FeSi<sub>4</sub>P<sub>4</sub> and CoSi<sub>3</sub>P<sub>3</sub>: Hidden Gems of Ternary Tetrel Pnictides with Outstanding Nonlinear Optical Properties
Ernesto Soto, Shannon Lee, Andrew P. Porter, Gayatri Viswanathan, Georgiy Akopov, Nethmi W. Hewage, Kui Wu, Victor Trinquet, Guillaume Brunin, Geoffroy Hautier, Gian‐Marco Rignanese, Aaron J. Rossini, Kirill Kovnir
Abstract
Metal silicon phosphides have shown promise as nonlinear optical materials. To be practically useful and cheap, earth-abundant 3 d 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 FeSi 4 P 4 and CoSi 3 P 3 . 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, FeSi 4 P 4 and CoSi 3 P 3 are semiconductors with direct band gaps of 1.3 and 1.6 eV, respectively, indicating low-spin d 6 electronic configuration for octahedral Fe 2+ and Co 3+ . Relative to reported sulfide materials, FeSi 4 P 4 and CoSi 3 P 3 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 AgGaS 2 standard but also demonstrate an LDT response of 2.3–2.5 times higher than that of AgGaS 2 (at 1.09 μm laser with a pulse width of 10 ns)─which is unprecedented for small band gap semiconductors.