Flexible polar nanowires of Cs5BiP4Se12 from weak interactions between coordination complexes: strong nonlinear...

The Cs(5)BiP(4)Se(12) salt grows naturally as nanowires that crystallize in the polar space group Pmc2(1), with a = 7.5357(2) A, b = 13.7783(6) A, c = 28.0807(8) A, and Z = 4 at 293(2) K. The compound features octahedral [Bi(P(2)Se(6))(2)](5-) coordination complexes that stack via weak intermolecular Se...Se interactions to form long, flexible fibers and nanowires. The Cs(5)BiP(4)Se(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 microm 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 degrees 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(5)BiP(4)Se(12) emerges.