Single-Crystal Growth and Characterization of High-Entropy Phosphides ASiP₂

High-entropy engineering was applied to ternary phosphide MgSiP₂ to synthesize (Mg, Zn, Cd, Mn)SiP₂ (A⁴SiP₂) and (Mg, Ca, Sr, Ba, Zn)SiP₂ (A⁵SiP₂) single crystals. These systems were successfully synthesized using the Sb-flux method, with A⁴SiP₂ retaining the space group I4̅2d, while A⁵SiP₂ crystallized in a new space group I4̅2m with multiple high-entropy sites. This structural transformation was driven by the inclusion of large atoms, such as Sr and Ba, causing significant unit cell distortion. From the composition analysis using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM–EDX), we found the A⁵SiP₂ sample had (Mg, Zn)SiP₂ region inside the crystal. This unique microstructure, where the binary phase was covered by the high-entropy phase, provides insights into the crystal growth mechanism of A⁵SiP₂. Owing to the higher stability of the MgSiP₂ and ZnSiP₂ parent phases, (Mg, Zn)SiP₂ nucleates first, serving as seed crystals for the subsequent growth of the A⁵SiP₂ phase. Because of their noncentrosymmetric structure, both A⁴SiP₂ and A⁵SiP₂ exhibited optical second harmonic generation response, implying that these materials have the potential to be used as nonlinear optical materials. They also exhibited band-gap narrowing relative to the parent phases, similar to what has been observed in high-entropy oxide systems. This demonstration of single-crystal growth of high-entropy phosphide opens doors for a new class of crystal compositions with potentially nonlinear optics.