Flexible polar nanowires of Cs ₅BiP ₄Se ₁₂ from weak interactions between coordination complexes: Strong nonlinear optical second harmonic generation

The Cs ₅BiP ₄Se ₁₂ salt grows naturally as nanowires that crystallize in the polar space group PmC2 ₁, with a = 7.5357(2) Å, b = 13.7783(6) Å, c = 28.0807(8) Å, and Z = 4 at 293(2) K. The compound features octahedral [Bi(P ₂Se ₆) ₂] ⁵-coordination complexes that stack via weak intermolecular Se⋯Se interactions to form long, flexible fibers and nanowires. The Cs ₅BiP ₄Se ₁₂ fibers are transparent in the near-and mid-IR ranges and were found to exhibit a nonlinear optical second harmonic generation response at 1 μm that is approximately twice that of the benchmark material AgGaSe ₂. The material has a nearly direct band gap of 1.85 eV and melts congruently at 590 °C. Ab initio electronic structure calculations performed with the full-potential linearized augmented plane wave (FLAPW) method show that the band gap increases from its local density approximation (LDA) spin-orbit coupling value of 1.15 eV to the higher value of 2.0 eV when the screened-exchange LDA method is invoked and explain how the long nanowire nature of Cs ₅BiP ₄Se ₁₂ emerges.