnc and then to monoclinic C2/c phase, which is mainly associated with the rotation and distortions within the [PtI₆ ]^2- octahedra. These findings represent a significant step toward revealing the structure-property relationships of (NH₄ )₂ PtI₆ , and also prove that high-pressure technique is an efficient tool to design and realize superior optoelectronic materials.

Pressure-induced bandgap engineering of lead-free halide double perovskite (NH4)2SnBr6

Lead-free halide double perovskites (HDPs) have recently been proposed as potential stable and environment-friendly alternatives to lead-based halide perovskites. Bandgap engineering plays a vital role in the optoelectronic applications of HDP materials. In this study, methods combining high-pressure techniques with density functional theory calculations were employed to implement the bandgap engineering of a classic HDP-based (NH₄)₂SnBr₆. Under high pressure, (NH₄)₂SnBr₆ exhibits a redshift of the bandgap with increasing pressure up to 6.3 GPa and a sudden blueshift up to 20.2 GPa, followed by a redshift at higher pressures, which is relevant to the cubic-tetragonal phase transition, direct-indirect transition, and amorphization, respectively. Our results enrich the understanding of the structural-optical properties of (NH₄)₂SnBr₆ and reveal the special role of NH₄⁺ cations in pressure-induced bandgap engineering, thus providing important information for application in optoelectronic devices and helping to design ideal materials with higher efficiency.