KCs₂ [Pb₂ Br₅ (HCOO)₂ ]: A Polar 3D Lead-Bromide Framework Exhibiting Strong Second-Harmonic Generation Response

Area of Science:

• Materials Science
• Crystallography
• Optics

Background:

• The demand for novel nonlinear optical (NLO) crystals is high for coherent light generation.
• Existing NLO materials often have limitations in performance or transparency.

Purpose of the Study:

• To synthesize and characterize a new NLO crystal with enhanced properties.
• To investigate the structural features responsible for the observed NLO effects.

Main Methods:

• Mix-solvothermal synthesis method.
• Characterization of optical properties, including second-harmonic generation (SHG) and birefringence.
• Theoretical calculations and analysis of local dipole moments.

Main Results:

• Successfully synthesized KCs2[Pb2Br5(HCOO)2], a unique NLO lead bromide formate.
• Observed strong phase-matching SHG response (6.5× KDP) and large birefringence (0.16@ 1064 nm).
• Identified a polar 3D lead-bromide framework with oriented Pb-Br layers contributing to NLO properties.
• Demonstrated a wide transparency window across visible and mid-IR regions.

Conclusions:

• KCs2[Pb2Br5(HCOO)2] is a promising NLO material with excellent SHG and birefringence properties.
• The specific crystal structure, featuring oriented Pb-Br layers, is key to its strong NLO performance.
• This work presents a novel strategy for designing and discovering new NLO crystals.