LaMg₆Ga₆S₁₆: a chemical stable divalent lanthanide chalcogenide

Divalent lanthanide inorganic compounds can exhibit unique electronic configurations and physicochemical properties, yet their synthesis remains a great challenge because of the weak chemical stability. To the best of our knowledge, although several lanthanide monoxides epitaxial thin films have been reported, there is no chemically stable crystalline divalent lanthanide chalcogenide synthesized up to now. Herein, by using octahedra coupling tetrahedra single/double chains to construct an octahedral crystal field, we synthesized the stable crystalline La(II)-chalcogenide, LaMg₆Ga₆S₁₆. The nature of the divalent La²⁺ cations can be identified by X-ray photoelectron spectroscopy, X-ray absorption near-edge structure and electron paramagnetic resonance, while the stability is confirmed by the differential thermal scanning, in-situ variable-temperature powder X-ray diffraction and a series of solid-state reactions. Owing to the particular electronic characteristics of La²⁺(5d¹), LaMg₆Ga₆S₁₆ displays an ultrabroad-band green emission at 500 nm, which is the inaugural instance of La(II)-based compounds demonstrating luminescent properties. Furthermore, as LaMg₆Ga₆S₁₆ crystallizes in the non-centrosymmetric space group, P-6, it is the second-harmonic generation (SHG) active, possessing a comparable SHG response with classical AgGaS₂. In consideration of its wider band gap (E_g = 3.0 eV) and higher laser-induced damage threshold (5×AgGaS₂), LaMg₆Ga₆S₁₆ is also a promising nonlinear optical material.