FeSi4P4 and CoSi3P3: Hidden Gems of Ternary Tetrel Pnictides with Outstanding Nonlinear Optical Properties

Metal silicon phosphides have shown promise as nonlinear optical materials. To be practically useful and cheap, earth-abundant 3d transition metals are preferred over their scarcer and more expensive 4d and 5d counterparts. We developed a synthetic method to produce polycrystalline bulk powders and millimeter-sized single crystals of ternary compounds FeSi₄P₄ and CoSi₃P₃. Both studied compounds have noncentrosymmetric and chiral crystal structures with ordered Si/P arrangements as was confirmed by single-crystal X-ray diffraction and solid-state NMR. Despite the presence of the transition metal, FeSi₄P₄ and CoSi₃P₃ are semiconductors with direct band gaps of 1.3 and 1.6 eV, respectively, indicating low-spin d⁶ electronic configuration for octahedral Fe²⁺ and Co³⁺. Relative to reported sulfide materials, FeSi₄P₄ and CoSi₃P₃ small band gap semiconductors demonstrate an outstanding combination of second-harmonic generation (SHG) activity and laser damage threshold (LDT). Both studied materials are phase-matchable with a 2.09 μm laser and not only exhibit 2.5–3.0 times stronger SHG signal than that of the state-of-the-art AgGaS₂ standard but also demonstrate an LDT response of 2.3–2.5 times higher than that of AgGaS₂ (at 1.09 μm laser with a pulse width of 10 ns)─which is unprecedented for small band gap semiconductors.