K ₃ YGe ₃ O ₉ : A Rare-Earth Germanate Nonlinear Optical Crystal with a Short Ultraviolet Cutoff Edge and Moderate Second-Harmonic Generation Response

Germanate-based compounds are emerging as highly appealing candidates for nonlinear optical (NLO) applications due to their favorable electronic configurations, robust structural frameworks, and wide optical transparency. However, germanate NLO crystals with a short ultraviolet (UV) cutoff edge and high second-harmonic generation (SHG) response are urgently demanded. Here, a new noncentrosymmetric rare-earth germanate, K₃YGe₃O₉ (KYGO), was successfully prepared via the spontaneous crystallization technique. KYGO crystallizes in the triclinic <i>P</i>1 space group and features a three-dimensional framework constructed from corner-sharing [GeO₄] tetrahedra and [YO₆] octahedra, with K⁺ cations filling the interstitial channels to ensure charge balance and structural stability. KYGO possesses a remarkably short UV absorption edge at 210 nm, corresponding to a substantial band gap of 5.36 eV, and exhibits a broad transparency window extending into the mid-infrared region. Thermal analysis reveals a negligible mass loss of up to 1400 °C and an endothermic peak at 1225 °C, indicative of a superior thermal stability. KYGO displays phase-matching behavior and a strong SHG response of 1.1 × KDP under 1064 nm laser irradiation. First-principles calculations further elucidate that the NLO activity predominantly originates from the cooperative electronic distortions of [GeO₄] tetrahedra and slightly distorted [YO₆] octahedra. These findings highlight KYGO as a promising candidate for NLO applications and offer a potential avenue for fabricating short-wavelength germanate NLO materials.