Mg Substitution in BaGa 4 Se 7 : Synthesis, Structure, and Property Modulation

BaGa 4 Se 7 is a promising infrared nonlinear optical (NLO) crystal, offering significant potential for mid-infrared applications. To further optimize its performance and explore new functional materials, we introduced Mg into the BaGa 4 Se 7 lattice, successfully synthesizing a series of novel quaternary solid-solution NLO crystals. It confirms that the synthesized polycrystalline materials retain the same structure as the BaGa 4 Se 7 crystal. Notably, Mg ions are incorporated into the crystal lattice by occupying two distinct cationic sites: the Ba 2+ and Ga 3+ sites. In the initial polycrystalline state, Mg preferentially occupies the Ba sites. During the subsequent slow, near-equilibrium single-crystal growth process, thermodynamic stability becomes the dominant factor governing Mg distribution. This leads to a redistribution of Mg ions: some Mg 2+ ions that originally occupied the Ba sites are expelled toward the tail end of the growing crystal. In the main body of the crystal, a more stable and optimized occupancy pattern is established, where Mg 2+ occupies both Ba and Ga sites in a specific equilibrium ratio. Remarkably, when this occupancy ratio reaches approximately 1:1, corresponding to the composition of Ba 0.95 Mg 0.1 Ga 3.95 Se 7, the crystal demonstrates a significantly enhanced second-harmonic generation (SHG) response. Its SHG intensity at the fundamental wavelength of 1.064 μm is measured to be 1.86 times that of pure BaGa 4 Se 7, while maintaining a high optical transmittance of up to 75% across the measured spectral range. This indicates a successful balance between enhancing the nonlinear optical coefficient and preserving excellent optical quality. However, as the Mg concentration is further increased, the substitution of Ga sites by Mg becomes dominant. This excessive substitution disrupts the optimal structural arrangement and charge balance, resulting in concurrent degradation of both nonlinear optical performance and overall optical properties. Our findings conclusively demonstrate that the nonlinear optical performance of BaGa 4 Se 7 can be finely tuned and optimized through trace Mg substitution, highlighting the critical role of precise dopant control and site occupancy in developing next-generation high-performance infrared NLO materials.