MnSiP₂: A New Mid-IR Ternary Phosphide with Strong SHG Effect and Ultrabroad Transparency Range

Mid-infrared (MIR) nonlinear optical (NLO) crystals are crucial in generating MIR lasers, which are important in civilian and military applications. Compared to the extensively investigated ternary and quaternary chalcogenide NLO crystals, ternary phosphide crystals have a large nonlinearity coefficient and a wide transparency range. For instance, the well-known ternary chalcopyrite phosphide NLO crystal, ZnGeP2, possesses a large nonlinearity coefficient (deff = 75 pm/V) and a broad transmittance range of 0.75–12 μm. However, its applications in the far-infrared laser range were hindered by its strong absorption near 9 and 10–12 μm. Here, considering that Mn2+ not only makes contributions to large second-harmonic generation (SHG) response but also extends the transparency wavelength range, a new ternary chalcopyrite phosphide crystal, MnSiP2 (MSP), was rationally designed by substituting a II-site metal (Zn or Cd) for Mn in the II-Si-P2 family. In this work, the synthesis and single-crystal growth of MSP were systematically investigated. The theoretical and experimental characterizations demonstrated that MSP exhibits a good balance between strong SHG behavior (6 × AgGaS2) and large laser damage threshold (about 70 MW/cm2). Specifically, the long-wavelength MIR cutoff edge of MSP is 18.2 μm, and the outstanding terahertz (THz) transmittance covers the range of 0.4–2.0 THz, indicating the ultrabroad transparency range. This work may provide a feasible strategy and impel one to search for promising outstanding MIR NLO crystals in the ternary phosphide family.