Shedding Light on the Structure and Characterization of K₂ZnGe₂O₆: A Phase-Matchable Nonlinear Optical Crystal

Nonlinear optical (NLO) materials have recently aroused great interest owing to their capability of frequency conversion in solid-state lasers. Herein, we report an acentric zinc germanate K₂ZnGe₂O₆ obtained successfully through spontaneous crystallization methods. It affords a novel three-dimensional (3D) framework comprised of [GeO₄] and [ZnO₄] motifs with K atoms located in the tunnels. K₂ZnGe₂O₆ displays a moderate second-harmonic-generation (SHG) intensity (0.73 × KDP) with phase-matchable behavior. Optical characterization demonstrated that it has a UV cutoff edge located at 368 nm with a large energy band of 3.23 eV, accompanied by a wide transmission window, covering a 3-5 μm atmospheric window. Moreover, thermal properties implied that it possesses intriguing thermal stability of 987 °C and a congruent melting nature. Additionally, first-principles calculations unveiled that the NLO performance was primarily attributed to the collective effect of [GeO₄] and [ZnO₄] building units. These findings indicate that K₂ZnGe₂O₆ is a potential NLO crystal.