A chemically stable divalent lanthanide chalcogenide, LaMg₆Ga₆S₁₆, has been synthesized. This compound exhibits unique electronic configurations and physicochemical properties due to the presence of La²⁺ cations. The structure of LaMg₆Ga₆S₁₆ is based on an octahedral crystal field formed by the [Mg/Ga-S]∞ anionic framework, which allows for the stable incorporation of La²⁺ cations. The La²⁺ cations are coordinated by six S atoms to form [LaS₆] octahedra, leading to the formation of a stable divalent La²⁺ ion with 5d⁴ conduction carriers. This unique electronic configuration results in an ultrabroad-band green emission at 500 nm, which is the first instance of La(II)-based compounds displaying luminescent properties. Additionally, LaMg₆Ga₆S₁₆ crystallizes in a noncentrosymmetric space group (P-6), making it active for second-harmonic generation (SHG), with a SHG response comparable to that of AgGaS₂. The compound also has a wide band gap (E_g = 3.0 eV) and a high laser-induced damage threshold (5× that of AgGaS₂), making it a promising nonlinear optical material. The synthesis of LaMg₆Ga₆S₁₆ was confirmed through various analytical techniques, including X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES), and electron paramagnetic resonance (EPR), which identified the La²⁺ cations. The compound's stability was further confirmed through differential thermal scanning (DSC), in-situ variable-temperature powder X-ray diffraction, and solid-state reactions. The unique electronic configuration of La²⁺ in LaMg₆Ga₆S₁₆ contributes to its luminescent and nonlinear optical properties, making it a promising material for applications in optoelectronics and nonlinear optics.A chemically stable divalent lanthanide chalcogenide, LaMg₆Ga₆S₁₆, has been synthesized. This compound exhibits unique electronic configurations and physicochemical properties due to the presence of La²⁺ cations. The structure of LaMg₆Ga₆S₁₆ is based on an octahedral crystal field formed by the [Mg/Ga-S]∞ anionic framework, which allows for the stable incorporation of La²⁺ cations. The La²⁺ cations are coordinated by six S atoms to form [LaS₆] octahedra, leading to the formation of a stable divalent La²⁺ ion with 5d⁴ conduction carriers. This unique electronic configuration results in an ultrabroad-band green emission at 500 nm, which is the first instance of La(II)-based compounds displaying luminescent properties. Additionally, LaMg₆Ga₆S₁₆ crystallizes in a noncentrosymmetric space group (P-6), making it active for second-harmonic generation (SHG), with a SHG response comparable to that of AgGaS₂. The compound also has a wide band gap (E_g = 3.0 eV) and a high laser-induced damage threshold (5× that of AgGaS₂), making it a promising nonlinear optical material. The synthesis of LaMg₆Ga₆S₁₆ was confirmed through various analytical techniques, including X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES), and electron paramagnetic resonance (EPR), which identified the La²⁺ cations. The compound's stability was further confirmed through differential thermal scanning (DSC), in-situ variable-temperature powder X-ray diffraction, and solid-state reactions. The unique electronic configuration of La²⁺ in LaMg₆Ga₆S₁₆ contributes to its luminescent and nonlinear optical properties, making it a promising material for applications in optoelectronics and nonlinear optics.