Hybrid Bismuth Halide with Rich Polymorphism and Second Harmonic Generation Response

Hybrid structures have emerged as a promising class of
optical
materials, due to their ability to couple the robustness of inorganic
and the tunability of organic compounds. However, their application
in nonlinear optics (NLO) remains limited, largely due to underexplored
factors that drive the formation of noncentrosymmetric structures
and NLO property characterization. In this work, we explore the formation,
structural and temperature polymorphism, and optical properties of
the (Et3NH)3Bi2Br9 composition,
which crystallizes as either noncentrosymmetric or centrosymmetric
polymorph. Structural analysis showed that the alignment of [Bi2Br9]3– units dictates the symmetry
of phases, as well as the nonlinear optical properties, with the triclinic
polymorph exhibiting a second harmonic generation (SHG) response both
in visible and IR regions (1.29 × KH2PO4 and 0.08 × AgGaS2). Thermal analysis reveals polymorphic
phase transitions and low melting points, making them melt-processable
and ionic liquid candidates.